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Thread: harvesting waste heat from my water heater

  1. #1
    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
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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
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    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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    Moderator & Master Plumber hj's Avatar
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    There are companies which already make units for the shower drain, although my impression is that they are more "feel good" than really effective. Unless the tubing was very small with a high surface area to volume ratio, there would be minimal heat transfer. And the heater often does NOT turn on until the user is almost finished with his hot water usage But in any case it is NEVER instantaneous when the hot water faucet is turned on, so the situation where the burner is running but there is no water flow would be the most common one.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The drain heat recovery systems warm the cold water so that you can use less hot water to get your desired temperature. They do work, but it depends on the size you are able to install, the length of the shower you take, and how hot you like your shower. It will do nothing for taking a bath, since there's no drain waste heat to recover while you are filling the tub. A WH does not immediately fire up when you start to use water, so as mentioned, a lot of the flue heat would be after you're already done. WIth no flow, all the heat you might have collected will just radiate into the house and not get to the WH anyway.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    General Engineering Contractor ballvalve's Avatar
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    All large steam boilers did and do pre heat the incoming water with exhaust gas. Starting back in the 17 or 1800's. I think they were patented as "steam economizers". But they were cast or ductile iron, and large structures. But could raise the efficiency to very high levels.

    Your first problem is you might have double wall exhaust pipe, so of course you have to change to single wall.

    You are NOT going to have a reverse flow of air, because the exhaust is not that hot [measured it?] and you wont extract but a percentage.

    You could double wall the pipe and blow air in with a computer fan, and likely get the most for the least. Get a good CO2 detector.

    If you had a power exhaust you could do as much as you like, even inside the pipe, and run it down hill so condensation doesnt go into the burner.

    You might have a condensation issue on your standard pipe on the outside wrap too. If I wrapped a section, I would over sleeve it with a perhaps 10" pipe and fill it with vermiculite.

    Perhaps cheaper is a new high efficiency heater. And if you are on natural gas, not propane, it probably won't pay back.

    The main issue is that cheap gas water heaters are junk, and one heat exchange pipe up the center is a bad joke. Everyones is afraid of a spiral except Polaris. LIkely your pipe is hot because the base of your heater has 3" of calcium in it, and your efficiency is down to 20%.

    When did you last REMOVE the drain valve and rod it out with a hose and hanger with water coming in the top? And adjust the burner gas flow level? Try to boil water on your stove with 3" of pea gravel on the pan bottom. Low burner level, unless you need the recovery, helps greatly with efficiency. A gas water heater with a large tank and about 6 pilot size burners inside would probably get to 95% efficiency.

    We are in the DARK ages with standard gas water heating.

    Polaris used to ship out their power vent, 95% heaters with a sort of "economizer", the air INLET 2" pipe ran through the 4" exhaust pipe for about 4 feet. I extended mine to about 20 feet, actually to gain some back pressure [poor ignition] and now the inlet air is fairly warm.

    I feed my Propane Polaris with an electric 40 gallon heater, because when gas is over $2.75 a gallon, electric is cheaper, and you can
    insulate the tank to r-38 in a moment.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Industrial and residential WH systems have totally different use patterns. Trying to compare them is not productive.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    General Engineering Contractor ballvalve's Avatar
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    Well, industrial was only the first paragraph and the point was to say that they cared more about saving fuel in 1800 then we do post "free" gas and electricity decades.

    With all the new natural gas finds, I doubt Obama will be allowed by our purchased congress to regulate efficiency of gas water heaters like autos. I am certain that all the cheap water heaters in the world add up to 10,000 times the large high efficiency commercial boilers in operation.

    The Eskimos managed to boil on a stone pot and later gas tins with a tiny blubber flame and heat their house at the same time. They were more patient than the average American teenager, and thought a house at 34' F was the ultimate in comfort. The shortage of 80 gallon jaq. tubs was a big advantage also.
    Last edited by ballvalve; 09-24-2011 at 02:53 PM.

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    In the trades Dana's Avatar
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    Extracting any heat from the exhaust stream of an atmoshperic-drafted water heater is just asking for trouble unless you're willing to do the real math on it. The maximum possible return on it is also ~20% of the source-fuel heat.

    With a drainwater heat recovery heat exchanger and a 0.60EF unit your net return during showers and other long continuous draw would be on the order of 30% of the source-fuel heat (on average), and it's been engineered to work, AND it won't cause flue condensation or increase backdrafting risk.

    Whether drainwater heat recovery ever pays off financially is a function of your fuel costs & burner efficiencies, and the volume draw of simultaneous hot-water/drain flows. In a car wash or commercial laundry it pays back pretty fast no matter how you're heating your hot water, but in a home in 50cents/therm-land and 6 minutes/day of 2gpm shower, it's just a weird ornament in the basement designed to make the inspector thing you're running a distillery. :-) I installed one so that I'd never run out of hot water in my kludged-up heating/hot-water combi system. Without it I'd have to crank the modulating burner to a higher, less efficient level for space heating to acheive the peak hot water heating goals. The payoff for me was in not having to hear the cursing by mi esposa if she jumps in the shower right after I stepped out. ;-) YMMV. At $2/therm it would pay off even financially in reasonable time periods with our family of 3, but at the current ~$1.25/therm it'll be awhile. I don't expect gas to remain this cheap forever though.

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    General Engineering Contractor ballvalve's Avatar
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    The place to get some heat is the septic tank, but I wouldnt want to ask the inspector to approve the exchanger.

  14. #14
    In the trades Dana's Avatar
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    Depends on where you are, but very often the temp of the septic tank isn't significantly above that of the surrounding soil- it's not a strongly exothermic process. In cold/very-cold climates reducing the temp of the entering effluent too much can slow the breakdown process, but at those temps the amount of preheating of the water in your septic-tank heat exchanger would be pretty minimal anyway.

    The temperature gain of a 500 gallon septic tank from a 20 gallon shower with 100F drain water isn't going to be much either. It takes a pretty tall drainwater HX to bring down the showedrain outflow temps much below room temp before it leaves the house, but it's at least possible. With even a 4-footer the potable outflow temps from the HX entering the HW heater are often above room temp.

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    General Engineering Contractor ballvalve's Avatar
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    I dont have a thermometer in my tank, but all those turds being eaten by bacteria makes a lot of heat. My compost pile can get too hot to touch. The minimum tank size in most areas is 1000 gallons and most use 1500 gallon tanks.

    With 500' of poly pipe in the tank, you've got a very low budget system, if you can concoct a system to make it safe.

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