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John Siegenthaler's simplified Combisystem for new construction

Kevin_in_Denver_2 Member Posts: 588
At PME magazine, Siggy proposes a pressurized drainback system for DHW and space heating.

I love how simple and economical it is, but as always, make sure you understand it before you install it.   The schematic is attached.

Some quotes from the accompanying article:

"The performance of this system has been simulated for a 1,500-sq.-ft. home for

two locations: Albany, NY, and Boulder, CO. The simulation was based on the

following system configuration:

<ol><li>four 4x8-foot flat plate collectors (128-sq.-ft. gross


</li><li>collector efficiency line intercept = 0.76

</li><li>collector efficiency line slope = 0.865 Btu/hr/ft2/ºF

</li><li>collector slope = latitude +15º

</li><li>collector azimuth = 180º (directly south)

</li><li>119-gallon, well-insulated storage tank

</li><li> DHW usage = 60 gallons/day heated from 50ºF to 120ºF</li></ol>

The design space heating load of the 1,500-sq.-ft., well-insulated house was

set at 15 Btu/hr/ft2, or 22,500 Btu/hr total, with an indoor

temperature of 70ºF and outdoor design temperature of 0ºF. This yields an

overall heat transfer coefficient of 321 Btu/hr/ºF.

The specified combisystem supplies almost 31% of the total annual space heating plus domestic water heating load for

the house in Albany, NY. The total solar fraction increases to 44% if the same

house were located in Boulder, CO."

"This closed-loop, pressurized drainback system eliminates the need and

expense associated with a heat dump — which is required for most antifreeze-based

solar combisystems. It also eliminates several hardware components such as

antifreeze, air separators, expansion tank, collector loop relief valve,

top-of-loop air vent and fill valves."

"Domestic water is heated instantaneously as it passes through the external

stainless steel heat exchanger. A flow switch turns on the small circulator

supplying this heat exchanger whenever there’s a demand for domestic hot water.

Hot water from the top of the tank flows through the primary side of the heat

exchanger and instantly transfers heat to the other side. This heat exchanger

is easily replaceable (and recyclable) if ever necessary due to scale or other

issues. The minimal quantity of domestic water residing in this heat exchanger

(probably less than one pint) reduces the possibility of legionella


The components shown in Figure 1 could (and should) be consolidated into a packaged

product. This would speed installation and ensure proper component sizing and

placement. It would also help consumers view solar combisystems as an appliance

rather than a highly customized/complex system.

Is the U.S. solar industry ready to supply products tailored for smaller,

energy-efficient homes? Is your firm ready to promote their virtues and install

them? Perhaps it’s time to gear up. The sustainable housing market is in need

of solutions."

In northern  Wisconsin, apparently, the tank water would have to be 30% antifreeze, which would add a few hundred dollars to the cost.

Siggy is coming to Colorado in Feb. to discuss this and other solar issues.
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

    Yep.  It's a packaged system, but unfortunately. like most "one size fits all" systems it's not really ideal for any particular option.

    It'd be substantially better if combined with infloor with delivery temps below 100F

    but then it'd not provide adequate service to the hot water end load.

    I think it's got potential, but I think I'd rather have it staged thru 2 tanks.  both to allow a little more storage, and provide real stratification.

    I'll think on my piping scenario. probably based on a HTP phoenix or solar phoenix. I'm all about using off the shelf components. unfortunately my drawings won't be nearly as nice

    well, if his intention was to get people thinking, I certainly am.

    BTW, I'm getting ready to build a 1600 sf superinsulated house here in central wi.  Unfortunately, I'm going to be limited by the location of my collectors (roughly 100 feet away, slightly downhill.) otherwise I'd be all about trying this out on myself.

    cheers, Keep pumping away.

  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    be careful

    I love siggy, but I have to wonder about maintaining solar tanks with an auxiliary source. you don't have any stratification if you're pumping the tank, as far as I know, and so this basically violates the basic idea of something like the phoenix with the separate coil for solar in the bottom so it can circulate without pumping the tank. Likewise I think heat extraction should require a coil in the top. otherwise you could pretty much envision this whole tank as one blended temperature as soon as anything starts pumping?

    and I *hate* the idea of losing solar storage to fossil fuel BTUs if you're trying to do heating. but I can't pretend to critique his energy analysis... I'm a bit skeptical, but I could be convinced. My belief is that allowing the tank to range its full natural temperature range should have a fairly big impact on solar collection vs maintaining even a third of it at a much higher temperature for DHW. thinking on demand/separate heat source there.

    but siggy's done solar for 30 years now. I could be all wet on this.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • hot_rod
    hot_rod Member Posts: 22,103
    pros and cons

    to just about any solar, or hydronic system for that matter. It's a good idea to have the explanation with the drawings to explain the pros and cons. It's hard to get all that out in a print article. Attending the seminars where you can discuss and get installer experience and input helps a lot.

    The Wall is a big help for that, but still getting together face to face really helps explain a concept. A room full of seasoned installers is really exciting.

    As long as solar has been around we are still learning new methods for piping, storing, and controlling the systems. There is a lot of smart and clever people taking a serious look at solar thermal again. As long as the market grows and stays strong we should continue to see some new and exciting products and methods developing.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fortunat
    Fortunat Member Posts: 103
    I agree with Rob about stratification and tank mixing

    I previously posted this in the other thread discussing the same system, but I'll repost it here because this seems like the more appropriate place.

    I think this system does have some tank mixing weaknesses...

    my original post below:


    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


    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.


  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    John Siegenthaler's simplified Combisystem for new construction

    I've been  modeling the proposed system with the sorts of temps that a radiator system would need (with no direct DHW preheat)

    say minimum production temp at 100F,

    required temp of 145 deg,

    with 160 SF of collectors with the same y intercept and slope as the spec'd panels,

    no Heat exchanger, and 120 Gal storage,

    the heat delivery from that array (in Wausau Wi, my design conditions) is 4.9 MMBTU

    the same array with an 80% eff HX, 300 gallon storage, 120 deg delivery temp and 80 degree minimum delivery temp, in Wausau wi.  heat delivery works out to be 10.6 MMBTU.

    Same load, lower delivery temperatures, even with the heat exchanger losses doubles the heat output of the array.  

    Retscreen has been used to calculate the heat output of the array,  Monthly loads determined by my own calculator. 

    there's probably some happy medium, price vs performance, as the second option needs an additional heat source, an addtiional DHW source etc.  and therefore more boiler room area, and more knowlegable installation personnel, and more costs.

  • Seimon
    Seimon Member Posts: 2
    Question about water level


    I've been looking at a system like this for some time - came across the MaxLean concept just last week also. Excuse me if I'm being dumb, but I hope someone here may be able to answer my question.

    If I have the tank with - associated air gap for drainback volume - on the ground floor, how does this affect the upstairs radiators? Do they not drainback also? Or am I missing something here with a pressurised system?


  • Royboy
    Royboy Member Posts: 223
    my take

    is that unless the water level drops to the level of the upper connection to the heating loops, there's no way for air to enter the radiator loops and allow drainback. air enters the solar loop via the branch line off the solar return piping that connects to the top of the storage tank (above water level). this is different from some configurations of drainback systems where the collector return piping connects above water level in the drainback vessel, and thus air can enter the return piping at that point. it seems like the radiator loops would be under negative pressure when their circulating pump is off and positive when the pump is on, but in any case drainback should not occur.
  • Seimon
    Seimon Member Posts: 2

    Ah, that makes sense. Thanks for taking the time to explain.


  • Simply Rad
    Simply Rad Member Posts: 184
    Whole picture

    About the combi-system.  I think we must take a step back and take about the basics.  Fortunate and Karl said it.  Before even thinking about solar combisystem you have to get the distribution system right.  Right being low temperature distribution controlled with a reset control.  So many times now I am asked to provide a solar bid.  I sit down and explain to them that they are better off to first replace the cast iron boiler.  Then reduce their heating loads with insulation or window upgrades.  Then I finally have to tell them that their radiant install.....is not a good match for solar.  I can't tell you how many times I see "naked staple up" (plateless).     "But Joe the plumber told me I can provide 75% of my entire space heating with 3 flat plates and he said nothing about the staple up"!

    Siggys system does not seem like much energy, but if you look at it over a 20 yr. life cycle it certainly helps.

    Sorry getting out of the solar subject, but I think its all part of it. 
    Jeffrey Campbell
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    it's all system thinking

    Radiant system design (especially solar-integrated) rewards system thinking.  In order to think clearly thru the system, you need to have a thorough understanding of the limitations and strengths of each particular technology.

      for example:  I do really well with solar.  but I don't know geo.  so I have the ear of an experienced Geo contractor.  he keeps me in check on the reality of geo systems.  fortunately we both have similar needs for heat distribution (IE low delivery temps) so we're on the same page for heat delivery.

    That said, when we have to integrate solar into a less than ideal delivery system, we will dump the heat into the space (say thru the retrofitted staple up tubes, because we're not going to redo the whole system) and with some fiddling around we can get heat into the house (which is the name of the game) between heat calls.  we won't meet design conditions, but we can extend the time between heat calls and use the existing delivery system.  it's just some controls. (one of my weak points, since I don't have a strong conventional heating background)

    I start with the heat delivery of the collectors at my target solar loop operating temp, and then figgure out how much emitter I need to use to get rid of the heat once the DHW tank is satisfied.  unless I'm sending it to a large tank, which simplifies things since I then have a large buffer.

    I will be doing a lot of the planning and design for the Midwest Renewable Energy Association's 400 level Solar Thermal Design Course, and will be asking around for information, mostly on heating topics.  Hopefully some here will be willing to share more of what they know, especially on my weak points. I'll probably start a new thread for that one.


  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    On the shelf water heater for the combisystem

    Maybe something like this is what Siggy has in mind for the heating appliance.

    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • tim smith
    tim smith Member Posts: 2,752
    Simple idea for drainback???

    What about using a 50 gallon or so condensate feed tank/pump set, vertical? you would have built in pump? as long as you could get correct size. you would have a float valve for feed water. Maybe add some taps for heat exchanger(s). Just a possibility? readily available. 
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    edited February 2010

    I don't see a single tank ever really offering the sort of performance we'd want from the solar at panel radiator temps.  I'm sticking to the radiant area that walls, floors and ceilings offer.   that is unless someone wants to make a 12' tall tank for good stratification

    how about the best of both worlds: 2 tanks, one a HTP Phoenix or similar, and the other a TT Smart 120.  that tank has 120G of domestic in the center tank, and then 43 gallons in the shell.  an additional small DB tank on top would allow for pressurized DB operation, the boiler ports would allow it to interface with the heating system, and a FPHE off the side of the Phoenix would allow it to bump up the heating water to the ODR required water temps.  and it'd give you 160 G of storage in the solar tank.

    this could all be done (if you set the DB tank above a 55G phoenix (at 55" tall), in a footprint of about 3x6' including access.  if you're set on storing heat in water, there's no way to get around the fact that you're going to need some footprint. 

    maybe you could use a TTE boiler with 14G inside DHW storage.  That'd reduce the footprint some and would cover the DHW load as well.

    or AIC makes a 120 gallon indirect instantaneous water heater.  120G boiler/solar water and 2? gallon DHW with 72 SF heat transfer area.  instantaneously heat your DHW to whatever the solar fluid temp is, and then bump it up with whatever you like, pressurized heating system, the top 6" of tank would allow you a drainack resevoir without sacraficing HX area.

    someone let me know if you want to try these out, and I'll consult for free.


  • hot_rod
    hot_rod Member Posts: 22,103
    mine is close to that

    I used an Ergomax as my drainback tank, mainly because I had one. they have a ton of copper HX coil in them.

    This feeds to the Phoenis as a DHW loop. The Phoenix then supplies the radiant via a Taco Radiant Mixing Block HX version. The 120 tank to the right is extra storage.

    My wood boiler can also warm the Phoenix and storage via the coils and 3 way zone valves.

    My 10 foot tall solar tank will be a 500 gallon old LP tank. It's connected to my wood boiler right now. This summer I will weld legs to stand it upright for a solar tank.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • michael_34
    michael_34 Member Posts: 304
    variable speed circulator

    I think the critical component on the solar loop is the variable speed circulator which must operate at exactly the right speed to create a 120F (probably a bit higher) output temp from the collector array regardless of the temp of the collector input temp. If this actually works correctly, then the "integral mod/con burner/heat exchanger" does not defeat the solar or vice-versa. (AW)

    What variable speed pumps are most people using? What controller? I use the Resol E.

  • hot_rod
    hot_rod Member Posts: 22,103
    The "E"

    seems like a lot of control if it is just switching two loads.

    Next week we should have the new Plus controller with drainback, variable speed, arr #10 for heat dumping, and the anti-legionella function all built in.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Add some low temp distribution to a staple up system

    A small fan convector can actually be tied right into the staple up loop very inexpensively.

    Find out where they actually "live" in the house.   Usually bedrooms can be much cooler.


    You could stick this one right under the couch where they spend 60% of their time:

    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    but if you do the variable speed to a fixed outlet temp

    all you do is make the collector reduce efficiency 100% of the time. I don't think it'll help you as the average temp across the collector will be higher than it would have been otherwise, so your collection efficiency must then go down.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    CFP's are plastic

    Usually a condensate feed pump isn't rated to 180F that you need for solar
    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
    Siggy's COSEIA Seminar

    I had to cut out just as the solar talk started.

    Did John have a tank manufacturer recommendation for the simplified combisystem?
    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
    No recommendations....

    But no one actually asked that question ;-)

    email me and I will give you his email address and you can ask him directly.

    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.
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