Comparing Energy and Water – Why Recirculation Pumps are Over Rated
Posted by Luke Fishback
When a hot water recirculation pump in my home recently stopped working, it challenged me to figure out whether this seemingly water-saving idea really made sense. In this post I’ll present my logic for not fixing it.
To get us all on the same page, a little background education: a recirculation pump is a pretty common way to produce instant (or nearly instant) hot water throughout a home. It tends to be used for larger homes where the water heater might be a long way from a shower or sink. This pump continuously pushes water from the water heater throughout the hot water lines in the home and then back into the water heater tank. It prevents the water in the pipes from ever cooling down, making for quicker hot water at the shower and tap. These pumps are marketed as water saving devices because they prevent homeowners from needing to run the water for a long time before hopping in the shower or using the sink. While I can’t argue with the convenience they provide, it turns out they might not score so high on water conservation.
My recirculation pump used a constant 150Watts of electricity 24/7. That’s the same as 3.6 kilowatt hours per day (150W * 24hrs / 1000 = 3.6 kwh). It takes a lot of water to make electricity. On average across all of our electricity sources it takes about 2 gallons of water for every kilowatt hour of electricity produced. This is mostly water lost to evaporative cooling. So, without even factoring in all of the energy it takes to keep that water warm, it already takes 7.2 gallons of water per day just to power the pump. Let me be clear, this 7.2 gallons of water does not show up on your water bill, it is used during the generation of the electricity to power your pump. For more info on this, check out the “2 gallons of water” link above.
Now, if I unplug the pump it takes 45 seconds longer for the water in my shower to get warm when the shower is at full tilt (around 2 gallons per minute). So I use an extra 1.5 gallons of water every time I shower. To justify my recirculation pump as a water-saving device I would have to shower (or do other similar hot water things) 5 times every day… and that’s not even considering the energy expended to heat that recirculated water.
Just to run this to ground, let’s do a quick estimation on the extra energy used to heat all of that recirculated water. In the previous test we figured there was about 1.5 gallons of water in the pipe between my shower and the heater. So we’re talking about 3 gallons or so of total water that had to be constantly warm (water lines to and from the shower). Even in insulated pipes it’s probably reasonable to assume that the water had to be cycled through every hour or so to keep it hot enough. Since the water that is re-introduced to the hot water heater isn’t exactly cold, let’s put a fudge factor in and say we only have to heat the equivalent of 1 extra gallon every hour, or 24 extra gallons per day. At 24 extra gallons of hot water every day, that recirculation pump may have been doubling my hot water bill! Wow! Too bad I don’t have real-time gas monitoring to check the savings…
If you do have a recirculation pump and can’t live without the convenience it affords, at least consider putting it on a timer so that it’s not running at all hours… after all do you really need instant hot water available at 3AM or while you’re at work? A simple holiday light timer will do the trick.