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Using potable water as heat medium

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Shelterman, Feb 18, 2008.

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  1. Shelterman

    Shelterman Member

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    Hi All,

    I've been reading this forum for a couple of months now and getting a good education.

    I have somewhat of a unique situation...I am currently heating my house with two high efficiency propane, 100,000 btu, Polaris DHW/Space heaters and am wishing to incorporate a Wood Gasifier to the system. The reason my situation is unique is that the water going to the heat coil in the plenum of the furnace is potable water since it comes directly from the Polaris. I want to keep the water going to the heat coil in the plenum potable as I want to be able to keep my existing system in tact as a backup. My question is which would be better, to use heat plates at the Polaris or send the potable water to the Polaris from a heat coil in the storage tank? I see advantages and disadvantages to both. I’m thinking the advantages are greater by using the heat plates, but my concern is that the plates won’t be able to keep up with the demand since the Polaris serves as both dhw and heating. I plan on routing the cold supply to the Polaris through a coil in the storage tank so that should help with the demand when using a lot of hot water.

    What do you guys think? I’ve attached a simplified diagram showing the basic setup if I used heat plates. I will add more detail later if I need to.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Welcome to the forum and to the boiler room, shelterman.

    Interesting system - a few questions:

    You have two DHW heaters, and the water circulating through the plenum heaters is potable water?

    Any chance of adding a second plenum coil for the wood heat?

    Might also consider sidearm rather than preheat coil given the distance between your storage and your house. Any chance of putting the storage closer to the house?
  3. Shelterman

    Shelterman Member

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    Thanks Nofossil. Yes, I added a very large addition to my house a few years ago making a total of about 4500 sq. ft of heating area. The Polaris heaters have a circulating pump outputting potable water to heat coils on my furnaces One of the heaters goes to a downdraft plenum the other, located in the basement to an updraft plenum. Adding an additional plenum coil never really occured to me...and would work on the downdraft unit, but the updraft in the basement would be more difficult I think. Might be a possibility....I will have to look at it closer later.

    The sidearm would work I suppose, it just seems as though it would be a greater expense to put sidearms on both units and another pump. The expense of putting copper coils in the tank is somewhat negligable as I already have plenty of copper to make any coils needed for the storage tank.

    The reason my distance from the house is that I am planning on building an attached 3+ car garage with radiant heat in the slab between the structure housing wood system and the house.

    Pook...yes, I am aware of the stagnant water concern. That was another one of the advantages to flat plate exchangers at the Polaris.
  4. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    I would say to definitely use a heat exchanger. Yes, it's an extra pump, but the safety factor is pretty critical when dealing with systems that mix heating and potable water.

    Also, just be aware that you need to use reasonably-sized piping to make that 100-foot run (really 200+ feet, since it's a loop, and there are fittings that count as extra flow restrictions). In order to get proper flow with too-small piping, the pump requirements can get pretty silly.

    Joe
  5. Shelterman

    Shelterman Member

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    Joe...When you say heat exchanger are you refering to the flat plate exchanger as opposed to a coil in the storage tank? I was planning on 1 1/2" pex for use as piping. The run to the most distant Polaris will be about 150'.
  6. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Actually, much as I like flat plate heat exchangers, I would recommend a shell&tube;design for your system. With that sort of loop length, you will be having some pretty significant head pressure from just the pex friction, so a higher-head heat exchanger like a flat plate would mandate either a larger pump, or that you double up those pex lines (run two supply and two return) which is expensive.

    Joe
  7. Shelterman

    Shelterman Member

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    I see. Though I am familiar with shell and tube heat exchangers, I'm not well versed on how they are utilized. I thought they were more for just maintaining the temperature in a water heater or hot tub. Will they transfer enough heat for my purposes? On a real cold day my 100,000 btu/hr Polaris heaters stay pretty active.
  8. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    They're available in all sizes. Just have to order one that has the performance you need.

    Joe
  9. Shelterman

    Shelterman Member

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    Thanks Joe for the idea. I will look into it.
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