Need Advice on DHW Design

[rickh1001]

With the EKO 60 installed and operating well for the past few months, I finally have time to devote to bringing the DHW on board.

I had a hot water coil installed into the 500 gal pressurized LP storage tank (actually two - one for next summer when I plan to install some solar heating). It seems I have at least two options. One, I could add a small circulator, and pump the entire contents of the water heater through the coil. I would be feeding an LP fired hot water tank. I would need some type of aquastat on the tank to tell the circulator to turn off/on with the temp. Would I use a standard Honywell rectangular aquastat like I have on my plenum heat exchangers, held against the side of the hot water tank (after I cut through the outer shell), or are there other, more compact controllers I could use?

Another way I am thinking is to simply route the incoming cold water through the coil in the storage tank. This would be less efficient, as I would have standby losses in the heater, and the 30 ft or so of connecting pipe would become cold between use. However, upon demand, that would simply get mixed into the hot water tank, followed by 170F + water. I realize this would still use some LP gas, but would avoid the need for a circulator/aquastat, and would cover most of the energy.

In either case, I assume I would need a mixing valve at the tank, to control the output water temp to 120F?

 

[mpilihp]

I havent used one but for your application it sounds like a Side arm would be the way to go to heat up your gas DHW tank. Then Id feed the cold supply for DHW through your coil in the storage tank and have the output of that feed into the gas DHW tank, pre-heat it so it doesnt run your tank down.

ANd yes id have a 3-way valve to temper the output to a safe level..

 

[Nofossil]

I agree with mpilihp on all points. That combination would give you the best of all worlds at a very low cost and simplest possible control complexity. (Note: no controls at all other than what you already have).

Be sure that you get a mixing valve that has integral check valves. Otherwise, your DHW can attempt to heat your storage via thermosiphon through the preheat loop.

 

[rickh1001]

mpilihp and nofossil,

Thanks for the input. I am a little confused however. Are you suggesting I put a sidearm HX on the DHW tank, and plumb the heating water for that through the HX coil I have in my storage tank? If so, wouldn't I still need a circulator to push the water through? Since I have the extra HX coil in my storage tank, couldn't I just circulate the entire contents of the DHW tank through that to keep it up to temp? With this extra coil, I can run potable water through it for direct heating, so I'm not sure of the purpose of the sidearm HX.

Sorry for being so dense. It doesn't take much to get me confused on plumbing stuff.

 

[Nofossil]

mpilihp and nofossil,

Thanks for the input. I am a little confused however. Are you suggesting I put a sidearm HX on the DHW tank, and plumb the heating water for that through the HX coil I have in my storage tank? If so, wouldn't I still need a circulator to push the water through? Since I have the extra HX coil in my storage tank, couldn't I just circulate the entire contents of the DHW tank through that to keep it up to temp? With this extra coil, I can run potable water through it for direct heating, so I'm not sure of the purpose of the sidearm HX.

Sorry for being so dense. It doesn't take much to get me confused on plumbing stuff.

No, two separate issues.

1) Run cold water through coil in your storage before it goes into the cold inlet of your DHW tank.

2) Set up a sidearm in some convenient section of your heating system so that when the wood boiler is running the sidearm is heating your DHW tank. The ideal place would probably be on the return end of the circuit, after the zones but before it goes back to the boiler. Really, anywhere that there's flow would work. Sidearms don't extract a lot of heat per hour, but they'll get your DHW pretty hot any time the wood boiler is running.