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.
![Reblog this post [with Zemanta]](http://img.zemanta.com/reblog_e.png?x-id=db13a97b-5862-48aa-8e50-cbe3295bd161)
6 comments:
I got a great email about this post. An excerpt is pasted below. My post failed to address demand hot water pumps like Metlund's system. These systems make great sense and will save water even when their electrical usage is considered:
[This post] ignores the kind of pump that I have (the Metlund pump), that
is started on demand by a push of a push-button and stops running as soon
as hot water is sensed by a thermostat in the pump, normally located under
the sink where the operator is standing. In other words, the electrical
power spent is only about 20-30 seconds per activation -- not a continuous
loss of power. The pumps I've seen for sale at Home Depot are the kind
that run continuously, wasting electrical power most of the time.
There is no loss of water in any case. While they are running all pumps
bypass the water from the hot water supply line into the cold water supply
line, and reversing the normal direction of flow in that line, back into
the water heater's tank.
My Metlund pump in Maine was a snap to install, and is the neatest
non-electronic gadget I have ever seen. Has worked perfectly for years.
Another bonus of demand systems like Metlund or Chilipepper is that they ought to work fine with all kinds of water heaters, including tankless and solar systems.
An excellent analysis on an interesting topic - I think it's amazing that the companies that sell these pumps don't have any problem designing something so wasteful.
And why classify the water that's cold at first as 'waste' water? Maybe you should man up and embrace the experience of having the first minute of your shower be cold. It builds character and makes one really appreciate hot water when it finally does arrive.
Luke -- As a non-scientist, I have always heard how miraculous the recirculation pumps -- instant hot water! What could be better than that? I am glad to know to be more wary of this modern miracle and also to have learned about similar, yet better alternatives (like Metlund or Chilipepper).
Of course, my neighbor, who has a traditional water heater, captures the cold water in her tub shower in gallon milk jugs and uses them to water her garden. While her method is not terribly convenient, I always admired her dedication.
By the by, the graphic designer in me loves your "Visible Energy" logo but wonders why it is buried at the bottom of your blog?
Amanda Keen Basler
Thanks Amanda and Chuck for two great points. 1- I should "man up" and bear the cold first minute of my shower or 2- I should use some buckets to capture the initial cold water. But I'm not doing either, at least not consistently. And neither are most people. I love that some folks are willing to take on some discomfort in their efforts to live greener and I definitely support it. Generally I see those measures as limited in their ability to become mainstream in the near future though. And, importantly, I think there is enough low hanging fruit in most of our lives that we can focus on doing the stuff that doesn't introduce much discomfort and makes (an often bigger) difference. That said, please heed the good advice of Chuck and Amanda if you're so inclined and save even more water.
Post a Comment