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Drainback or closed loop solar hw?

Post in 'The Green Room' started by 4acrefarm, Apr 19, 2013.

  1. 4acrefarm

    4acrefarm Member

    Joined:
    Jan 11, 2009
    Messages:
    159
    Loc:
    western ma
    I have an old chicken coop that is solid except for the roof. I am planing to build a 400 sf panel on roof and barn wall inside coop I plan on building a 770 gal tank. It is the perfect place for a drain-back with tank right under the panels. I would in-close and insulate the coop and turn it into a greenhouse. The tank water would then be pumped 100 feet to the house for dhw and radiant.
    Any reason that i should not use drain-back?
    I think 3/4 pex in collector running vertical up roof and onto wall 12 inch spacing.
    Any deas on insulated pex? It would be 50 feet through unheated barn and 50 feet through unheated celler.
    I think I will use side arms to a pair of 80 gallon electric tanks. one is for dhw and the ther conected to wood boiler radiant loop.
    All suggestions and comments are welcome. thanks in advance.

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  2. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
    291
    Loc:
    SW Montana
    Hi,
    It does sound like a very good setup for a drain back.
    I like drain backs whenever they can be used as they are simpler and cheaper with less maintenance.

    400 sf is a lot of collector -- should produce a lot of heat.

    A good flow rate for good efficiency is 0.04 gpm per sqft of collector, or 16 gpm for 400 sf.
    It is possible to use lower flows down to about 0.02 gpm per sf with some loss in collector efficiency, but allows using a smaller pump and smaller plumbing.

    3/4 inch to and from the collector might be on the small side. You can run through this pump/plumbing sizer to find out for sure: http://www.builditsolar.com/Projects/SpaceHeating/PipeSizing/PipeSizing.htm it will also give you an idea what size pump you need.
    If you have any trouble going through it, let me know.

    Not sure what this sentence means: I think 3/4 pex in collector running vertical up roof and onto wall 12 inch spacing.

    On running the pipe back to the house, I have a similar situation, and for insulation, I basically boxed the pipe in foam board -- pictures here: http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/trench.htm This works fine and is a lot cheaper and higher R value than the commercial products. I used CPVC on mine, but I think that PEX would be a better choice.

    What kind of collector construction do you plan to use?

    Gary
  3. 4acrefarm

    4acrefarm Member

    Joined:
    Jan 11, 2009
    Messages:
    159
    Loc:
    western ma
    Gary, thanks for the response The 3/4 was referring to inside the collector, I have since looked at prices and am now leaning towards 1/2" inside the collector. I plan to run the colector tubes up the roof then up the wall which is above the roof . I would build the manifold inside the collector all with pex. The supply lines would be 1".

    I assume you would recommend double glazing (poly carbonate dual wall) and foamboard behind it.
    Thanks for all the work you have done and continue to do in this feild. I have read and been inspired by most of your projects. I currently have hot air collectors on the house that have been very productive for several years now.
  4. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
    291
    Loc:
    SW Montana
    Hi,
    Glad to be of help -- I get a lot of ideas from the forum.

    Usually 1/2 inch for risers is fine -- each riser gets so little flow that even on long risers the pressure drop for half inch risers is small.

    PEX inside collectors can be a problem. If the collector stagnates (no water flow) the temperature in the collector can go over what PEX can take pretty quickly. The only ways that I know of to use PEX are build the collector with a high tilt angle (70 degs or more) and to only use single glazing -- both of these help to keep the stagnation temperature in check. Its tempting to think that you can avoid stagnation by just keeping the collector pump running, but there are lots of failures (pumps, controllers, leaks, ...) that will eventually result in the collector being stagnated. Its also nice to be able to have the controller shut off the pump once the tank temp gets up to a high value (say 165F) -- this protects the tank if its the EPDM lined type.

    There have been discussions about some type of automated venting that would open and let air flow through the collector when the temperature gets above a certain level. This seems feasible to me, but I've not heard from anyone who actually has a working system.

    So, if you want to use PEX for the collector, I'd avoid double glazing, as it will raise the stagnation temperature. You are in a cold area, and the double glazing (like twinwall) would help the performance, but a single glazed collector will still do well.

    You do want something like 1 inch of insulation board behind the absorber, and, as you likely know, it needs to be polyiso -- the pink and blue stuff will melt in a collector -- I have the pink sculptures to prove it :)
    Polyiso (Polyisocyanurate) is available at a lot of lumber years, but they may not know it by that name -- just look on the sheets for the polyisocyanurate label. Its a little more expensive, but also has a high R value, and its blown with a greenhouse gas friendly blowing agent, which is not true of the blue and pink stuff.

    Gary
  5. 4acrefarm

    4acrefarm Member

    Joined:
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    Loc:
    western ma
    So single glazing would work ok, and avoid high temps? It would also save me some money. an automated vent system is possible I have access from the barn to get to the back of the panel. What would you recommend for pumps? Is there a self priming pump I could use for the collector?I want to avoid all all holes in liner. My transfer lines would be 1" (?) 100' into sequential sidearms. Thanks, My name is also Gary
  6. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
    291
    Loc:
    SW Montana
    Hi Gary,
    I think the combination of steep tilt and single glazing can get the stagnation temps down to OK levels, but I think that a collector tilted at (say) 45 degrees with single glazing is still going to get to hot when stagnated in good sun. You need both the steep tilt and the single glazing to get temps down to what pex can take when stagnated.

    I know you are not wanting to hear the copper word, but it does have a lot of advantages for collectors -- just costs more than pex.

    Single glazing will perform well even in cold climates -- my shop heating collector is single glazed and it still puts out a lot of heat down at 0F in good sun.

    For the collector pump, for such a large collector, you probably want one of the Grundfos or Taco HVAC pumps -- there is a rundown on some of the comone ones here: http://www.builditsolar.com/References/Pumps/PumpCurves.htm
    These HVAC pumps are good reliable pumps and even though they are not paragons of efficiency, they are not bad and pumping energy will be a very small fraction of the heat energy collected. If you do want to look into a more efficient pump, one to look at is the Grundfos Alpha series.

    Basically for a drain back, you need a pump with a startup head (zero flow) that is about 1.2 times the vertical distance from the tank water level to the top of the collector. This is to get the flow started. Once the flow is started and the full circuit is full, you need a pump that will deliver the about 0.04 gpm per sf of collector with the pipe friction you have -- 16 gpm in your case. It is important to get a pump that meets this startup requirement in that if the pump is not able to pump the water to the top of the collector at sartup, the system will not work at all. A Taco 008 might be a little on the small side, but might be OK -- it won't quite make the 0.04 gpm/sf, but maybe close enough -- this is a good pump -- I use it on my Solar Shed collectors. Or, a Grundfos 15-58 three speed pump on high speed might do it -- again, it won't quite make the 0.04 gpm/sf, but probably close enough. I've got three of the 15-58's -- very nice pumps.
    If you want to go through the whole pump sizing routine, there is a page here on it: http://www.builditsolar.com/Projects/SpaceHeating/PipeSizing/PipeSizing.htm

    The pump to the house is a tradeoff: if you go for a small pipe diameter, it keeps the pipe cost down and reduces pipe heat loss, but increases the pipe friction, which makes for a larger pump. A larger diameter pipe reduces pipe friction and allows a lower head, lower power, less expensive pump to be used.
    I ran numbers for several pipe sizes for my Solar Shed setup, which has over 100 ft of tank to house pipe. This is a 240 sf collector. I ended up with 3/4 inch pipe as the compromise. The pump I use to pump the water from the tank to the house and then through the radiant floor loops is just one Grundfos 15-58 running on high speed. This works OK in that while the flow rate is on the low side, it is able to transfer all the heat from the fully heated 400 gallon tank to the house after a sunny day well before the start of the next collection period.

    Since your collector is larger, a 1 inch pipe might be better for yours.
    For 250 ft of 1 inch pex at 5 gpm, the pressure drop is 3.95 psi, or 9.1 ft of head.
    For 250 ft of 3/4 inch pex at 5 gpm, the pressure drop is 13.15 psi, or 30.4 ft of head -- quite a bit of head loss.
    There would be some additional pressure drop for fittings etc.
    5 gpm with a 10F temperature drop will transfer about 25,000 BTU per hour.
    Your collector might do 320000 BTU on a sunny day, so the pump would need to run about 13 hours to move it all to the house.
    All rough numbers.

    Self priming pumps that work well in solar service are hard to come by, but you can avoid the tank wall penetrations by using a U-tube -- as on this system:
    http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Plumbing.htm
    The water stays in the U-tube up to the water level inside the tank, so the pump does not lose its prime. Its important to locate the pump as far down as possible so that it has a little positive head at its inlet. You do have to protect the U-tube from freezing if the tank is in a cold area.

    Gary
  7. 4acrefarm

    4acrefarm Member

    Joined:
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    Loc:
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    Gary , Thank you very much. You have given me lots to think about. I am thinking about venting the back of the panel into the barn if needed. I will Look into copper.
    Thanks Gary

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