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Thread: harvesting waste heat from my water heater

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    DIY Junior Member Ian Miller's Avatar
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    Default harvesting waste heat from my water heater

    So I have this idea about how to use the heat that my gas water heater is shooting out the exhaust to preheat water going into the water heater, thereby using less energy. I noticed that the exhaust pipe gets pretty hot, so I was thinking about wrapping a soft copper pipe around it and running the inlet water through the copper pipe with the outlet of the copper pipe going into the cold inlet of the water heater. I figured I could get around 50 feet of pipe around the exhaust pipe, which I figure would raise the inlet temp of the water to the heater significantly. My question is if I should be concerned about adding a T&P valve somewhere, in case the water in the pipe would get too hot from sitting next to the exhaust when the heater was heating and no one was using the water. Does someone who knows plumbing know if this could work and how I would make it safe? Thanks.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    An atmospheric vent needs the heat to draw properly. I don't think this would work very well. You might find that an automatic damper slowed any losses up the flue when the burner wasn't running.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    An atmospheric vent needs the heat to draw properly...
    Without enough heat to maintain the draft, you risk the flu reversing and carbon monoxide poisioning. Also, condensing natural gas appliances are made of a higher quality steel to resist corrosion from acids in the condensate.

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    Forum Admin, Expert Plumber Terry's Avatar
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    I can see that wrapping the pipe around the outside of the pipe cold preheat the incoming cold.
    The water heater has a T&P already.
    That's pretty much the basic idea for a "heat exchanger"
    It would be interesting if they marketed something like that.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Plus, you'd have two dissimilar metals, not a very good transfer path, and the possibility of electrolysis causing corrosion. Yes, I do think you could prewarm the water some, but without a properly engineered solution, I do think you'd create more problems than you'd gain in efficiency. Copper isn't cheap, either. What WOULD work, if you're trying to prewarm the water and you take long, hot showers, is a drain heat recovery system. This is installed in your drain line and extracts the heat from the hot water going down the drain. Depending on the size you can fit in, it can extend the length of the shower by quite a bit.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member Ian Miller's Avatar
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    So, my plan was to put the pipe on the outside of the flue, so it seems to me it wouldn't be robbing that much heat from the gases escaping the flue. it would just be capturing the heat that radiates from the flue. At least this is what I think. Am I wrong? Also, the copper pipe wouldn't have any contact with the vent gases, so I don't think corrosion would be an issue. Or would it? I've thought about this arrangement on the shower drain, but I think there's more heat to be harvested from the water heater and the shower drain would be harder to get to. Maybe I'll just have to be the one to design this and market it. Or maybe there's a reason it's not on the market already. Thanks for your inputs. Any further thoughts are welcome as well.

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