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Drainback design questions

RyanW Member Posts: 31
I am doing my best to design a new drainback solar hot water

system to provide domestic hot water and help heat the 1500sq ft concrete radiant floor in my

house in central Virginia.  I know that it will not entirely heat our very insulated passive solar house but I am looking to reduce my fire wood consumption.    I want to avoid summer overheating so a

drainback system seems like the best choice.    

I have room for (8) 4’x8’ AET 32 panels mounted flat on my 5/12 pitch

south facing roof and am looking to install a 120 gal solar storage tank with

an integrated heat exchanger.   Sadly I

don’t have room for a larger tank.   I am

hoping to set it up to automatically send the excess heat to the radiant floor

during heating months.   For safety I

want to make the currently open radiant floor system a closed system which will

add an extra pump and heat exchanger.    

For non heating months I will simply turn off the controller that kicks

heat to the floor so that the system will drain back when the tank hits its max

temp.   I am attaching an image of how I

see the system and would appreciate any advice.  

 A few Questions:

1. is 1" copper the correct sizing for feeding 8 panels or should I save a bundle and use 3/4" pipe

2. It looks like most 120 gal tanks have a 20sq ft internal

coil heat exchanger. Is this enough to handle the heat from 256sq foot of


3. should I add a pressure tank and pressurize the drainback

system to reduce the chance of steam  if

the system turns on mid-day and runs fluid through 8 super hot panels

4. is the Taco 009 the correct pump for only 9’ of vertical

head?    I calculate a total head of 19’

(9' ft vertical head + 50’ of 1” copper pipe + the heat exchanger + a safety of 4’)

5. any advice on sizing the flat plate heat exchanger for

the floor.  

6. I currently have an I link SP-81 pump controller for the

floor pump and would like to pair it with an aquastat that would kick the floor

pumps on when ever the 120 gal tank hits 140* and off when it drops to

130*  any recommendations?

 Ok that is enough in the question department for now.   Thanks for any advice!




  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    HI Ryan,

    I'll try to help with this as much as I can.  I'll answer the ones I have time for tonight, and others (including drawings ) tomorrow If I can.

    the simple questions first:

    I don't think you need anywhere need 8 collectors first.  based on your drawings, you have the ability to put 4 collectors side by side.  I would go with 4 4x10s, tilted up to somewhat higher than 5/12.  even if you can get to 45 deg. that'd be a lot better.

    at that latitude, you'll get something like 10% more heat in the coldest months of the year. 

    if you don't have the ability (visuals? covenants?) to tip the collectors up more, I would still go with 4 4x10s, as the arrangement you show on the drawing won't work for a drainback system (never tried it, you're welcome to, and tell us how it works, but I know an engineer who built a system like that and had all manner of problems getting it to work)

    I'll suggest looking up the Caleffi Idronics manuals.  3 and 5 have a whole lot of good information on integrating solar and space heating. 

    after that rambling, I'll answer them in order

    1:  1" pipe is good for 8 collectors.  about 1 gpm per collector and you're getting to the 8gpm flow rate which will establish syphon and work well.

    2:I would recommend either doing an internal tank with a larger coil (HTP

    SSU 119 or smart 120 or SME 120)  the rheem you;re looking at has a

    coil with high head loss thru it, and getting proper flow takes some

    serious HP.  you could also: run the DB fluid thru 2 flatplates, one circulates to a

    120gal storage tank, and the other has floor fluid going thru it.  if you have the room for a little more equipment, that's going to be more efficient all around, as you can put more SF of heat transfer space in the system.  if your time isn't too expensive, the cost would be similar or cheaper.

    3: yes, pressurize the system, and you don't need a separate tank.  put in a pressure relief valve (#50 probably)

    4: the taco 009 is a great pump for most DB situations.  use a variable speed controller to save some electricity and allow you to get the most out of your system.

    5: Go to Flatplateselect.com   you have to sign up, but it's a great sizing tool for their flatplates.  others will be similar. 

    6 what else is heating the floor? let;s sort out the other details of the system and then look at the controls.

    I have had good luck with: solar fluid from the collectors hits a diverting zone valve, a and A: goes to a flatplate HX in the floor loop (cold) and then back to the pump to go back to the roof.  floor pump activates when the diverting zone valve goes to the floor. or it goes to B: thru a coil in the DHW tank, to the pump, and back to the roof.

    Set in the winter mode: when the incoming fluid hits 120 it switches to go route A and turns on the floor pump (even if it's on) the supply temp to the floor never gets more than 130,( with the return 20 deg cooler)  that's actually on a pressurized system, and it works well, because when the solar temp hits 180, we shunt the rest of the heat to the floor, and because it's a basment, it never really heats up much.  keeps it dry actually. 

    160 SF would do nicely I think for a 1500 SF house in Virginia. 256 might cook you out.

  • RyanW
    RyanW Member Posts: 31
    Re: design

    Thanks Karl for the insightful comments.  I love figuring out this kind of stuff and have lots of time to install the system during my off season.   I built my own house years ago and last year I installed a 4kw PV system which was also a fun learning experience. see http://themouseworks.com/Solar.htm for photos.    This system will replace a 30 yr old Reynolds system that I installed used years ago and I fear is on its last legs.  I have it heating an 80 gal tank which gives up hot water most of the year with a manual bypass to an on demand hot water heater for backup when it is needed on cloudy winter spells.    I will move that system to spend its last days heating a greenhouse floor.  

       So  I am in a strange position where I have a state rebate that will virtually pay for the parts for the entire system.   The rebate is based on the size of the collectors so the extra four will basically be free.    I know that I am over sizing the system but I currently live in my workshop and am planning on building a house either as an addition or nearby and moving half of the collectors onto a much steeper roof what would have better solar gain and then the square footage would better reflect the panel area.   Free panels now to supply heat later on is too good to pass up! 

      I have a standing seam roof and don't want to penetrate the roof.   I will be using S5! clamps and given the high winds that we often experience I worry about tiling the panels and thus adding massive wind-loading to my roof.  A 10' by 16' sail on my roof scares me.    I was orginaly looking at installing two rows of 4x6.5 panels (less wind load)  tilted at 55 degrees but then did calculations with PVWATTS and found that the same number of 4x8 panels flat to the roof (23 degrees) would give the same winter output without the wind loading issues.   The price after racking is nearly the same.

    So I am curious what specifically would not work with the panel layout for drain down.   Is it the T junctions on the roof on the supply and return lines feeding the two rows of panels running in parallel?    The way I see it the the extra length in the upper panel supply line would equal the extra length in the lower panel return line and  thus the flow rate would be the same for both sets.      If the two rows of panels are tilted to drain and all of the piping is tilted what would keep it from draining?     I have not interest in the water not draining correctly and thus blowing up the panels!

    Would it be better if each row of four panels had dedicated set of 3/4" piping with the HTF joining at the draindown tank and then splitting after the pump.    I could see a flow meeter on each return pipe and ball valves to balance the flows if needed.    Or perhaps two seperate pumps each paired with a 15gal heat exchanger drain back tanks with  with one system feeding the 120 gal water tank and the other feeding the floor.  The floor panels would thus be dormant half the year.   More equipment!

    2.   I like the two flat plate heat exchanger idea.   I was recently told that they have difficulties with mineral buildup with the combination of high temp situations and domestic hot water. Have you had that experience?  I have well water with no noticeable mineral buildup on the tea pot I have used every day for 5 years.    

    3.What PSI would you pressurize a system to?   Don't they naturally pressurize a bit when things get hot in the system?

    6. Right now I have my on demand water heater set up to heat the floor in an open radiant system.   We never use it as I heat mainly with a wood stove (renewable) to save on natural gas.   But I would like to keep the on demand water heater as back up heat if I ever get injured and can't cut wood.

    The system that you described is it a drain back or antifreeze?

    Thanks for the tips on the Caleffi  manuals.  I will look them up tonight.


  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    Given the situation you're in, I would then say to use 2 separate pumps for each array.  I think you'd see the upper array not fill properly if you used a single pump.  if you are going to pull off some of the panels later, you could put the 2 arrays in series, IE pump into one array, and then thru the second array.  not optimal, but definitely doable. 

    the appropriateness of the flatplate option is how hard your water is, and how on top of the system maintenance you are willing to be.  most people want a "turn it on and forget it" system.  this is usually what we end up providing.  at most there is a thermostat or switch that the customer changes twice a year.   we will put isolation valves and drains to flush vinegar thru the HX if necessary.  Just keep in mind that any heat source (like a HX in the DHW loop) needs to have a pressure relief valve that can't be isolated from it.

    I usually pressurize to 30 PSI, with a 50PSI PRV

    we are required to fill drainback systems with 30% antifreeze.  at your latitude, i would not worry about using pure water. 

    check out the system on page 35 of Idronics 6.  That would be a slick situation for you. 

    you would have to do different things on the heating side, but on the DHW and Solar side you'd be in good shape.

    for controls, I would design something that would enable the solar at 100F, and force it on at 140, so you can keep collecting during the heating season.


  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588

    You have shown a system that doesn't look as simple as it might be.

    At BuilditSolar.com, Gary has documented a system that does the same thing that you want to do but he does it with just 2 pumps and only one heat exchanger which is just a roll of PEX.

    Many of your questions are answered in all the background information. Study it, and I think you'll agree with his design philosophy and the solutions he uses:


    Included is control design, pump and pipe sizing info. Don't let the DIY approach put you off. Gary is a longtime product design engineer and has measured and reported the performance of the system better than most manufacturers.

    The only real constraint you have is your space available for a tank. Here's a tank that is reasonbly priced, fits your space, and will perform the functions that you need:


    Your collector array piping is just fine, drainback or closed loop, it's the right way.

    There are no "safety" reasons for having a closed loop in your floor slab.

    Your storage tank size is about 5 times too small.

    All of today's solar controllers have a lockout setting that will prevent the solar pump to start up if the collector sensor is above the boiling point of water. Pressurizing raises the boiling point, but doesn't eliminate the possibility of boiling.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    I'm not saying that there's no way that array would work, I just have concerns about it flowing properly.

    ABSOLUTELY have a closed floor system.  It can be the same water you are using in the solar loop.  WIsconsin plumbing code forbids the returning of heating water to the potable system, with good reason.  there has been lots written about it elsewhere.

    I really like Gary's system, for a lot of reasons.  There are a ton of ways to skin this particular cat, and none of them are the "right" way, but there are certainly "wrong" ways.  open floor loops with potable water in them is one of those things.

    given his constraints on mechanical room space, and collector mounting, I like the system in Idronics 6, page 35. 

    you can certainly hook up the collectors any way you like, but if you hook them with 2 pumps, you can use one, both, whatever.
  • ABSolar
    ABSolar Member Posts: 41
    Drainback details

    Hi RyanW,

    1) Yes, 1" copper is the correct size for 8 collectors.  Use type "L" for outside, optional for inside (but nice and heavy duty)

    2) No, 20 square ft of hx coil isn't enough for 256 square feet of flatplate collectors

    3) PRESSURIZE a drainback system!?!  I never heard of this - WHY?  Don't worry about the flash steaming upon pump startup.  The collectors can handle it - it's just a brief event - the collectors will rapidly cool down to the tank temperature- all will be well. Drainback is drainback.  Steamback is a pressurized glycol system.  Drainback is self regulating.

    4) No, the Taco 009 pump is undersized for that flow and head.  (check the pump curves).  Either step up to 0011 with variable speed pump control from the solar controller or use 2 pumps in series - again controlled to drop out the booster pump once siphon is achieved.

    5)  Great advice on that flatplate heat exchanger website.

    6) I don't like the potable domestic in the radiant slab.  You can eliminate the heat exchanger and keep solar water and radiant slab water the same.  Consider a Turbomax 120 gal tank to prefeed your on demand hot water back up.  A simple motorized 3 way valve should be able to switch back and forth between slab and tank for solar heat storage. I difer to others regarding the controls of such...Kevin? Karl? Hot Rod? 
  • RyanW
    RyanW Member Posts: 31

    Thanks everyone for all of the good input.  

     Karl I figured that the upper panels would fill all of the

    way as the hot return from them would join with the hot return from the lower

    set and then go UP the roof about a 18” before turning down into the attic

    through the ridge cap.   If the piping

    continues to gain altitude above the highest panels wouldn’t that force the

    panels to fill before it could turn and head back to the drain back tank?  

    Code here allows open radiant systems and that is what I installed when I built my house on the cheap.   But I like the idea of turning into a closed system if I can.   

    Kevin.    I sure wish

    that I could fit a larger tank.  I would

    if I could but I am having to bust into a wall just to fit the 120 gallon

    tank.    I do love the looks of the Simple Drainback 3rd

    Generation System.   I need to think

    about that some but the $4500 price tag is intimidating.  But the simplicity would be wonderful and

    might be worth it.     I love the DIY technology and grew up with

    two different homemade theromsiphon systems that my dad made.   But every thing  has to be SRCC rated for the rebate!    Right now it looks like I could purchase the equipment that I need for nearly the same cost as the rebate so my only cost would be my time installing the system!   Feels too good to be true.  

  • ABSolar
    ABSolar Member Posts: 41
    collector piping layout

    Hi again RyanW,

    Nice drawings, btw.  Your piping schematic was correct for proper drainback function  and balanced flow between collector arrays (reverse/ return piping).  I see no reason why the system shouldn't drain back.

    One item on the radiant floor schematic - it appears that you connected the bottom of what looks like a Spirovent type air eliminator with the return loop from the radiant floor, in essence, by passing the heat exchanger.    Is this intended to be a mixing valve point? I would think that this is not what should be done for temperature mixing control.  Can you clarify?
  • RyanW
    RyanW Member Posts: 31
    Mixing valve

    That should  be a three way mixing valve to mix hot water from the heat exchanger  with the cooler return water from the floor(pre heat exchanger) to an appropriate temp for the pex in the concrete floor.   I hear that it can cause expansion and contraction problems and also hot and cold spots.    It is my understanding that water above 120* is not a good thing.   So lets say that the heat exchanger was supplying 140* to the floor it would temper it down to 110 or so.    If the heat exchanger was supplying cooler temp then it would not mix any cold in.    

    Perhaps this is overkill and not necessary?

    One local supplier recommended a vacuum breaker on the top of the roof system to help with drain back.   That seemed unnecessary to me and was wondering if there was any reason for one?   I know that it would mean that the pump would have to be bronze or stainless steal as it would low oxygen in to the system.

  • RyanW
    RyanW Member Posts: 31
    another option?

    I found a local distributer who can supply AET equipment at

    a very reasonable price and they have suggested a few changes which I am


    Version 1.

    Every one says that the 20ft heat internal heat exchangers

    in 120 gallon tanks is probably too small to handle the heat from 8

    panels.   So they are designed a system that would use two AET 15 gallon drain back tanks with internal 15’ heat exchangers

    mounted side by side such that the Hot return from the solar would split just

    above them and fall into each one and then join below  and get pumped back up to the panels with a

    Taco 009s pump. They say that without the solar flowing through the HX the Taco 009 pump wold work with 9' vertical head. 

    I would use a regular 120 gallon tank that would have a taco

    008ss pump pumping domestic water in a loop from the tank through both drain

    back heat exchangers in parallel and back to the 120 gallon tank.    The rest of the setup would be the

    same.   Sounds like a lot of careful

    installing to make sure that both drain back tanks get equal flow from both

    solar and hot water sides but seems doable with careful measuring.   They

    want me to use a Steca/sunearth 0301u controller .   They recommend a vacuum breaker mounted at

    the top of the panels sytem to help with draining which seems odd to me.


    Version 2.   Would split

    the system in two so that when I build the new structure it would be easy to

    move on system.  I am attaching a PDF of this system.

    System 1 would be one set of 4 4x8 panels would supply a AET

    TMSS 120 gallon domestic tank with internal heat exchanger.    A taco 009s  pump would pump distilled H2O through the

    panels and back to a 15 gallon AET drain back tank and then into the heat exchanger.  I think that all of the piping would be 3/4”

    copper.   This would be controlled by the

    Steca/sunearth 0301u controller.   I

    would have the on demand water heater as backup for this system


    System 2 would have the other 4 4x8 panels and would only run

    in the winter months  to heat the

    floor.    This system would use the same

    Taco 009s pump to pump distilled water through the panels and back into a

    15gallon AET dranback tank WITH a heat exchanger.   A second pump (grunfus 15-42 that I already

    own)  would circulate water in a closed

    system through the drainback tank heat exchanger and then through the floor.   I think that the 1500 sq foot floor would

    soak up all of the heat and keep the system running cool and thus I understand the

    panels would operate very efficiently.    I need to do some research as to if the radiant

    supply water would would get too hot 130* and require a 3 way mixing valve to

    temper it.   I don’t know if the Steca/sunearth

    0301u controller would be the correct thing for this job.  Ideally this system would run when ever the

    sun shines in the winter with a indoor thermostat in line with the controller to

    keep it from running if the house got way too hot like 75-80*.   I don’t think that would happen and I don’t

    mind opening windows for fresh air either.  

     This system would have no backup

    heat which is fine as we use a wood stove and have never used the radiant floor

    with the on demand water heater in the current open system.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    edited March 2011
    Simple Solar Drainback Tank, etc.

    Dan doesn't want us to discuss pricing.  I disagree with that because

    cost is the second most important thing about system design.

    Anyway, the SHEM Simple Drainback Tank itself is less than half the

    price of the HTP solar tank.  The number Ryan mentioned must be retail

    for everything including the panels.  The shipping cost of a tank is an


    If you use the SHEM tank or an atmospheric tank, there's no potable

    water in the collector loop or the radiant loop, so there's no legionella safety

    problem.  This design also eliminates the need for a drainback tank, expansion

    tank, load side heat exchanger, tank side pump, etc.

    Not "everything" needs to SRCC rated for the rebates.  Only the collectors for now.

    Pressurizing a drainback system is often a good idea.  Oftentimes the

    tank isn't pressure rated, so you can't pressurize those systems.  I

    like closed drainback systems because they minimize the storage tank

    water lost to evaporation.   Also, a flash steam event can be VERY noisy, even scary sounding.

    If you study Gary's system, you can see that the size of the collector side heat exchanger isn't an issue because there isn't one.  The local distributor you found is doing their job well.   But remember, their primary job is to sell equipment.  The vacuum breaker is another really bad idea for a lot of reasons.  The air gap that Gary describes performs the function of breaking the siphon.   Here's the description of the collector loop piping with some other important information, like removing the check valve in the pump, etc.:

    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • ABSolar
    ABSolar Member Posts: 41
    Split sytems

    RyanW -

    Not a bad idea - splitting the systems for simplification and future changes - I like it.  Vacuum breaker up top!?!  NO NO NO!  System will not operate correctly - no vacuum breaker up top.  BUT, make sure there is an air gap on the collector pipe returning inside the drainback tank (AET drainback allows for this...) Also, Gary says same thing in Do it yourself solar - see his diagram for the tank return pipe.

    Regarding pump sizing, follow Gary's step by step to verify if the 009 will work, even on the split system.  Choose your target flow rate - SRCC testested AE-32 @ 0.0193   gpm/ft2 = .62 gpm/ collector.  Gary @ doityourself solar says you can get better performance using .07 gpm/sqr ft. which then becomes 2.24 gpm/ collector - big difference!  Now your flow rate will be halved if you use 4 collectors for each system and the friction head head changed because you can now drop to using 3/4" pipe.  But costs rise again for double the piping & 2 pumps...

    If you stay w/ version 1, then you still have 8 collectors, 1" pipe and maybe the 009 will work - double check  the math!  In both version 1 & 2, regarding the radiant loop mixing, I think you can use a simple 3-way anti scald valve w/ built in temperature adjustment indicators (Caleffi has a nice temp gauge to add on theirs) to blend return water w/ supply - be sure to install the 2 tees for this mixing bypass pipe in reverse/ return format for optimum flow balance.

    For your controller, does the Steca have heat quantity measuring?  Does it also have variable speed pump functiopn or 2 pump drainback function?  If not, take a good look at the Caleffi/ Resol BXplus units - I think you'll like these...

  • ABSolar
    ABSolar Member Posts: 41
    pressurizing drainback

    Kevin  - can you clarify how pressurizing a drainback system works and why one would do this?  Is the intent to assist the pump in sending water to the collectors? Exactly how does pressurizing help in the water draining back to the drainback tank?
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    edited March 2011

    I think the double array may work fine.  I tend to be conservative in my system designs that I sell to customers, and not sell designs I'm not comfortable about.  therein lies the difference between the professional and the DIYer or experimenter.  my house is my lab, not my customers' houses.

    I will certainly encourage the use of the same water for the solar and floor loops.  it's never a bad idea to use a DB pump with bronze or SS internals.  especially if you don't have O2 barrier pex or the tank is atmospheric and vented. 

    do not use a vacuum breaker on a drainback system.  that's a hold over from the draindown systems which aren't used up north any more.  I don't know about other parts of the country.

    you might look at american solartechnics for a 200G tank that will fit in a small area.  you could drainback directly thru the tank and use a copper preheat coil in the tank for your dhw.

    If you are doing a drainback and have a tank that's too small to accomodate all the heat you could produce on a spring or fall heating day, I would recommend using a 2 stage control strategy.  enable the solar heating to the floor when the tank reaches 120F and then force the floor heat on when the tank reaches 140, with a floor sensor as your high limit. 

    I don't know the steca controllers at all.  I use Resol/Caleff and or build my own heating controls. 

    Take a look at Page 34 in Idronics 7.  That's what I'd shoot for, and look at the caleffi Isolar Plus for control duties.  It has plenty of flexibility for what you need, and with the addition of a setpoint control, you'll be in good shape.

    I usually start with the collector, then work my way back, in the design process.  figure out what the piping needs to do, and then work thru the controls.  sort out how you want the system arranged and want it to work, then design the controls to make that happen.

    FYI the suggested flow rates for the AET 4x8s are between .5 and 1.8 GPM.  I usually shoot for 1gpm per collector, and 1.25 per 4x10 collector.  Most AC pumps will still over pump most systems we install, thus the use of the VS controls.

    Google TACO TD-10 for a great circulator sizing worksheet.

  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Pressurized Drainback

    Most of the small drainback tanks for solar are closed systems. As they heat up, they pressurize themselves. There isn't an expansion tank, just an air cushion. That's the same air that goes up to the collectors at night. These systems can have added air pressure if necessary in this rare case: In sunny high altitude climates, these can water hammer in the afternoon when the pump shuts off. That's a callback. 30 psi will do the trick

    The above probably doesn't happen much outside of Colorado, Montana, etc.

    Another type of closed drainback system (my favorite) uses a large, closed storage tank to contain the collector water and air. This eliminates one of the pumps required by the small drainback tank system described above. It's dead simple, the DHW picks up its heat from a huge one-pass coil of copper , (that has a significant volume of heated water inside) : http://www.solarheatexchangemanufacturing.com/NewFiles/120galdb.html

    Again, it pressurizes itself when it heats up. If you use a pressure-rated tank like this, you keep it closed for at least two reasons: Water evaporation and the ability to use a less expensive cast iron pump. (Gary Reyes uses cast iron pumps in his open tanks, so far with success. He reckons they're semi-closed because it's the same water all year, and only a small amount of new air is drawn in daily from the contraction of the water when it cools)

    The air is at a constant pressure inside the whole system. It pushes down on the water just as much as it pushes up. Therefore pressurized air doesn't help the pump send the water up, nor does it rush up to the panels faster to speed up draining when the pump shuts off.

    So: pressure on the system can help prevent pump cavitation, the occasional steam hammer event, and (not sure if I can find a reference on this one, H.R.?) makes any air bubbles in the system much smaller and therefore less problematic. Air is a fact of life with drainback, so you can't eliminate it (even if Ryan's supply house would like to sell him a few air vents and scoops :) )
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    A couple more notes

    In residential drainback, if your pump has enough head to fill the collectors, it usually has more than enough flow for the collectors if your pipe is sized right. Therefore you should almost always use the smallest pump that will still fill the panels.

    Having a large collector flowrate increases collector efficiency, yes, but it comes with two penalties that can severely diminish the system efficiency: 1. Higher Pumping power and 2. Less Tank stratification due to more mixing . To understand tank stratification, ask yourself which tank has the most usable heat: 1. A 100 gallon tank with the upper half at 100F and the lower half at 40F, or 2. A fully mixed 100 gallon tank at 70F.

    Another drawback of a high collector flowrate is excessive collector pump cycling. A pump that cycles 30 times per day probably has a shorter lifespan than a pump that cycles twice.

    Thermosiphon systems work great, but their collector flowrate is typically less than .2 gpm/collector.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • CC.Rob
    CC.Rob Member Posts: 130

    Been following this thread with interest.

    I have a simple SDHW system with an SSU-119 and Steca 0301U. Love the SSU-119, matched to 80 sf of collector at 41 north. Works great.

    The Steca is a very, very simple 3 input, one output differential control. If I had to do it again, I'd go with a variable speed control like a tekmar 157 or one of the Caleffi's as has been suggested. I suspect the Steca might be a bit simple for what you need to do, particularly if you need to control anything else. But if not, the control has been solid for just about 3 yr and has very low power draw.

    Good luck. Sounds like a great project.

    p.s. Nice hats!
  • hot_rod
    hot_rod Member Posts: 22,121
    edited March 2011
    pressurized drainbacks

    are the best way to go as long as the components, like the tank are rated for pressure. The pressure raises the boiling point. The pressure will prevent the water column from breaking up at the place where the temperature is the highest and the pressure the lowest. Which is typically at the top of the collector.

    Closed pressurized DB eliminates any loss of water from evaporation and keeps the system from taking onn fresh oxygenated water.

    With an unpressurized DB you depend on the pump to pressurize the loop. The pressure should prevent the water from flashing when it hits a hot absorber plate. You'll know that sound when you hear it, as will the neighbors.

    The pump in an unpressurized system may not put enough pressure inn the collector, even when it is running, under a full sun condition, it could vaporize. When the pump shuts off you can actually pull a negative pressure at the top of the collector loop

    I think 20 psi is a good pressure for DB, watch the pressure increase as it heats up to assure you have enough air space for expansion. Or use the solar thermal expansion sizer formula. For a single tank, no drainback tank, I can include the math for determining how much space to leave at the top of the tank for expansion.

    The DB water can be tied directly into the heating system if it is pressurized and the heating system is suitable for that pressure.

    Some of the new solar controls have drainback function, options. It allows a dual pump, or one pump on a variable speed control. They also provide some drainback specific start up features, like filling time and stabilization to prevent short cycling.

    You need to size the piping to assure at least 2 fps velocity to be able to push the air bubble back to the tank. So with 1" copper you need to flow at least 5.5 gpm.

    Credit: John Siegenthaler Appropriate Design for the drawings

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RyanW
    RyanW Member Posts: 31
    re-designed system

    Thanks everyone for all of the good advice.   I have been reading, researching, and mulling over the wisdom and decided to try to simplify everything.   Kevin in Denver you will be happy to hear that I am now looking to follow your advice and use the Solar Heat Exchange Manufacturing 120 gallon simple drainback tank.   I love how it eliminates heat exchangers and pumps.       Yes you were right the price that I found on-line was a good double the real price!    So now I am looking to tie it to all 8 panels piped in the reverse return fashion and use the same solar water to heat the floor.  The SHEM tank comes with a Wilo Star S 21F pump that appears to be large enough to pump my 11ft of vertical head.  I am attaching drawings of the much simpler system.   

    But now come the questions:

    1.   Any good/bad experience with the SHEM tanks?

    2.  I will use the drainback water to run through my radiant floor.   It does not look like the SHEM 32 controller will let me set the second pump settings like I want.  It only allows for a temperature differential rather than set temperature settings.   I want it to cut on when the tank reaches 140* and off at 130* so that we will have domestic hot water for a cloudy spell.   I think that I need an Aquastat for this job.         I currently have a thermostat linked to an  "I-link sp81" which turns the floor pump on.   I would like to have the aquastat do this or perhaps there is a controller that is better suited for this job.  Any recommendations

    3.   Any good three way mixing valve recommendations that don't have too much resistance and are durable for tempering the solar fluid down to 110*?

    4.  AB Solar you mentioned to make sure that I use a two close Ts on the radiant loop near the mixing valve.   As I don't have a back up heater on that loop so do I need to get into that fancy plumbing?  

    Thanks everyone.

  • hot_rod
    hot_rod Member Posts: 22,121
    mix valves

    You will want a good, listed, 3 way thermostatic valve on the output DHW piping from the tank, and a second one to mix down the radiant temperature.

    It's possible for a solar tank to run much higher temperatures than you want to feel at the faucet.

    With that many collectors you may want to run that solar tank to 180F to assure good DHW production, and to store as much energy as possible. Even in winter months you could see 130- 140F with that much hp.

    Caleffi is one of many brands of thermostaic 3 way valves. We build the 521 series which has a handy temperature gauge option for ease of setting.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    I think you've got a good thing going there. 

    I would love to see one of the big name tank manufacturers come up with something like this tank, with a coil usable for DHW preheat.  This is essentially how I'm going to plumb up my SHW system at my new place, the radiant is currently run off a mod-con tank style water heater, with a FPHE to the floor loops.  I'll probably use a flatplate to go from the solar to the DHW.

    So, for Controls on the heating side, I would do something 2 stage.

    enable the floor heating with stage one thermostat (air sensing set to 65? ) when the tank reaches 90 or 100F, (that way if you only get a couple hours of sun, you keep your tank temp down if the house needs heat, and the pump will run a little longer) and then you can force the floor pump on once the tank gets to 150, and let it turn off once it drops to 140.  (it will still provide lots of DHW, and store some of that heat in the concrete, for use overnight) the high limit for that scenario should be a floor sensing tstat set to maybe 80F?

    That tank is only going to store some of the BTUS that the array will produce most of the year, and your floor will effectively store many more BTUs than the tank. 

    I think the Caleffi Isolar BX will do nearly all that except the thermostat function, But I haven't used one yet.  I use the Ranco 2 stage digital aquastats pretty often to do this job, and just put a toggle switch on  the upperstage, and have the customer switch it twice a year (they could also just turn down the second stage thermostat, but this forces them to at least look at the system twice a year.


  • RyanW
    RyanW Member Posts: 31
    mixing valves

    Thanks Hot Rod for the mixing valve tip.  

    We are definitely going to have a mixing valve on the DHW side plus it will be one of the few things that our inspector will look for.    My current old cobbled together system does not and thus I am well aware what 180 degree water feels like.   It sure does clean pots though!    After the inspector leaves I might bypass the tempering valve for the clothes and dish washers as the extra heat sure helps clean.  

    I forgot to mention that is was really nice to see that others also felt the vacuum beaker was a bad idea.   I was surprised that they included it in their quote.

  • ABSolar
    ABSolar Member Posts: 41
    radiant return & bypass

    Hi RyanW,

    No, I wasn't speaking of closely spaced tees for the bypass piping (that's for use in primary/ secondary piping).  But I was trying to say that the the orientation of the side port on the tees are used in reverse/ return format to equalize piping friction.  Just like you do on the collector piping....so if you use a side port of a tee to go from the radiant return pipe back to the radiant supply pipe, then use the end port of the tee on the supply piping to receive the bypass pipe.  But if you are using a simple 3 way mixing valve on the supply piping, then just make sure that you alternate w/ the tee pipe configuration on the return...sorry, I'm trying not to overcomplicate my description.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    What are the reasons to temper slab temperature?

    Isn't the reason that radiant distribution loops have a 3-way tempering valve is to protect flooring from high temperatures?

    If you have a bare slab with no flooring, is it necessary?

    I know that copper pipe can bust up a slab with thermal expansion, but PEX doesn't have the strength.

    Room temperature overshoot might be a reason, but a hotter slab can heat quicker.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • ABSolar
    ABSolar Member Posts: 41
    reasons for radiant mixing

    Kevin, I think the maximum floor surface temperature reccommended is 85 - it's a comfort thing - anything beyond becomes uncomfortable to the human radiator.  Dan Holohan talks about this in his hydronic radiant heating book.  Remember not to confuse floor surface temp w/ supply temp - you still can feed 120 mixed supply at design temp load (10,5,0,-5,-10 etc) in a slab to get the result of 85 floor surface temp - depends on room heat losses.
  • ABSolar
    ABSolar Member Posts: 41
    pressurized db

    hi HR,

    You mentioned the water column "breaking up" - at the top of the collectors - is this because of a steam flashing event?

    I can see that a volume of pressized air (either in a seperate db tank or combined db/ storage) on top of (effectively "behind") the drainback pump will aid the pump in pushing the column of water up to Where do you introduce the air pressure? At the db tank top? Does it matter? 

    How does pressurizing a db closed loop system prevent evaporation?  Don't all closed loop systems prevent evaporation?  Wouldn't increased pressure try to push minute water out through fittings more so than equalized atmospheric pressure?

    Is there a difference in the effect(s) of pressurizing db systems between using seperate drainback tanks or combined db/ storage tanks?  ...just trying to wrap my head around pressurizing a closed system that has a volume of air that gets mixed around in the system as opposed to staying in an expansion tank...

    I love that you could use the db pressurized htf in the radiant system, effectively eliminating an HX and a pump! 
  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    yep on the fluid temp. 

    in solar heating cases where we have a large amount of solar heat to deliver from a large tank, we can deliver the same number of BTUS slowly with low Delta T (supply to floor temp) and that works nicely.

    in this application, we would like to deliver the heat to the slab quickly as it's being produced.  I would therefore mix the floor loops to say 120 or so.  the heat delivery will be pretty rapid, and the floor won't overshoot too badly. 

    the goal of the solar application is to keep the collector temp down while not overheating the space or tank.  the collector efficiency will decrease as the temp goes up, and so usually we shoot to keep the collector temp below 120.  in your warmer ambient temp, you could go to 140 with out any big problem. 

    you could also design your controls with a master switch that will turn on all the pumps, so when you have snow sitting on your panels, you can dump 10 min worth of heat to the panels to get the snow to slide off.  then you can restore to normal operation for the rest of that day.  you would only do this on a sunny day, when you stand to gain much more than you will waste heating the panels.

    works great.

  • RyanW
    RyanW Member Posts: 31
    Sweat or union panel connections?

    Do folks prefer connecting panels together with sweat couplings or using unions?  In my plans I was thinking about putting the panels together with brass o-ring unions such as  the Heliodyne dyn-o-seal  such that they would be easy to remove down the road for any service needs and hopefully easier to install on the roof.      Also is there any danger with the drainback water seeping into union crevices and cracking them as they will be exposed to night time winter freezing?

  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    edited March 2011
    Sweating is difficult on the roof

    Unions make it easy to force things into alignment.   No long-term problems have been seen.   95/5 solder required due to high temperatures, some guys prefer brazing.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited March 2011
    Some CITIES prefer brazing...

    Our fine little city on the prarie (Denver, CO) doesn't allow anything BUT high temperature brazing/silver soldering for connections on the solar collector.

    THe AHJ looked at the stagnation temperature potentials, and decided that we need to do high temp brazing/silver soldering on any solar collector connections.

    The rubber grommets where the copper passes through the walls of the collectors LOVES to be set on fire... I tired to argue, to no avail.

    And the use of split ring escucheons only SLOWS the flames.

    Don't ask me how I know. I am trying to forget as hard as I can :-(

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • hot_rod
    hot_rod Member Posts: 22,121

    we have been sending our collectors out with union 1/2s sweat on them for about 4 years now. I haven't heard of any issues with the fiber gasket union connection.

    Many of the German collectors assemble with a stainless flex tube and o-ring seals.

    Another California collector brand has been sending out those o-ring unions for at least 20 years that I know of.

    Maybe if the union or connector is a factory installed fitting, and tested by the listing agencies it is approved by the AHJs?

    I like the unions as you can assemble them on the bottom connection while they lie flat on the roof, then lift and assemble the upper connection.

    Tough to solder that connection on the roof with merely a 1" stub sticking out. Harder to disassemble and remove, also.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RyanW
    RyanW Member Posts: 31
    And How about the fluid of choice?

    What to you folks use in drainbacks?

    I know that some areas require antifreeze in drain backs but here in sunny central Virginia they allow water and that is what I am planning on using.    I have read that many folks fill their drain back systems with distilled water but 120 gallons to fill the SHEM 120 tank seems like too many grocery carts.    It will be also connected to the slab on grade radiant floor which would be impossible to drain and re-fill anyway.

    I am imagining using my slightly acid well water run through the acid nutrilizor and then through a 1 micron filter.   I would use it to blast the radiant floor clean one zone at a time and then fill the tank.      Is there a better process to clean out an old  radiant open system that has to have some scum in it?   Perhaps a quick vinegar flush like the recommend for flushing  on-demand water heaters?

    Thanks for the union advice.   The local installers around here sewat their panels on the roof which seemed like a difficult process and even more difficult if they ever need to be removed.    I am still in search of a decent source for some good one s with with O rings.   Any leads?  

  • Karl_Northwind
    Karl_Northwind Member Posts: 139

     caleffi Will sell you unions no problem.

    for the last 10 years I've only used standard NIBCO (Brand) sweat unions.  Never had a problem with those unions. 

    we don't ever silver solder or braze the unions, but you may be required to by your AHJ.

    I use distilled water for filling drainback systems, but if you know the water isn't hard, and you check the PH and make sure it's neutral or slightly basic, you should be fine. I'm no expert on fluid chemistries, so I'll defer to others.

    I'll be using well water for mine.

    One thing that I've found makes lots of sense is when assembling the unions, put a tiny bit of thread dope on the union threads and on the backside of the male half.  it's just lubricant, but makes the connections go together easier..

    and use 2 wrenches on that union.  the headers can and will twist if not treated nicely.

  • ABSolar
    ABSolar Member Posts: 41
    to unionize (or not to...)

    I have had difficulty w/ plumping supply unions for collector alignment, because the tiny bit of slop (play) that the sweat end on the collector has, combined with any slightly different manufacturing tolerance of the distance between header pipe stubs and subtle pipe stub orientation, can make union thread matching difficult.  I have also experienced leakage at unions, perhaps due to innital over tightening, or perhaps manufacturing quality control.  I have not used the Caleffi unions, but I will try them - unions do provide for collector swivel and final mounting flexibility.  I also like that they allow changes fairly easily.  That being said, I have done a number of sweat slip couplers and yeah, the gaskets do start to catch fire and yeah, it would be nice if the manufacturers gave us a little more pipe sticking out at the headers.  Even trying to align slip couplers can be problematic if the header pipes aren't perfectly matching.  Now, if labor time is cheap, then slip couplers are the least expensive connection.  But, if time on the roof is important, I'm sure the new gasket type unions are the way to go......
  • ABSolar
    ABSolar Member Posts: 41
    fluid of choice

    I think that, because of your combined radiant volume and your storage tank volume and collector loop volume, that distilled water is impractical.  I would flush your radiant tubes w/ trisodium phosphate and then rinse w/ well/ street water.  For final system fill, I would use well/ street water w/ a dose of baking soda to counter copper/ flux acidity.
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    gasket unions

    we've had good luck with the caleffi collectors and their gasket unions.  my only complaint is the blind ends always look a little clunky, and we've started painting the shiny brass end.

    I've worked on a lot of systems with the heliodyne collectors with the o ring unions, and have only seen leaks where they've not been properly tightened.

  • hot_rod
    hot_rod Member Posts: 22,121
    solar fluids

    Distilled water us generally over-kill. To get true distilled water you boil it and collect the condensate. It is a very expensive, slow and energy consuming process. I suspect some of the bottled distilled water you see on the grocery store shelves is just filtered water.

    DI or DM, deionized or de-mineralized is fine for solar or blending glycols or conditioners.

    This is done with a filter bed much like a softener, but no brine recharge. After so many gallons flow through the filter, they are re-bedded. Some contractors have portable DI units that they take to the jobsite for filling or blending chemicals in hydronic systems.

    I buy DI water from a few different water treatment shops. Rhomar Water here in Missouri sells DI water, I think they lease the multi tank equipment from Culligan? I just take a 55 gallon plastic barrel, or 5 gallon plastic jugs and they fill them. Pay something like 10- 20 cents a gallon maybe. Here are a couple links to better explain the process.



    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RyanW
    RyanW Member Posts: 31

    Aw shucks Hot Rod.   I was starting to look forward to making up a small still (water only!) to make some cheap pure water for my system.   10-20c a gallon is probably a better use of my time and less risk with the feds.  

  • hot_rod
    hot_rod Member Posts: 22,121

    those small countertop water distillers are usually 550- 600W and make about 1 gallon over a 6 hour period.

    Multiply that by you KWh cost. It's an expensive water treatment device for large, or small quantities. I don't think distilled water tastes very good either :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RyanW
    RyanW Member Posts: 31
    System works great! Thanks for all the help

    This post is quite a bit late but I just wanted to give a shout out that the solar system I installed in 2011 has been awesome an I thank you all for all the input. The two rows of four panels in a drain back setup sharing the same feed and return has worked great. see:


    Thanks so much Karl for the heads up with the Simple Drain Back System. http://www.solarheatexchangemanufacturing.com/

    I installed the 120 gallon system and other than enlarging the flow meter to a 1-10 gpm meter to accommodate the increased flow for 8 panels it has been great. I did a bit of innovative plumbing with the tank to allow us to use the same solar fluid to heat our floor such that solar almost keeps our house warm all winter...we burn about 1/2-1 cord of supplemental wood in our stove on the cloudy days. In the end I ditched all the fancy aqua-sensors and put the floor on a simple $6 timer that runs it from 8 am to 1pm and then the rest of the afternoon heats up the tank for evening/ early morning domestic use. We are loving the warm floors this winter!