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Anyone tried putting a coil in an attic for DHW?

Post in 'The Green Room' started by BoilerMan, Aug 23, 2012.

  1. BoilerMan

    BoilerMan Minister of Fire

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    This has been on my mind for quite some time. Use black poly-ethelene or possibly it's modified variant PEX, but I don't think the temps would be that high. The idea is pipe suspended from the rafters, the volume of water would be around 10 gallons, but legionella, and condensation seem like issues. Anyone actually tried this, or have any other simmilar poor-man (read cheap guy) solar ideas?

    TS

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  2. woodgeek

    woodgeek Minister of Fire

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    Gary at "build it solar" has a bunch of projects that might interest you....and he will likely be along shortly...

    Overall, seems like you are shooting yourself in the foot by not getting significant output except in the summer....you will still need to run a lot of conventional backup.
  3. fossil

    fossil Accidental Moderator Staff Member

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    It's hot in the attic in the summer, it's cold in the attic in the winter. A collection system outside the roof can take advantage of the available insolation (solar energy) year-round. Rick
  4. BoilerMan

    BoilerMan Minister of Fire

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    It's only for the summer, will drain in winter, all DHW is provided by wood in winter/spring/fall. I'm planning on some active solar when funds allow, see sig, I'm setup for it, and even ran copper into attic from boiler room during construction. I was just curious if it would pick up enought BTUs for use, as I only burn oil in the summer for DHW.
    Thanks all!

    TS
  5. fossil

    fossil Accidental Moderator Staff Member

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    Got it, Taylor. I tend to believe that it (the attic thing) will disappoint for the time/$ put into it...but I could easily be wrong. My inclination would be to skip the attic thing and save my money for the future solar installation. But if you've got money & time on your hands and you like to tinker (and who with a gassifier doesn't like to tinker?), then by all means try it and see what it can do. Whatever you decide to do, let us all learn from your experience. Rick
  6. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    This comes up quite a bit, and I think its workable, but maybe not just by hanging a coil in the open attic.

    The attic varies quite a bit over the day in temperatures, and can get pretty cool even on summer nights -- this is the log of attic temps in my house in SW MT: http://www.builditsolar.com/Experimental/Attic/AtticTemps.htm so, in some seasons, an open coil might loose quite a bit of the temperature it gains during the day to cooler attic air at night?
    I think (but never tried it) that putting the coil of pipe in an insulated box in the attic, and having a temperature controlled fan (like an attic vent fan) blow hot attic air through the insulated box when the attic hot, but not when the attic is cold might make this scheme work pretty well for three seasons, but probably not in the winter except for a mild climate.
    You do need to think about freezing, but the coil in an insulated box might be pretty resistant to freezing.

    Given that this makes the whole thing more complicated, I'm not sure it would not make more sense to just pull hot air down from the attic via a duct and blower and use it to heat a coil of pipe (or metal tank) when the attic air is hot. That has the advantage that you can put the coil in a non-freezing location.
    Randy uses such a scheme to pull hot air from his attic for clothes drying: http://www.builditsolar.com/Experimental/AtticToDryer/AtticToDryer.htm
    Here is a scheme to use solar heated air over the outside of a metal tank for heating water -- I guess the solar heated air could come from at attic: http://www.builditsolar.com/Projects/WaterHeating/SolarAirWaterHeater.pdf

    The attic air is not likely to get the water up to as high a temperature as a regular solar thermal collector, but that does not mean its not worthwhile -- if warms the water from 50F up to 80F or so, that cuts your water heating energy in half.

    If you do this, would love to get some pictures.

    Gary
  7. nate379

    nate379 Guest

    I gave some thought to doing a solar collector for water when I looked into panels and decided for the amount of hot water I use it just wasn't cost effective. (couple showers a day, maybe a load of clothes and dishes every 2-3 days).
    THOUGH... summer for us is really just a few weeks at best.

    I ended up putting photovoltaic panels up.
  8. BoilerMan

    BoilerMan Minister of Fire

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    GaryGary,
    The clothes drying idea seems like the best thing to do with all that hot air..... I think that'd be worthwhile. My thoughts on the attic DHW was to use the second coil in my indirect with a circulator switched by a thermostat in the attic space. If it's above a temp I deem useable it turns on the circ. This would be a closed loop system, with the water only going through the coil in the indirect, and the attic pex etc, same as if it were an active solar collector on the roof..... I do have 300' of 1/2" black polyethelene pipe kicking around with nothing to do, but the rest would have to be $$$ spent. Bronze circulator & line voltage thermostat. Ah well

    Thanks for the links and ideas all, thats what I love about this forum so much!!!

    TS
  9. btuser

    btuser Minister of Fire

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    Condensation from the cold water and warm (moist) air in the attic could be a problem and complicate setup. You've also got a lot of pressurized water in your attic waiting for an excuse to ruin your house, so a simple cheap setup should not be considered.
  10. Seasoned Oak

    Seasoned Oak Minister of Fire

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    I would think it made more sense to put it(coiled black plastic pipe on the roof. More sun .hotter water,no chance of leaks ruining house. The nice thing about solar HW panels is it shades the roof to some extent as well. I agree though it is a lot of effort for minimal savings unless electric rates are unusually high(hawaii) or you live in the south.
  11. Frozen Canuck

    Frozen Canuck Minister of Fire

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    I would pass on this one. Upside limited to an as mentioned small heat gain. Doubt you would even recover labor costs if you paid yourself min wage. Downside could have you calling someone like me saying I need you to gut my house down to the studs & rebuild it. While you & your family move out. Normal condensation in an attic causes enough structural damage on its own when it goes unnoticed, piping all that cold water up there wont help that situation....just sayin all that extra condensate has to go somewhere....most likely into dry wood. Nature always seeks a balance.
  12. BoilerMan

    BoilerMan Minister of Fire

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    Condensation is my biggest concern as well FC, but I won't call anyone about water damage, I built my own house!! LOL. I just thought I'd ask and see if anyone else has done it. I might try it as an expierement, as it'd only take a few hours since I have pipe run to the attic already for future solar. I'll let everyone know if I do decide to try it and how it works if at all.

    TS
  13. Seasoned Oak

    Seasoned Oak Minister of Fire

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    In summer your incoming water is much warmer and does not need pre-heating.Plus in summer i dont like my domestic water scalding hot. 105-110 is fine. In winter the warmest place i the house is next to or above my heating appliance. Any pre-heater would probably go there. I would not want to deal with all that condensation up in the attic.
  14. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    Our summer water temperature is only about 5F warmer than winter -- about 55F, but I guess it depends on where you live.

    It seems to me that the condensation is something to think about, but could be dealt with with a drainage or evaporation system. Again, a lot depends on climate -- where we are its very unlikely to be a problem at all.

    Pulling hot air from the attic (as in Randy's scheme above) to use to preheat water elsewhere avoids the problem altogether, and has other advantages.

    If you figure 60 gallons a day of hot water use, and manage to cut the cost of heating this by 50%, the savings per year would be in the area of $250 -- depending on what you have on hand already, an around 1 year payback. If the system lasts 20 years, the total payback with fuel going up at 5% more than inflation is around $9000 plus about 1 ton of CO2 emissions saved per year.
    http://www.builditsolar.com/Experimental/PEXColDHW/Cost.htm

    Seems worth trying to me.

    Gary
  15. woodgeek

    woodgeek Minister of Fire

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    Hi, I'm a wet blanket. In ME, lets say that the attic is warm enough to heat DHW on sunny days, maybe 6 mos/year. Let's say that is 100 days, and the attic is only really hot 12 hours per day. How to insulate the pipe/tank? If you insulate it, it doesn't heat. If you DO insulate it, then it cools off at night. If you shower in the evening after dinner, you get 100 free showers, if in the AM, they you are out of luck. So, if you have a family of 4 (say $400/yr for elec hot water and they are willing to rejigger the water usage severely, you might save 100/365 = 30% of your bill, or $120/yr. If you have a smaller or less cooperative family, you will save proportionally less to none.

    Of course, a tank in an insulated box with a big blower on a thermostatic switch avoids the night losses, but then you have a big pressurized tank in your attic. IF you run the heat hydronically from the attic to a basement HX in the tank, you are working almost as hard a solar system, for a lot less output.
  16. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I guess after looking a little more carefully at my attic temps and Dave's attic temps:
    http://www.builditsolar.com/Experimental/Attic/VATemps.htm
    http://www.builditsolar.com/Experimental/Attic/AtticTemps.htm
    I'd have to agree that my 50% number above is optimistic.

    As mentioned above, I do think you need to have a way to have the tank or pipe coil not lose heat when the attic temp is too low to heat, but it seems like the insulated box in the attic with a thermostatically controlled fan, or the scheme where you just pull hot air from the attic peak to a tank or coil not located in the attic and insulated would be workable without too much complexity.
    One could improve the attic heat collection by using a simple inside collector like Randy's: http://www.builditsolar.com/Experimental/AtticToDryer/AtticToDryer.htm This would get you hotter air for a longer period.

    Besides the low cost, the really nice thing about this scheme is that you don't have to have a collector on the roof, which is a problem with some of the local housing police associations.

    As a VERY rough cut, If you had a 150 cfm flow of 100F air over a tank with fins or a spaced out coil, and you managed to get the exiting temperature down to 80F (perhaps a bit optimistic), that's (150 cf/min)(0.075 lb/cf)(100F - 80F)(0.24 BTU/lb-F)(60 min/hr) = 3250 BTU/hr -- enough to raise the temperature of 40 gallons of water by 10F per hour.
    The other thing that (maybe) lends some credibility to this is that Randy is able to dry clothes with a attic hot air supply to a regular dryer -- this is typically about 7000 BTU over an hour -- pretty good delivery rate?

    There was some work done by Steve Baer back in the 70's about several methods to transfer heat from solar heated air to a water tank -- its in NMSEA Sun Bulletin, Volume 2, Number 5 1977 I can't find it now, but will ask NMSEA if they have a copy online.


    Does not cost much to give it a try?

    Gary
  17. woodgeek

    woodgeek Minister of Fire

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    Ok. There is nothing wrong with moving the air, just requires a big chase depending on your home layout, and avoids the condensation concerns here.

    I guess I still prefer the idea of getting some surplus hydronic baseboard HX (copper pipe plus fins) and sweating up a block of that with a fan blowing over it, (or a junked A-coil) and putting the whole thing in the attic over a tray for condensation, and then hooking it to a secondary coil in my DHW tank (if I had one). That way I have no plastic pipe running all over my attic, just nice sweated copper, and if it leaks it only drains the loop, not the city reservoir.
  18. BoilerMan

    BoilerMan Minister of Fire

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    This is exactly what I said i was going to do, minus the copper in the attic. It would be a closed loop system. Water circulated through "collector" in attic (plastic, copper, or otherwise) and through the second coil in my DHW tank. Same as active solar on the roof, but with it in the attic. I'm gonna try it, and RAndy's dryer Idea is next on the list, that is pure free money saved IMO.

    TS
  19. woodgeek

    woodgeek Minister of Fire

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    S'cool. A fan blowing over the coils in the attic (operated by the same circuit as the pump) will help a lot with the heat xfer, fins or no fins. The dryer thing sounds ok, given that it is straightforward to schedule dryer loads to hot attic periods. And there is no end of laundry in the summer.

    Let us know how it goes!!

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