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Options for Heating Domestic Hot Water from Storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by arngnick, Feb 28, 2013.

  1. arngnick

    arngnick Member

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    I am trying to determine the best easiest method to use to heat my domestic hot water from my thermal storage. Since I am not willing to add a demestic coil in my tank I have pretty much decided to use a flat plate exchanger with 2 circulator pumps to heat my water controlled by an auquastat located on my existing electric hot water tank. Open to more options or details.

    My current idea is to pull cold water from the bottom of the tank and pass it through the plate exchanger then inject the hot water into the port where the blow-off valve is located. I think the best place to put my auqastat is on top where the water leaves the hot water tank. My questions...
    1. Can I use the internal thermostat that is on the electric tank?(I do not want to comprimise the electric functionality)
    2. If not where is the best place to locate the auquastat?
    3. Best injection points givin the lack of ports?
    4.Locate the plate exchanger closer to the storage or domestic tank?
    5.Any other thoughts or better ideas?

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  2. bmblank

    bmblank Minister of Fire

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    I was considering heating my domestic with a heat exchanger like that. I have a coil in my tank, but that doesn't really help when the tank is cold. My intentions were to use a plate exchanger (cause they're small and cheap) with a flow switch in the domestic side connected to a small circulator on the boiler side. Basically it'd act like another zone, but instead of a thermostat it has a "flowstat". Turn on the hot water, the flow triggers the circulator, hot water pumps through the exchanger warming the domestic.
    It wouldn't be instant hot water, given all that opportunities for lag, but i would expect it to be enough to tide the family over until storage gets back up to temp.
  3. arngnick

    arngnick Member

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    Where would you inject the water in the hot water tanks? Preheater?

    The guy that sold me my boiler said depending on the size of the plate exchanger and the storage temp will determine how many times I will have to circulate the through the exchanger. He said I could easily make hot water from 130 degree water in storage. Even in a single pass through the exchanger if it was big enough.
  4. bmblank

    bmblank Minister of Fire

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    I have a thermovar valve in my system that, as i understand it, will send the water through for another pass if its still above 140. I don't really care how hot the water going in is because i have a mixing valve in my domestic hot water line. I would just add it a if it were another zone, except instead of a manifold and pex, it'd just run through the heat exchanger.
  5. arngnick

    arngnick Member

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    Same loading unit I have on my system I have the 810. So you will run a seperate "zone with a simple circulator through a plate exchanger controlled by a flowstat. On the domestic side of the exchanger you will come out of the "hot" side of the tank and straight to the point of use?
  6. bmblank

    bmblank Minister of Fire

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    Oh,i understand your question a little better now.
    I actually don't have a water heater tank, I'm all storage at the moment.
    I was also considering an indirect water heater, which basically sounds like your endgame. To retrofit something into an existing water heater would be interesting.
    I guess the way i would imagine it is to create a t, or H rather, between the inlet and outlet of the water heater. In the leg between them that's where you'd want the pump and heat exchanger. Out would just circulate with the tank. If you drew from the cold side, where there is a tube that goes to the bottom of the tank, and pumped it through the exchanger into the top of the tank that would ensure the greatest delta t and be the most efficient use of the exchanger. Unfortunately this would also have the effect of completely bypassing the water heater if someone were unlucky enough to want heat when the tank was trying to heat. Flipping it around, you'd be pulling water from the top of the tank and dumping it back into the cold side. Less efficient, but there shouldn't be any cold water surprises.

    Keep in mind, I'm just making this indirect indirect water heater up as i go. I'm not sure it would work the way i think it would. Its late and i should probably be in bed.
  7. arngnick

    arngnick Member

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    I agree an indirect is the best option but very expensive(probly not bad once I buy all the parts to retrofit my current tank). I was thinking the exact same way as you first described but the fact that when someone turned the water on it could cause an issue. That is why in my design I was going to return the hot water from the exchanger to the side port at the top of the tank where the blow-off valve is. Then the hot water will rise to the top and with a 5 or 10 degree swing on my auquastat the system will not likely short cycle. Thoughts?
  8. maple1

    maple1 Minister of Fire

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    I'm using a sidearm exchanger. Got it from OFS, advertiser on here - the one with the multiple tubes inside.

    'Cold' DHW comes out the bottom of my electric tank via a T, and after it goes up though the exchanger it goes back into the electric tank at the pressure valve port (another T there). That is all convection flow, no pump - but your exchanger needs to be mounted as low as you can get it for decent convection, ideally the bottom of exchanger at the same height as the bottom port on your electric tank (found that out the hard way & had to replumb & lower my exchanger to get it to work right - it's now just a few inches higher than my bottom port). I've got an 80 gallon tank - have used no electricity since I got it replumbed. It builds up/banks heat over night & when nobody is home most of the day. The boiler side is just plumbed into my supply & return headers. I have a zone valve on that circuit, which is controlled by an extra electric hot water thermostat that I just strapped onto the body of the ball valve that I have on my bottom electric tank port, as close to the tank as I could get it (right up against it). I have it turned down as low as it will go (110?) - there's not good thermal contact there (flat thermostat on round valve body), so it isn't as accurate as it could be. But it's working. I have the thermostat well insulated with pipe insulation, I got some thermal grease to put in there too but haven't done that yet. So far, my regular zone circ is getting enough flow through there to heat the water, during heating season, when the house is calling for heat. I will see if that side will also work good enough just by convection when heating season ends. If not, I will just run a wire from the end switch on the zone valve to start the circ when the thermostat calls for more DHW. So no added circs for DHW. I also got a NO thermal switch to strap onto my top of storage somewhere, so it will close the exchanger zone valve if storage gets too cool (130?) to avoid heat loss from the DWH tank to storage if I happen to get lazy with fires - that's also not installed yet.

    So far, so good.

    One thing I have found, at least in my case, is the 1" boiler ports on the exhanger caused most of my zone flow to short circuit through there when heat was calling, so I've got the ball valves in that circuit choked off some.

    EDIT: Another early mistake I made was plumbing the outlet of the exchanger into the top port on my DHW tank. That caused water to short circuit from my well water pump through the exhanger to the tap when there was a hot water tap open, and bypass the tank - the exchnager couldn't keep up and the water would go cold. Also, I decided against indirect as I wanted a fully functional electric hot water tank for fall back in case of extended periods of no fire burning - it's the most efficient way for me to heat hot water (aside from with wood).
  9. arngnick

    arngnick Member

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    I think that is a good option! I did a good amount of plumbing yesterday and I made the same mistake as you. I plumbed directly in the hot side on the top of the tank. I can see how that could be a bad thing so I am going to move it to the top side port. I will put my auquastat on top on the hot outlet(unless someone convinces me that its a better idea somehwere else). Like you I like the idea of the electric backup even though I plan to burn through the summer.
  10. ewdudley

    ewdudley Minister of Fire

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    If you're lucky and it works out for you, best easiest is to have a (electric) tank in the house one floor above storage with a sidearm above storage and below the tank. Gravity feed from storage to sidearm and gravity feed from sidearm to tank
    Floydian likes this.
  11. Floydian

    Floydian Feeling the Heat

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    This has been my plan all along. Just remember the scald valve. You definitely don't want 160::F+ delivered to your faucets!

    Also, I recall a lot a HW tanks are only rated to about 165::F or so?.... and you'd need to replace the cheapo plastic drain with something that can take the heat.

    Noah
  12. maple1

    maple1 Minister of Fire

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    Definitely the way to go to get the most hot water as easiest as possible, and I'd have done that if I could have. I've got two temp probes on my electric tank, stuffed in the element openings. The way mine is laid out, the hottest I've seen it get is about 155 at the upper one, and 135 at the lower one, in the morning after it does its thing all night. Before I finally got the plumbing straightened away, it was a constant 113 on just electric. I did put in a mixing valve to avoid scaldings - yes, don't forget that. I've also got about two rolls of pex stuffed in around & on top of my storage tanks within the insulated enclosure that preheats the DHW before it gets to the DHW tank or the exchanger - that alone does a lot of my DHW heating. I'm a bit leery of how much heat the pex will stand, but I don't usually push my storage to the limit either.
  13. arngnick

    arngnick Member

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    Just to followup I have been heating my water using the design I explained above and it is working great. I have the auquastat set and 120 with a 5 degree swing. could not be happier. (the two circ pumps and plate exchanger make it a fairly expensive setup but works well)

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