15 December 2010

Green in Action: On-Demand Recirculating Hot Water Pump

Now that we've been in the house for almost six months, and we've experienced both summer and winter(ish) conditions, it seems like a good time to evaluate some of the green features we incorporated into the design.

Let's start with the on-demand recirculating pump that's tied in to our plumbing system.

What Is It?
You know how you have to let certain faucets run for a while to get hot water? And how, the farther the fixture is from the water heater, the longer you have to wait? In our condo, it took probably a full minute for hot water to get from the water heater in the attic, through the walls of the second story, and down to the kitchen on the first floor. That's because the hot water lines between the water heater and the faucet filled with hot water the last time you used it, but since then the water got cold, so you have to wait for all of the cold water to flow out before the new hot water can get there from the water heater. Imagine if you were to put a bucket under the faucet to collect all that wasted cold water, then you took it to your water heater and somehow poured it back in to be heated up and used again later. That's what this system does, only without the risk of spilling buckets of water all over the house.

How Does It Work?
30 seconds or so before we need hot water, we push a doorbell-type button located near each of the sinks. Here's the one in the kitchen (with one of the signs that I put up when we had the neighbors over to check out the house):

And here's the one in the front bathroom, which we tucked away below the end of the counter:

A person who needs to use the bathroom would press the button on the way in. The button triggers this pump that's connected to our water heater:

The pump sends hot water from the water heater through a loop of hot water piping that goes to the kitchen and all of the bathrooms, sending the cold water back to the water heater (saving you the trouble of carrying that bucket around). This requires a lot of extra plumbing. Instead of just a hot water line and a cold water line to each location, there are additional pipes to bring the cooled water back to the water heater. Here's what's going on inside the walls of the front bathroom (we used Pex pipe; blue is cold and red is hot):

And the piping to/from the water heater is even more complicated, as you can see from the pipes that were laid in every direction before the foundation was poured:

Fortunately, the water seems to know where it's supposed to go, and once the hot water has made its way around the loop, there is only a tiny bit of cold water left in the line (where it branches off from the loop, about two feet worth in the bathroom picture above). Hot water flows almost instantaneously!

The Pros
  • We save water.
  • Our hands don't freeze.
  • We don't have to stand around waiting for the water to heat up.
The Cons
  • This kind of system isn't cheap. Since the piping cost was rolled in with the main plumbing bid, I don't have an exact number, but it wouldn't surprise me if it added about $2,000 to the cost of the house...which will probably never repay itself in water savings.
  • The system uses a small amount of electricity. (When these systems were originally designed, they were made to recirculate all the time -- no buttons. Hot water was always available, but the energy usage ended up outweighing the water savings. The on-demand system is the best of both worlds...as long as you remember to press the button. Steve has used it for washing his hands, etc., since we moved in, but I'm happy to wash my hands in cool water, as long as it's not cold, so I only just used it for the first time last week and am still getting into the habit...which leads me to...)
  • You have to remember to press the button.
The Verdict
Without knowing exactly how much electricity this system uses, I am confident that the water savings outweighs the energy use. And now that winter is on its way, having hot water at my fingertips (literally!) is a nice treat. I continue to be amazed at how fast the gloriously warm water flows. If I had it to do over again, we would definitely include an on-demand recirculating hot water system again.  I give it a solid A.


  1. That is sooo cool. I mean HOT.

  2. Yet another example of the novelty that is indoor plumbing. I like.