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Old 10-20-2018, 09:53 AM   #1
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Tempering Hot Water

I have a 12 gallon Raritan hot water heater with a built in heat exchanger.

The water gets real hot by engine heating--basically a good thing because adding cold water makes the 12 gallons go further. Except some people don't know how to handle it and I would like to install a thermostatic mixing valve near the heater to avoid scalding someone. Any one done this? Looking at a valve on Ebay and the label says @45psi there is a 3.1gpm loss--can't handle that. Any valves that would work?
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Old 10-20-2018, 10:21 AM   #2
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Is your plumbing 3/8" That seems very small to me. I expect the tank ports are at least 1/2", and more likely 3/4".


I think a mixing valve is a must-have on an engine heated water tank for exactly the reason you mention. Ours was installed right at the HW outlet on the tank, and the cold water was brought to it. Honeywell, Watts, etc would all be suppliers to consider. Should be available at any local plumbing supply house. Don't waste your time at Hopeless Depot.
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Old 10-20-2018, 10:27 AM   #3
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Low budget alternative for showering...mix hot & cold water in a solar shower bag. This results in zero wasted water while one tries to find the proper temperature.
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Old 10-20-2018, 11:22 AM   #4
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I've got one on the output of the hydronic heat exchanger. One thing you want to do is thermally isolate it from the hot water tank or heat exchanger. This keeps it from getting heat soaked through thermal contact with no flow (which shuts off the hot) giving you a burst of cold before the wax pellet responds to the flow once a tap is opened.

Isotemp water heater tanks come with a thermal tempering valve as standard.
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Old 10-20-2018, 01:45 PM   #5
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When I installed our hydronic heating system I added a thermostatic mixing valve at the water heater. It made a big difference. No more scalding hot water when showering.
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Old 10-20-2018, 02:04 PM   #6
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Yes, I installed one. Personally, I learned how to operate the hot and cold taps to get the desired temperature around the time I was old enough to use the sink unaided. But I got so many complaints from people who apparently never figured that out, I decided to add a tempering valve. My other motive was to make the hot water last longer.

I ended up with one like the picture below, from Lowe's. It works great. It is isolated from the water heater by PEX plumbing. Some days I miss not being able to use really, really hot water for some tasks, but overall I'm glad I installed it.
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Old 10-20-2018, 02:25 PM   #7
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I have tried a tempering valve in my system and found that it worked after a short run but after a long run, my cold water tank which is also in the engine space (tho not heated) would also be quite warm and therefor could not supply enough cool water to temper the hot.
In the end I removed it as an ineffective and unnecessary complication to the system,
Better, would have been a remote thermostatic valve, which upon sensing the tank temperature would shut down the coolant flow to the loop in the tank thus preventing the hot water from getting to 180F (coolant temp)
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Old 10-20-2018, 04:16 PM   #8
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There are a variety of thermostatic mixing or tempering valves available, pick one that will connect to your existing piping. If you can't sweat copper joints, use threaded adapters to your piping. As others mentioned, pipe the mixer with a "heat trap" to prevent thermostatic siphoning or migration of heat from the hot water source to the mixer. It should be specified/described in the instructions that come with the valve. If you do end up with a sweat connection, pull the guts of the valve to prevent damage to the thermostatic element during soldering.

It will absolutely solve the too hot water issue. It will last if you heat trap it.
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Old 10-20-2018, 05:47 PM   #9
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Honeywell makes a good mixing valve so does Danfos, my preference is Danfos, Stainless Steal, a recerculating line would be a good idea aswell.
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Old 10-20-2018, 05:48 PM   #10
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Honeywell makes a good mixing valve so does Danfos, my preference is Danfos, Stainless Steal, a recerculating line would be a good idea aswell.
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Old 10-20-2018, 06:06 PM   #11
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When I installed my new water heater I added a Taco tempering valve. Several of the online plumbing supply companies stock them. It's adjustable, works extremely well, very satisfied.

It's important to understand that when the water gets heated to 170+/- degrees, it will expand pushing hot water back into the pressure tank. Without a hot water expansion tank and check valve in your system, you will want to open the cold water faucet and let the water pump cycle at least once to get cold water to the tempering valve. I added a hot water expansion tank and 2 check valves to make my system not push hot water back into the cold water side.

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Old 10-20-2018, 06:53 PM   #12
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This forum is great. Lots of good responses. The thermo isolation and the pushback of hot into the cold line are really worth considering. Might even install a hot water expansion tank and an inlet check valve. Thanks for all sharing their insight and helping me get out of hot water=bad pun
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Old 10-21-2018, 09:52 AM   #13
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There are two types of valves , tempering and anti scald.

The anti scald variety will shut off the hot water if there is not enough cold to bring the temperature down.

The tempering valve only mixes what water is supplied , loose the cold supply and 180F water can be delivered .

Much ungood.
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Old 10-29-2018, 05:34 PM   #14
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It’s an excellent idea. Isotemp Spa Marine water heaters come with a tempering valve.
Not only is it safer but by making the water hotter than would normally be safe, you can get a lot more usable hot water out of a small tank.
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Old 10-29-2018, 06:32 PM   #15
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Quote:
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Itís an excellent idea. Isotemp Spa Marine water heaters come with a tempering valve.
Not only is it safer but by making the water hotter than would normally be safe, you can get a lot more usable hot water out of a small tank.
Sadly, some water heaters come without a settable thermostat. There is no way to set them hotter than "normal." And, presumably thanks to the lawyers, "normal" means luke warm nowadays.
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