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Switching between two water heaters automatically

I've been in a bit of a quandary about switching between a 120 gal indirect I heat with wood in the winter, and a Toyotomi OM180 which I use to heat water directly in the summer.  As of now I have only one mixing valve, and simple a set of ball valves to isolate the unused heat source.  The question is with the fast recovery of the oil miser, and the storage of the indirect, is there any way I can pipe the two with maybe two mixing valves so which ever one is producing the hot water will be fed to the hot supply.  I have low voltage switches at all hot water faucets which control the oil miser so it's only switched on when there is a hot water need (20 seconds and endless hot water).  In the shoulder seasons it gets dicey since I can store enough hot water for 3-4 days, but it's not cold enough for a fire in the boiler for heat in the house. I don't want to use oil to heat the indirect since I can use less oil heating it on demand so to speak.  I want to just be able to switch on the oil miser and have hot water and not use the ball valves.  I've thought of using a three way zone valve but if it can be done simpler with one more mixing valve I"m all for it.

Thanks for this great site guys!

Taylor 
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!

Comments

  • Charlie from wmass
    Charlie from wmass Member Posts: 4,195
    If you feed the oil from the indirect

    it will not run if the indirect is hot enough and only a little bit if it is just warm.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
    https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,278
    True only when....

    there is a call for for hot water.  However when all is just sitting and no hot water call the Toyotomi fires quite often, about every 45min but only for about a minute due to low water volume (5 gal) and 145,000 btu burner.  I have thought of this setup, but with an aquastat on the indirect to keep the OM from starting whenever the indirect was over 120F.  My only reservation is the cold slug of water in the OM having to go somewhere on a call for heat, effectively making a really long pipe run from the indirect to the faucet. The OM seems to draft the heat right out of the tank out it's direct vent quite effectively, and some type of stainless flap in the exhaust like an on demand gas might prevent this, but I'm not going to re-engineer the OM. Maybe I'm just going to have to stick to wasting some oil or manually switching.  Sorry for being so cheap, and long winded. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,278
    edited April 2012
    Double :(

     
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Henry
    Henry Member Posts: 988
    Dangerous!

    One must keep the storage tank at 140F. Legionella exists up to 139F and tons of other bacteria will survive at 120 and below. Alternating between tanks creates a dangerous situation. You must not use and drain the tank between seasons.

    I am not sure where you reside, but in our area we refuse to install instant hot water heaters as they cannot meet demand with a stable temperature.We stopped in 2005.  In N.A. we do not have all the flow restricted faucets that they have in Europe or Asia. Our flow rates vary too much and in early spring our water temperatures still hover in the low 40s compared to Asia and Europe 60s!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,278
    edited April 2012
    160-180

    Both oil and the indirect are heated to 160 minimum, which is why there is a mixing valve.  The indirect is so large it holds water for 3-4 days between boiler firing. The Toyotomi is used in the summer, or when I have not has a fire in the boiler for several days and I'm impatient for everything to heat up (over an hour from cold).  Are there any issues with doing this?  I do occasionally drain both at different times. You can see in the right hand picture the two red ball valves used to manually switch the two before the Watts mixing valve (air intake hose removed for clarity).  The pic on the left gives perspective of where the 1/2" pipes go.  Both the indirect and the OM are 316 stainless and I am in northern Maine with soft well water. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • icesailor
    icesailor Member Posts: 7,265
    DHW Mixer/HW Extender Valve:

    You did not read the manufacturers instructions for the Watts 70A hot water extender. It must be heat trapped or the element will burn out and it will suffer from unwanted circulation. It must be installed at least 6" to 12" below the cold water inlet or the top of the water tank. You should have used a Honeywell AM series thermostatic mixer or a Califfi.

    Why some Heaters only should not be doing plumbing. It's a nice piping job.

    If you had a problem with hot water control, and you couldn't figure it out, and you called me, you would give me an argument.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,278
    edited April 2012
    Read my origional post please

    This is my own house, and if you read my original post you'd have noticed the the oil miser is only used as a true on demand hot water heater.  It's timer contacts are wired to switches at all major hot water usage points, so it's only "on" when I turn a switch on...... However the indirect is hot all winter and you'll notice it's properly heat trapped from the indirect, just from the oil miser since it's cold all the time. Also there is a swing check in the cold pipe just before entering the 70A hard to see in the right pic but it's there, so no convection loops here.  Anyhow, I'm not trying to give anyone an argument, only asking if there was a way to automatically switch with perhaps another 70A or some other valve.  Solid fuel is just my preferred fuel (transferred through water of course) it is not my occupation. You guys on here are much more experienced and knowledgeable than I in this matter. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,195
    There should be very little drafting

    with the OM unless the vent and air intake are set apart allowing gravity venting due to the height of the vent outlet, like trying to syphon between two buckets at the same elevation. Icesailor's suggestion of the AM mixer is a good one and the trapping needs to be from any heat source feeding the mixer not just the primary heat source. I must admit I have very limit knowledge of the OM so if anyone has a better idea and are more familiar with these units I would differ to them.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
    https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating
This discussion has been closed.