Hot water tempering tank

Trent Hardy

In the Brooder
5 Years
Jul 30, 2014
Newfoundland, Ca
Hi All,
So for anyone who isn't familiar with the concept of a tempering tank, there is plenty of information online, but basically one uses an already present source of heat (such as the sun or a wood stove) to warm water in a small tank. Then, when you draw water from your hot water heater (for a shower etc), it is replaced by already warmed water - thereby reducing the amount of electricity needed to heat water again. I had only recently heard of it and was very interested in the concept, and so have started to try it out.

This summer I took my first delve into the realm of the solar hot water tank and failed miserably :). I constructed a large (40 gallon) batch tank to use as a tempering tank. Not sure if my positioning was off, or the tank was too large, or if the weather was just too crummy (we basically had a three week summer this year), but I did not get the kind of results I was hoping for. Oh well, moving on, right?

I've decided that for my next project I will take the solar water tank idea and instead construct a tempering tank to place behind my wood stove for the winter months. This is what I am going to do.

1) I have cut a large portion out of the original 40 gallon hot water tank (my solar heater), and welded the top and bottom together. The tank now holds around 10 gallons. Using a hot water tank in this manner allows me to retain use of the various fittings (cold water supply, relief valve, drain valve, and sacrificial anode).

Unfortuantely, I'm just learning to weld, so my welds were NOT water tight. After a lot of head scratching and cursing, I finally managed to stop the holes using a tube of silicon that is rated for >200 degrees fahrenheit (I purchased it in the wood stove section of a local hardware store)

2)Next I will place the tempering tank on a small rack directly behind the wood stove. I will not have them touching as I have a few concerns regarding this:
a) contact may cause water to get too hot, either causing the releif valve to set off frequently, or scalding someone in the shower,
b) the silicone sealant I had to use is rated for 200 degrees, which I think would be exceeded if contact were made. I'm hoping that a small distance will decrease this.

3) I plan to use copper piping to run from the tempering tank to my hot water tank. Although at some point I would like it to join into PEX piping (cheaper). However this will be something I'll evaluate as I go.

Please feel free to add any suggestions/questions/comments/experiences. I have never done this before, so I'd love to get input.

I'd like you to remember however that I am not setting this up as a primary water heater. We only burn wood as a supplementary heat source (i.e. evenings and weekends) so anything geared towards making that sort of system would probably be a great idea for another thread.

Thanks in advance :)
Sorry to hear that your solar pre-heater didn't work like planned.
I have heard of people having success running the water through a zig-zag of tubing that is attached to a piece of plywood painted flat black and sealed in a shallow box with a clear cover and tipped up to catch as much sun as possible.
This link is for a site where you can calculate the angle that your solar panel needs to be at to get maximum sunlight for heating.

Are you going to make this a part of your potable water for the house and run it under pressure? That may cause a real problem for you.
A safer thing to do may be to use a tank of water that you heat, and run a coil of tubing in that water to attach to your house water system. That way, you will not be directly heating a sealed system, that will build up pressure, but you can use the hot water in the tank to temper the water in the tubing. Also, no risk to contaminate your water system with a welded tank and sealer out of a tube that may not be safe for consumption if it leaches into the water.

Watch your tank temperature carefully when you start running the system. If you have are directly heating the tank as part of the sealed system under pressure, having the tank too close it will start to boil and you may have problems with too much pressure from the steam and blow the welds out of your tank.

Hopefully, this makes at least a little bit of sense to you.
Hey, thanks for the suggestions. I never thought of the method you suggested. I assumed that the pressure relief valve on the tank would take care of excessive pressure. I also never considered the issue of leaching - something I'll definitely have to look into, as yes I had planned on making this part of our put able system

I plan on revisiting the solar tank again next summer using something similar to what you've suggested
Although a household water system is not a really high pressure, it is under pressure and if your welds aren't that great, there is probably a good chance that they will fail under pressure. I hope that your welds hold.

Can you do a temporary set-up, without hooking into the house water system and do a trial run to see how it works? How long to heat the 10 gallons, how close to the fire without creating steam, and such stuff. Maybe get a feel for it before compromising your household water supply.

Good luck! and let us know how it goes.
I'm thinking the welds will hold, but God only knows. I do plan to include a series of valves, so if the tank does break I'll be able to quickly turn the water supply off. We're still about a month away from burning wood so may be a while before I figure this out.
can you rig up something to connect it to a air compressor or hand pump and get the tank up to pressure and see if it holds?
It looks like a standard home water system (with municipal water) is set at 50 to 75 psi.
I could, but I'll probably just hook it up and see what happens. I've got a manifold constructed (from the solar hot water tank) that will allow me to isolate it from our water supply if need be and still be able to get water to our taps. If the welds let go it'll just be an excuse to mop the floor :). Also, we will only be using this when we're home (at least for now) so I'm not too concerned about the water pressure issue, but am wondering a little regarding the leaching you mentioned earlier. Do you know if silicon is safe for potable water use? I had originally just assumed it was...

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