Pre-Heating Water

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Jeff Howard

New Member
Sep 30, 2013
3
Hello -

Hopefully this is the right place to post this question. I saw the 'water boiler' section, however this technically isn't a boiler, just a regular wood-stove that's pre-heating our water so the electric water heater doesn't have to work as hard.

I've seen a few DIY projects online, but none are all that detailed and the designs are all very different. I wanted to run my concept past the forum to see if anyone else has tried this and can confirm (or shoot down) my design.

1) Does anyone know if a good online resource for people who are pre-heating water with their regular wood stove? The concept is pretty simple, but I want to make sure I learn as much as I can from other peoples projects (mistakes!) to make sure it's safe and efficient as possible.

2) I've done a lot of reading on copper tubing and it seems like "Type L" is the best option for this project. I plan to use two runs of 1/4" (Y'ed apart from a 1/2" line pre-stove, then Y'ed back together to the 1/2" line post-stove) for the best coverage. The top of the stove where I'd run the copper tubing averages between 400-600F (when the blower is off), which is far below any temp that should affect copper, and the tubing should always be filled with water keeping it less than 200F.

The best price I can find on copper tubing is Mueller at Home Depot. It says it's rated to 700psi @ 400F. Since it's going to be filled with water, I can't see a scenario where it would get above 400F, and if it does then there's no water left in the pipe so a rupture really doesn't matter at that point!

3) I plan on installing 2x in-line pressure-relief valves, but I'm trying to picture what will happen when/if the pressure relief kicks in. The valve will release, and the high-pressure steam should move towards the valve since it's now the lowest-pressure location in the system. So playing the scenario out, all the water between where it started boiling and the pressure release valve would be evacuated and cooler water from elsewhere in the system would take the place of the water that just boiled off. That seems pretty safe to me, but it would be nice to hear some experiences from other that have tried it.

4) Our Lopi 1750 has a high-CFM blower that blows air across the top (hottest) part if the stove where the copper coils would run. When the blower is running, the surface temp falls dramatically and honestly we run the blower pretty often to keep the air moving (else it's about 85F in our TV room next to the stove!).

So I guess that's :) Thanks in advance to anyone who's interested, and I'll be sure post the results of my project if people are interested!
 
Too easy to blow everything to pieces. Water boils at 212 and your stovetop can be 800.
 
A friends' parents have had a setup like that for 30 years. They have a pre-water heater that brings the water up to ~50*F, then a circulation pump that pumps it to/from the main water heater.

The only time they have an issue is when the power goes out and the circulation pump turns off. Then the relief valve releases pressure under the house (apparently it makes quite the noise !!!).

IIRC, they have a line coming up through the floor, then it spirals up around the flue (with a metal shield around the pipe to keep some heat on the pipes), then it goes through the pressure relief valve, and back down into the floor. The relief valve has its own line to direct overflow under the house. I don't know what they have down there to contain any mess. I think they also have valves so they can bypass/drain the stove if necessary. I also think they have a special pump that can handle the higher water temperature that it is moving.

I wouldn't do it without a circulation pump, which I didn't see mentioned in your original post.
 
Thanks for the info Hoozie.. I actually do have a design with a circulation pump, and I had planned to power with with some TEG modules (http://www.tegpower.com). I wanted to see if anyone had tried a design without it before going that route since it's more complicated and costly.

The only big question-mark for me on that design was how the circulation would work on an already-pressurized water system. As long as the pump itself was rated for a higher pressure than the municipal pressure (which is 50-60psi in my area), I think it would work fine.

I also have a tough time visualizing what happens when someone starts using the water... The pump would still be running, so it would pumping some of the hot water back into the cold side. Which I guess really doesn't matter, just seems odd.

Do you have any idea what they based their design off of?

Thanks,
 
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