Tuesday, March 31, 2009

Tom and the drain water heat recovery unit

On the Greenmonk Energy & Sustainability show http://www.ustream.tv/recorded/1319778 30-Mar-2009, Tom Raftery talked about someone's ideas on shower water heat recovery. I did a little back-of-the envelope calculation, then went to the Internet to check my assumptions.

Let's assume I use equal amounts of 50 degree cold water, and 130 degree hot water for a ten minute shower, using 3 gallons per minute. The water mixture is 90 degrees, starting from 15 gallons cold and 15 gallons hot (assuming my thermodynamics are correct).

If I had a magic heat exchanger that warmed up the cold water 10 or 20 degrees, I might be able to change the cold/hot ratio so I used 18 gallons of not-so-cold water, and 12 gallons of hot water. I can't raise the drain water above 90 degrees, as that's what it is already. Though I think the 20 degree rise is optimistic, the resulting savings is: 3 gallons of hot water not used. Or, to put it another way, a shower about 2 minutes shorter. Well, perhaps a shower head shut off valve would be cheaper and easier to install.

Space? Where do I put this unit? In my case, it would need to go somewhere in the first-floor kitchen, which is beneath the shower unit. So, we're talking some major renovation to open the walls, find the supply and drain lines, and repipe both. Probably a serious time delay and inconvenience.

Cleanout? Supply lines are under pressure, drain lines are not. Generally, keeping wastewater flowing is desirable so that solid deposits don't build up. I've snaked the drain line a few times to get out the hair and other clogs, and don't really want to picture what a unit that coils cold water inside that will look like in a few years.

Cost? For best heat exchange, we're probably talking metal, so replacing plastic drain lines with copper is a big investment. My house is old enough to have copper supply lines; newer ones probably have plastic. And it's likely the supply lines aren't near the drain lines, so a large rerouting is in order. A U.S. Government site (eere.energy.gov) says "
Paybacks range from 2.5 to 7 years" yet with no published basis for this claim. Maybe my 3 gallons of hot water conserved each day will add up to several hundred dollars electric usage avoided over 7 years?

Here are several links I found:

There is not a lot of hard data in these links, with statements such as "Overall savings for heating hot water to satisfy all household needs will be much less." Uh, then what? The most data I found in this admittedly short search was in the greenerhouse.info site. Those measurements support my quick calculations, with a couple more caveats:

  • Start up time could be a couple minutes, as the cold water isn't being heated up until enough hot wastewater passes through.
  • They estimate a savings of 20% hot water (just like I figured).
  • They don't show water volumes used
Another useful site I found is http://www.bio-radiant.com/WaterCal.html to calculate actual energy costs of hot water usage, with projections of alternate sources. It appears we're spending several hundred dollars per year on hot water for showers, dish washing and clothes washing. Time to measure these a little more.


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