newbie drainback questions

sunny

my questions:

why do i need a seperate heat exchanger for the heating loop? my drainback tank is also going to be my storage tank (about 225 gall for my dhw and heat). can't the same fluid that circulates through the panels circulate through the heat zones?

how do i keep my heat at (not over) 140* for the radient floor. i've read that you should keep the temp at about 140* for a joist spacing subfloor application and that u can max 170* but i don't like maxing anything.

part of my radient loop will be going up and serpentining in a wall (typ. wall application) do i need an air vent / scoop at the highest point?

assuming it's ok to use pex for my site-built dhw heat-exchange coil, how do you keep it's shape in the tank? do they use copper at the top because of higher temps or can the pex go straight to the existing hw tank?

thanks all

Martin Romero

Drainback System Specifics

Sunny,



Please give a few more specifics on your system, etc.: 



1. What type of panels and how many will you be using?

2. What part of the country are you in?

3. Will you be using a backup heating system?

4. How big of an area do you plan to heat?



Take note of previous posts discussing the use of pex in solar systems.





Peace,



Martín

Mark Eatherton

You could, but....

and there's a butt in every crowd ain't there..



You would have to have a ranch style home for it to work. However, I wouldn't suggest it because of the possibility of environmental fouling (bacteria) and the ills associated with that whole shebang. Too many things to go wrong. It was tried many time sin the past, and it ended up causing so many problems that they got converted to closed loop pressurized systems.



The storage tank can approach temperatures as high as 200 degrees F during the summer. I wouldn't use any PEX within 10 feet of the solar storage unit.



As far as providing lower temp to the RFH circuits, there are a hundred ways of doing that, and the least expensive method is to use a mixing valve to blend the cooler return water from the floor with the hotter water from the storage tank.



You have a lot of homework to do on basic hydronics before you start trying to blend the two systems together, and this web sites owner has a lot to offer in the way of books.



Lastly, I wouldn't put any PEX in the tank. you are asking for trouble in the form of melted plastic. Yes, that is why they use copper. Don't try and re-invent the wheel. Use the ones that have been perfected over the last 30 or so years and run with that.



ANother great resource is the Caleffi i books on hydronic and solar.



ME

sunny

specifics

Martin, i plan on using 3 evacuated tube panel of 30 tubes (sunmax). i live in colorado @ 8000'. no backup heat, backup dhw. heating about 500sqft.

thanks for responding

sunny

butt...

i would have to have a ranch style for what to work? thanks for the mixing valve tip, i'll start looking for those. what two systems? dhw and heat? why do they sell copper/pex hex heat exchange coils? i'm not tring to reinvent anything, that's why i'm asking questions and trying to sort out all this info. thanks for the book tips... 

Mark Eatherton

To be able to use the contiguous storage water...

If you had multiple storey home, how would you keep the upper floors heating systems from drain back into the tank on the lower floors?



I was not aware of anyone selling PEX coils for storage tanks, but it would not surprise me. You can find ANYTHING on the internet. Not that it would be appropriate to use it, but you CAN find it.<<br />




br />



Where exactly in Colorado are you?Proceed with caution...



ME



PS, the two systems are hydronic radiant floor heating and solar hydronic heating.



The biggest problem with solar thermal is when you need it the most, you get it the least. Consider looking at extruded aluminum heat transmission plates to go below the sub floor. Much better bang for the buck, and will operate with lower temps, extending the solar utilization.



ME

sunny

solar rocks!

i see your point in the heat loop now. a lot of companies are selling the pex heat exchange coils, i didn't have to go looking for them, but have been wondering about them all the same. yeah, definatly doing the heat transfer plates. don't believe the "need it the most, get it the least" statement. we have lots of cold sunny days here... i got a couple of hydronic heating books from the library since it doesn't look like i'm going to get many questions answered here

Kevin_in_Denver_2

It rocks after some experience

Don't slam the door on your ad hoc advisors.Get Tom Lane's book, read the Caleffi stuff, etc., then come back with some piping drawings. Then these guys can help iron out any remaining potential problems.It takes some real world experience and training to get a system like this right, but you're right, it's not rocket science. It may be worth hiring someone at least for some of the basic system design.Why no backup? Surely a fireplace?

Kevin_in_Denver_2

Piping diagram

The system you want is being analyzed by Siggy at PME magazine:

http://www.pmengineer.com/Articles/Column/BNP_GUID_9-5-2006_A_10000000000000609705



Attached also is the drawing you seek.



He's simplified it as much as possible.

Mark Eatherton

since it doesn't look like i'm going to get many questions answered here

Huh? Looks to me like every question you asked was answered. Probably NOT what you WANTED to hear, but we can't change the laws of physics. All we can do is EDUCATE.



Copper is 4 times more conductive than plastic, meaning you will need 4 times as much PEX to transfer btu's from the tank to your closed loop heating system.



And there IS significantly LESS hours of daily solar availability during the winter than any other time of the year...



Just because you can find it on the internet doesn't mean it is the best nor most suitable material available.



Good luck on your project.



ME

Karl_Northwind

we'll answer your questions

That doesn't mean we'll tell you what you want to hear.  that's part of being a professional.



first: you shouldn't be planning on doing a drainback system for evacuated tubes.  most evacuated tube sytems are designed to have fluid flow thru them whenever there's sun. 

only one manufacturer I Know of will allow it.   the temps they can reach can damage themselves if left dry in the sun (like summer)



Especially with the evacuated tubes, you shouldn't use any PEX in the solar loop.  I have done it, but only in high mass systems, where there's 100 feet of copper between the collector and pex. (soaks up the high temp spikes with 50 lbs of copper)



Coils in the tanks should be copper.  Pex I think could be used in situations where you're pre-heating DHW, because the storage capacity of 300' of 3/4 pex would help offset the low heat transfer efficiency. 



I wouldn't do un pressurized heating loops unless I had a single loop (say to a fan coil or single raidiator)  or you were servicing each loop with it's own ciruclator.  then you should use stainless or bronze circulators if you want them to last more than a few years (there will be some oxygen getting in the water in a non-pressurized system. 



oh yeah, you'll also want to put a big wye strainer before the pump intake, with a clean out shut off.



all of a sudden, a couple of coils of 3/4 copper to pick up the heat from the tank doesn't seem too bad (use 60' of 3/4 type L soft to keep the surface area up, and head loss down)



reliability is key.  and closed loop pressurized systems have proven it.  but it's your house, so do whatever you want.

post your schematic and I'll post mine.



karl

Kevin_in_Denver_2

Evac. tubes & dry stagnation?

Karl,



Which manufacturer allows dry stagnation? Those tubes can reach >420F, which would toast the foam that they use in the header . Most of them have been tested to SRCC OG-100, which evaluates any damage at dry stagnation, but I don't know how long they let them sit.



Siggy's drawing above shows how to have pressurized drainback (PD), which eliminates the load side heat exchanger. Two more advantages: 1. PD is probably the best way to mitigate the most annoying maintenance issue with atmospheric drainback systems, tank evaporation, and 2. PD is a closed system, theoretically allowing cast iron pumps. The downside of PD is tank cost, which would be $5-$8 per gallon, with a 120gal max. per tank. I've heard of using decommissioned propane tanks for that, but I haven't done it.

Karl_Northwind

drainback evac tubes

Thermomax has their propriatary internal heat regulator that shuts off the evaporation/condensation cycle in the heat pipe.

I think it's maxed out at 110C.  but don't quote me on that.

I don't know that they are recommended for drainback, but I seem to remember reading that. 

I don't do any work with evacuated tube systems.  durability and snow shedding and all that.



I pressurize my drainbacks, if only to eliminate the possibilty of cavitation and steam popping.



I suffer thru a single heat exchanger between the solar fluid and tank, mostly because here in WI, we are required to use 30%antifreeze in DB systems, and the price of antifreeze.



karl

Karl_Northwind

Siggy's schematic

Maybe I need some explanation on this one,

but I don't like a bunch of things with Siggy's schermatic. 



first: the solar hot in should be near the middle of the tank. avoid tank mixing.



second: where is the tank sensor for the DB pump? location is going to be critical in this sort of single tank system.  tank sensor for burner operation?



third: how big a HX are you going to need to heat 5 gpm from 45 F to 120 F with say 125 deg water (keeping temps down so the solar will actually do something.)

I come up with about 35 SF area for a FPHE.  single wall mind you.  like a 5x12x75 plate.



unless you're going to pay for another water heater?  ouch.



also what about the air in the system getting absorbed into the fluid.  there's a lot of water.  isn't this why we moved away from the compression tank, and amtrol made the air control valve?



is he proposing putting this together? and actually monitoring the heat output of the solar to the heating system?   I'd love to see how it runs.



karl

Mark Eatherton

I have a question on it too...

And hopefully John will see these questions and respond.



The return pipe looks submerged to me, potential issue there?



Interesting packaged concept there tho. With mechanical rooms getting tighter and tighter, might be worth looking into. There is only one manufacturer of that style of tank at present.



ME

Kevin_in_Denver_2

More to come in pme Magazine

I'm expecting Siggy to describe the specially placed 1/4" hole in the next edition. 



Karl, I'm sure he's not trying to heat 5 gpm.   Considering the low probability of two showers going at one time, 2.5gpm should do the trick.  My new free showerheads from Xcel Energy are 1.5gpm



Can't blame him for  a slightly incomplete design, because filling in the blanks is how consulting engineers like him make their money!



Are you saying that Wisconsin doesn't trust plumbers to slope the pipes properly in drainback?

Mark Eatherton

Siggy's going to be in Denver...

At CoSEIA Annual Meeting in February. I will be sitting with him on a panel discussing hydronic and solar thermal interfaces. I will ask him the questions then if he doesn't show up here and answer them before hand.



Anyone wishing to attend can contact me and I will forward the event information to them. He is also doing a whole day seminar thanks to Ferguson Hydronics and Dennis Bellanti for making that happen.



ME

Karl_Northwind

glycol in drainback

Plumbers always know best, right?

seriously, though.  the AF is cheap insurance in the frigid north.  water is allowed in specific cases, in the southern part of the state.  I sleep a lot better with antifreeze in my systems.



The requirement is actually from our rebate agency.  if you want funding, they want to make sure the system isn't going to suffer from a catastrophic failure a couple years down the line.  $10,000 worth of scrap copper on a roof, if the installer isn't around any more is no good.



If I recall,water heaters are usually sized by highest use, not "oh, they'll probably never use that much water at once"

 not too many customers are going to foot the bill for a large solar heating system plus hydronic heating system, and then be told they can only take one shower at a time.

I certainly wouldn't.

now, he does show the system using panel radiators, so we cold expect the tank temp to need to be 140F or so, which would reduce the HX size for the DHW heating, but would seriously reduce the heat output of a typical flatplate collector.  and having to do an evacuated tube system eliminates the drainback option, as discussed above.

add to that minimal storage.   If this is just a couple of collectors designed to handle the dhw load in the summer, and help standby losses, it worries me a lot less.



again, just a few musings from a solar thermal installer/instructor/designer. 

Should we move this to a new thread and invite siggy to join in?   I don't want to piss anyone off, but I feel strongly that drawings that are out there should be really well thought out, because someone's going to take it as gospel that anything designed by an engineer is going to be great, when it's all in the details.



cheers,

karl

Kevin_in_Denver_2

new thread

Good idea.  Sunny, the original poster, ran off anyway.



 I'll copy and paste some of article that goes along with the system drawing.

siggy

response to questions on drainback system

Hello Sunheads…



Someone e-mailed me that there were questions and comments on the schematic I published in the December PME column.



Let me try to address them.



1. DHW heat exchanger sizing. This of course depends on the scale of the system, the load, and the temperature maintained at the top of the tank. I can't prescribe a specific HX. However, as a starting point, I suggest the top of tank should be maintained about 10 ºF high the DHW delivery temperature (i.e burner maintains top of tank at 130 F if 120 F supply water is needed. Also keep in mind that this delta T issue is present regardless of whether an internal or external HX is used. I've become a fan of external heat exchangers based on the limited choice of tanks with internal coils, their serviceability (or lack thereof) at some point in the future, and their high rates of heat transfer per unit of surface area with forced convection on both sides (versus natural convection on tank side of internal coil heat exchangers.



2. The return line is submerged below the water level in tank, with an elbow to turn the flow horizontally. The latter is to prevent a vertical "jet" downward into the tank, which would break up stratification. When the solar circulator stops, air exits through the small pipe at top of tank to allow drainback.



3. I've not shown all the details as far as sensors or controls. Not an intensional omission. This is a concept, not a finished product. This concept counts on a well stratified tank, perhaps with internal baffles, or other details. The heated water at top provides excellent buffering of what could be lots of small "microloads" within the delivery system as a whole.



4. I have use a pressurized drainback system at my own house for 29 years. It's been through -30F in winter and is still working - although I have to imaging the performance of the collectors is somewhat degraded from when they were installed.



5. The initial dissolved air in the water will either produce very minor oxidation (oxygen), or collect in top of tank (nitrogen). I haven't found any problems with this air getting entrained through the remainder of the distribution system - note that water supply connection from tank is a few inches below water level.



6. Another goal of this concept is to get away from glass-lined, concrete-lined, or stainless steel storage tanks. Keep the cost down with a carbon steel tank. Probably max size of 119 gallons to avoid ASME ratings.



In summary, I'm confident this concept would work. It certainly needs "specific detailing" based on the defined load. My goal in showing this was to get the industry thinking about "appliance-like" solutions for solar+hydronic heat+DHW, rather than hundreds of individual components going into one-of-a-kind systems on each new job.



Stayed tuned for more on this in future columns.



siggy

Kevin_in_Denver_2

Thanks Siggy

Some of us thought we might be able to run out and buy all these components including the unique tank-style boiler, so this makes sense now.

Mark Eatherton

Thanks John...

I figured you had it figured out, and I knew the devil was in the details.



Thanks for responding as usual.



ME

Karl_Northwind

thanks for clarification

thanks,

Yep.  I guess most of my concerns have been addressed.  I was definitely trying to figure out how to make this work with a polaris, aosmith, phoenix, voyager or any of the other modcon water heaters out there.



how 'bout moving the boiler components up on the tank, and make the return from collector dip tube return to the middle of the tank.  it ought  to significantly improve the solar performance.



my concern is that the entire tank will usually be over 120 degrees, and the solar performance will be severely degraded because of that.  it's all in the performance slope, and the performance in the SRCC cat D rating is the heating zone  (category D is temp inlet is 90F above ambient, with tested flow rate) 30 degrees out, 120 degree fluid temp is pretty standard winter operating temps, and I hate to get it much higher.



I regularly run a secondary loop off my solar primary line to feed solar heat to a fan convector or furnace coil.  one HX to DHW preheat, and no HX between space heating and solar fluid.



So many ways to move that heat around and so little time.



thanks again

Karl

Fortunat

tank stratification

John,



Thanks for answering the questions here and for posting your schematic. I agree with you that the system you've drawn will work fairly well. As you know, these types of systems are favored in parts of europe (austria and germany) where large volumes of DHW storage are disallowed because of fears of legionalla. I do have a couple questions/comments though:



1- In our experience, how you return the water to a solar tank is absolutely critical to real world performance of a single tank system. The elbow on the solar return is definitely an improvement over the vertical 'jet', but still very problematic. Every morning when the solar pump first starts, you'll be splashing fairly cold water back on top of your gas heated hot water, which will quickly make the gas water heater heat the entire tank and solar loop. That is, unless you hold the solar pump off until the collectors are >130, which is pretty wasteful too. I believe superior solution is to use a stratifier lance (in the awkward german parlance) on the return. I know you are familiar with them, but for others, that is basically a low velocity perforated dip tube on the return to promote tank stratification. Do you have any experience with them?



2- I understand the good reasons to limit yourself to a 119 G tank, however that makes for a pretty puny solar space heating contribution. Consider that on a typical fall/spring day (when the solar should be contributing to space heating) the top 50 G start at 130 deg and the bottom 70 G start at 50 degrees, you really only have (21,000 + 76,440 97 kBTUs of solar energy storage available if you limit the tank temperature to 180 degrees. That's a pretty modest solar array and though it will help somewhat, it doesn't make much of a dent in a typical overnight heat load.



3- Finally, regarding heating DHW in a single pass flat plate HEX, I can't seem to find it right now, but I read a good paper (maybe by ZfS - Rationelle Energietechnik GmbH in germany) about the control challenges of this arrangement in large SHW systems. I suppose if you just bang the pump to full speed anytime the flowswitch on the potable side registers any flow it would work ok, but you'd be mixing the tank up unnecessarily. The germans tried to variable speed to pump based on heat exchanger exit temperature and (not surprisingly) found the control to be quite challenging because of the odd and spikey nature of DHW demands. Have you actually tried it?



Thanks and thanks for posting your ideas here and elsewhere. I think it is useful to have a good technical discussion about the merits of a variety of systems and we all are well served when the good technical ideas float to the top of the pile.



~Fortunat

www.revisionenergy.com