I want to get a hot water recirculation line added to a house. plumber is proposing a timer to turn on/off the pump.

I'm thinking a motion sensor method would be better. are these even common and how do most people install them in terms of where to put sensor(s). this is for a single story 2 bath house. would it be best to have a sensor in each bathroom as well as kitchen?

  • I like the concept. I might try the same thing. Can you tell me how long this has been in operation and how it is working out for you? Sep 20, 2018 at 16:21

4 Answers 4


Timer: the other way is too slow in many cases.

Using this chart of water contents(gallons per foot), we see that a 3/4" pipe of 70 feet has around 1.75 gallons of water in it. Using the fact that a lavatory sink emits out either 1 or 1.5 gpm, split between the hot and cold water pipes, means that it will take 1-2 minutes for the hot water to reach the tap, making the system useless.

A timer can circulate the water relatively quickly ever few minutes and then turn off, trading some electricity for your time. The motion sensor won't have enough time to circulate the water before you need it.

Nominal Pipe Size
(in)   Water Content (Gallons/ft)
Type K  Type L  Type M
1/8     0.0014  0.0016  0.0016
1/4     0.0039  0.0040  0.0043
3/8     0.0066  0.0075  0.0083
1/2     0.0113  0.0121  0.0132
5/8     0.0173  0.0181  0.0194
3/4     0.0226  0.0251  0.0268
1   0.0404  0.0429  0.0454

http://www.engineeringtoolbox.com/water-content-steel-copper-pipes-tubes-d_1617.html http://www.epa.gov/WaterSense/pubs/faq_bs.html


The most unusual solution I have ever seen this problem is a manual version of what you suggest.

They had installed a 'doorbell' system. The button was slightly hidden, under the lip of the counter, or some other convent place The 'ding-dong' part was replaced with a low voltage relay connected to the hot water circulation pump.

Before tap was turned on, you the button for a short time. (the kitchen was count to 6)

JimmyFix-it's comment about the thermostat control is the most common by far. His remark about pipe of your choice and vacation switch are both good.

The motion sensor, I like the idea, but it is likely the overkill geek in me. Again jimmy and the false positive thought seems right.

There is the idea of motion sensor that points at a place that you have to think about, a small area under the counter, easy to find, but not easy to use by accident.

I think the easiest idea is the remote thermostat. The next one would be a low voltage button, that started a 30 second timer for the pump.

  • thanks to both for responses... believe it or not in California.. i think this motion sensor method is now required. if i use a thermostat, then basically the system is always "on" except if i flip the vacation switch. this means hot water is running thru the loop 24/7 which seems like overkill when you are sleeping or at work. I thought the 'traditional' method was a basic timer.. where you set it to at least go off during night hours? Mar 24, 2015 at 18:34
  • i guess i dont have a feeling how much 'cost' it is to run it 16 hours/day vs turning off/on 4min burst everytime you walk by the sink or push the button. any thoughts? Mar 24, 2015 at 18:41
  • Feel free to hit me for this answer. Put a paper and pencil by the sink, and mark each time you walk by it. Maybe for a week, to give an average for 'normal.''
    – Some Guy
    Mar 24, 2015 at 19:15

Most common (in my area) is a thermostat control that mounts on a pipe of your choosing, with a "vacation" switch to shut down the pump during extended absences. I have also seen "thermo-siphon" set-ups with no pump.

I have never seen one hooked to a motion sensor. If your intent is to save energy by having the pump cycle on only when someone enters the bath/s or kitchen, I am thinking it would come on so often that it would defeat that purpose. Every time someone enters the bath or kitchen it cycles on, so it runs practically all the time anyway, and you have a power spike due to inductive load (and a lot of wear and tear) each time it kicks on. Multiple motion sensors might be a problem also, might you have to set up some type of relay system or a 4-way circuit?

Maybe use the normal thermostat, wired through a timer. It should be rather simple to set up and you could keep it from coming on when you know you do not need it (like at night).

  • I have used multiple motion sensors for many years with no problems at all the only caveat is they have to be on the same leg. In my case they are almost always on the same branch circuit so you don't have to label fed from more than 1 source.
    – Ed Beal
    Sep 20, 2018 at 18:06

I am currently in the process of installing re-circulation pumps (2 of them) in our new home and I believe the best fit for us will be motion sensor switches coupled with thermostatic valves under each sink. Before you install a pump however, you should know that by installing a re-circulating pump, you raise the water pressure by at least 5 psi. this is required to force the water back into the cold water line and back into the hot water tank. I have a pump under each sink and a motion sensor switch in each of the rooms. I took one of those motion sensor wall switches that turns on the lights when you enter the room, which leaves the lights on for a few minutes (make sure wattage rating is appropriate for your pump) and wired it in as the main power switch for the receptacle to where the pump is plugged into. I put the sensor/switch by the sink (minimal required distance from water taps) and left the pump/timer switch on the pump to "always ON", so when we are near the sink, the pump will have power and recirculate the water, only if needed of course, depending on the thermostatic valve under the sink, which came with my Bur-Cam Genie re-circulation pump. This will circulate hot water only when we are in the kitchen or bathroom and only if the temperature of the water in the pipe is below the thermostatic valve's setting. This way, the pump will never come on if we are not home or not using any of those rooms when we are at home. I know this solution requires a separate pump for each room, but it also saves energy as it only recirculates the water in those pipes for the room you are in. With the single pump installed at the hot water tank, you would be constantly recirculating water to all sinks where a thermostatic valve is installed.

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