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Old 03-05-16, 06:37 AM   #10
Steve Hull
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
Posts: 829
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Oil pan,

Hate to be a wet blanket, but your return in $, heat or whatever is going to be negligible. Up north, where entering cold water is COLD, then heat recovery from waste water can be rational.

I built one for my shower when living in Michigan about 30 years ago. The cold water coming in was about 40 F and the drain water in the shower was about 80 F. I plumbed it so that the recovered heat, preheated the cold water to the shower valve. After about ten minutes in the shower, I noticed I could turn down the hot water valve just a tiny bit.

It worked - but not to my expectations.

The key is understanding the difference in water temperatures. Assume your incoming water temp is about 60 F. Assume that waste water is about 80F. The difference that is there (delta T) to recover is about 20F or so. The recovery is no where near 100% so let's assume 50%. That means that a 10 F difference is there. Now multiply by the average number of quarts per minute going down the drain when this is working - let's call it two gallons (8 quarts) per minute. The number of BTUs saved is tiny! About 80 BTUs per minute.

As an example, the heat from one burning match is about 1 BTU.

Now go to a more northern place where incoming water temp is about 40 F (all else same). The delta T is now 40 F - double that in your area. The recovery is also double, but still small.

I love the idea of recovering heat, but sometimes the cost (or % recovery) to do so is just not there. Waste sewer water might only be 60 or 50 F.

But as an interesting and fun project? There I am with you 100%!!

You will need to mount your recovery tube in a vertical orientation so the water swirls down on the pipe surface (water "skin effect"). Copper will be a far better heat transfer media compared to other materials. Even better would be to make a coaxial heat exchanger with ripples or dimples on the exchange metal to expand the available surface area for heat exchange. A long pipe (10 feet) would also help to get efficiency. But count on a maximum of maybe 50%.

consulting on geothermal heating/cooling & rational energy use since 1990
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