Drain heat recovery

July 10, 2009 | categories: energy, engineering | View Comments

For the last few months, I've been thinking that a drain heat recovery system might be worth installing in our basement. The idea is that while you take a shower, you run the cold water refilling your water heater through a copper coil wrapped around the hot water going down the drain. There's a nice diagram on the EERE site. (Unfortunately, the EERE site also estimates the payback time at 2.5-7 years, which I think is bunk.) This kind of system doesn't work to recover energy when the draining and refilling doesn't happen at the same time, as with a dishwasher-- when the dishwasher releases the hot water, your water heater has long since refilled (or your on-demand system has turned off).

In April, I measured the ground water temperature in Somerville at 7 C (45 F); I suspect that's close to the annual mean ground water temperature. I also measured a hot shower at 38 C (100 F). This is a little conservative-- I think Sharon usually likes something more like 40 C, but let's say that on average we require water to be heated at least 30 C above the ground water temperature.

The heat capacity of water is 4.2 J/(mL * C), so with a 30 C difference, we're losing around 126 J for each mL poured down the drain. I measured the flow rate of our shower at 2 L in 20 s, which is 100 mL/s. This means we're using 12,600 J/s, or 12.6 kW. A ten minute shower uses 600 * 12,600 J = 7.5 MJ, or 2 kWh.

Drain heat recovery system vendors claim that they can recover almost half of the energy available to the heat exchanger. If we optimistically say we'll always get 50% of the energy back for the next shower, that would be about 1 kWh per shower, or 2 kWh per day, since we each shower every day. Natural gas for our water heater is currently $1.55/therm or $0.053/kWh, so that would save us $0.10/day, or $0.20/day if we assume our water heater has an efficiency of around 50%, which is typical of the crappy gas models like the one in our basement. At the flow rate I mention above, I'd need a heat exchanger about 40 inches long to hit 50% recovery, which would cost around $600 plus installation. If that totals around $1000, the payback time is 5000 days, or at least 13 years, even with the optimistic assumptions I've made above. Unfortunately, even if the system lasted 13 years without corroding, the chances of us living in the same house until then is small enough that I think I'll hold on to my $1000, or put the same money toward a more efficient water heater.

On the other hand, it does suggest that a doubling in the cost of natural gas plus self-installation would make it a win. If you own a drain that handles more than ~4 showers per day in the Northeast (like in gyms or apartment buildings), you'd have to be an idiot not to install one.

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