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Old 02-29-16, 12:37 AM   #4
mejunkhound
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I'd probably do what you are doing if I had 'free' stainless steel (or copper) pipe and disregarded the salvage value.

Using your epa numbers, , you are leaving out an awful lot of factors.
https://www3.epa.gov/watersense/pubs/indoor.html

70% used indoors, so that drops the 400 to 280 gal

Leaks and toilet are not heated water, epa says those are 40%, leaves 120gal day

epa says clothes washers are 22%, will assume warm water left at end, I'll grant your 30 F there, but we wash in cold water. 1/2 of faucet taken as cold water, 8%, leaves 120-32 = 88 gal per day that may be warm enough, say 100F (70+30) to recover heat.
Even engineered heat exchanges for liquid to air need about 10F differential,

so, max you get is 20F * 88 gal *8.3# = 14,608 BTU per day, about a pint of fuel, or say 45 cents a day if you have an elec WH, less if nat gas heater. A long long time for ROI, if ever, just based on scrap value of SS pipe.
Take away the clothes washer or use a modern one, the number drops to near zero since the water going down the drain from a shower is not sure 100F unless you have alligator skin.
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