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Solar DHW tank with gas backup?

I've never understood the idea of solar DHW tanks having electric backup, as a large portion of the solar guys are off-grid and may not have that electricity available. I have a bit of a mess coming up here, guy wants to integrate solar thermal with an outdoor wood boiler and have a gas backup to heat his new shop (radiant slab). The collectors are making enough heat to require approximately 140 gallons of storage for full utilization, but cost is a factor as well so at this point we're looking at a 120 gallon solar DHW tank with the single 4500W element (4.5kw will cover his heat load if need be). He really hates the idea of the electric backup, but hates the cost of a storage tank and LP boiler even more. Why does nobody make a solar tank with a gas backup? Or maybe they do and I can't find one. Pointers?

Comments

  • Larry WeingartenLarry Weingarten Member Posts: 1,716
    Hi, How about using a non-pressurized tank with a copper coil in it as your heat exchanger? Non-pressurized tanks cost much less. Also consider using a heat pump as backup, to cut the electric demand to 1/3 of what direct resistance would be. Years ago, I think it was American Water Heaters who did make a solar tank with an elevated gas burner in it. Now long gone. :s

    Yours, Larry
  • hot_rodhot_rod Member Posts: 12,898
    They were available for a few years. The HTP Solar Phoenix was a nice condensing one.

    Basically the typical gas fired solar tanks just put a port mid way up the tank and put the gas valve/ aquastat there. So the burner turned off when it saw the top 1/2 of the tank hot. Solar coil in the bottom would see any cold incoming water. Typical mid 70% efficiency I suppose.

    Certainly a compromise appliance, which limited solar harvest by keep 1/2 the tank out of play by the gas fired component.

    Why not separate components, much more selection and better control as to which source runs and when.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Member Posts: 886

    Hi, How about using a non-pressurized tank with a copper coil in it as your heat exchanger? Non-pressurized tanks cost much less. Also consider using a heat pump as backup, to cut the electric demand to 1/3 of what direct resistance would be. Years ago, I think it was American Water Heaters who did make a solar tank with an elevated gas burner in it. Now long gone. :s

    Yours, Larry

    In addition to being a budget oriented system, we also have a time limit and it needs to be relatively pretty as this is his place of business and will be exposed for all to see. Nobody is going to mess around building an unpressurized storage tank and then have to insulate it, etc only to end up with an eyesore. As far as buying one, the few I priced with dual coils were actually more costly than a DHW tank. At 15,000 BTU design load, it would take 7 lifetimes to break even with a heat pump financially; that's not in the cards.
  • GroundUpGroundUp Member Posts: 886
    hot_rod said:

    They were available for a few years. The HTP Solar Phoenix was a nice condensing one.

    Basically the typical gas fired solar tanks just put a port mid way up the tank and put the gas valve/ aquastat there. So the burner turned off when it saw the top 1/2 of the tank hot. Solar coil in the bottom would see any cold incoming water. Typical mid 70% efficiency I suppose.

    Certainly a compromise appliance, which limited solar harvest by keep 1/2 the tank out of play by the gas fired component.

    Why not separate components, much more selection and better control as to which source runs and when.

    Thank you. As far as separating the appliances, it's a cost thing. He doesn't want to pay the price to add in an LP boiler. The wood burner and solar are existing, and what I've gathered is that the solar was simply supplementing the OWB by backfeeding to the 400 gallon store in it while the radiant slab ran 24/7 with no means of tempering the water, it's quite a work of art. He wants the option to kill the OWB and still maintain functionality inside, even in low sun times, which is why we're adding the storage. I am fairly certain we're going to end up going the way of the 4500W backup, I was just hoping there was an explanation for the lack of a gas fired solar tank. Seems like it'd be a lucrative racket, for the off-grid folks

  • kcoppkcopp Member Posts: 3,433
    https://www.bradfordwhite.com/ecostor2™-sc-solar-indirect-system-single-coil-eco-defender-gas-backup-models-closed-loop
  • GroundUpGroundUp Member Posts: 886
    kcopp said:

    https://www.bradfordwhite.com/ecostor2™-sc-solar-indirect-system-single-coil-eco-defender-gas-backup-models-closed-loop

    I did actually look into those, and both my supplier and the BFW website say they are discontinued. Very similar to the Combi-Cor from what I gathered, which is also discontinued for some reason. There is a Laars version of the Combi-Cor called a Combi-Heat, but maxes out at I believe 75 gallons
  • hot_rodhot_rod Member Posts: 12,898
    I thought the BW Combi2 are still available?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Member Posts: 886
    Not according to my suppliers. That Laars unit I mentioned above seems to be the same unit as the Combi2 with a different sticker, but I spoke with two different salesmen about it and they both said it had been discontinued. Maybe in lieu of the Laars? Who knows. Or maybe they were both misinformed, which would not surprise me
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