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Hey folks,

I'm hoping a couple of you experts can offer me a little guidance. I'm soon to be building a new home that will be heavily insulated and relying primarily on a solar thermal system and a wood pellet boiler for space heating and for DHW. One indulgence I'm allowing myself is a 6-head shower system that will have the capacity to bleed through a lot of hot water--as much as 15 gpm when all of the heads are going full bore. Although I'll probably never be using more than 10 gpm at a time, I still want to size the system to be able to provide a good long shower @ 15 gpm.

My questions are about indirect water heaters. I've resigned myself to the fact that my best solution is going to be to have 2 - 120 gallons indirects, with one connected to an 80-evacuated-tube solar thermal system and the other to a 32 kW wood pellet boiler. I'm also assuming that I'd go with the solar tank as a pre-heat, and rely on the wood boiler tank as the primary source of DHW. What I don't understand are the following:

1) Do I go with single coil tanks for each indirect, or should the tank for the boiler be a dual coil, using energy from the solar thermal system as well?

2) If I go with a dual coil for the boiler, can the tanks be configured so that If it's a cloudy day that some of the hot water in the boiler tank can be moved in the opposite direction to the solar pre-heat tank, just to maximize the amount of hot water I could store up for a good long shower? My goal with the indirects is to get each tank to 190F before a shower to give me as much hot water as possible.

3) I'll be using a GFX shower drain heat recovery coil to recover some of the energy that would otherwise go right down the drain. Should the water being pre-heated by the GFX go solely into the solar tank? Would this pose any potential problems?

4) The house is going to be heated primarily via a radiant slab in the garage and six hydronic wall-mount radiators throughout the living space above it. Off of which tank would the water for the radiant systems come? I'm looking at heating the radiators with an EWT of 130F. Could water going into the radiant slab in the garage be at the same temp, or would that just cause problems?

5) Finally, I'm likely to have days where the solar thermal system is producing more energy than either of the indirects is able to absorb. Rather than going with a dump tank and wasting that energy, could I somehow have it transferred to the garage slab or (in the colder months) to the radiators in the living space without causing problems or making the system way too complicated or costly?

A lot of questions. I'd really appreciate it if a couple of you HVAC folks would give me your 2-cents. Much appreciated.



Litawyn Eco-Loft
Union, Maine 04862


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I have no clue about your situation and issues. I think that since you've got solar involved that your questions might be better answered on a forum devoted to that area of expertise (not that some here may have some info). Overall, I wonder of the economy of your set up; seems having two 120 gal boilers for shower water is a bit excessive no matter how much "free" preheating of the water is done by solar. Might not an on-demand tankless gas system be more prudent (could also be served by preheated solar)?
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