I’ve been reading about hot water circulation pumps. The articles will typically have a sentence like “The sensor valve lets the hot water circulating valve know when to shut down and turn on the pump.”

But, I can’t find anything that tells me HOW the sensor valve informs the hot water pump of its state. It can’t just be whether water is flowing through the system or not since anyone else (or anything else in the case of a laundry machine or dishwasher) in the house could be using water concurrently. But the sensor isn’t powered, so I don’t see how it could be sending a message back to the pump.

How does a hot water circulation pump sensor valve inform the pump that it should turn off?

  • 1
    Probably some sort of bi-metal switch that clamps onto the pipe and opens and closes. As written, this really isn't about home improvement, more about curiosity.
    – JACK
    May 7, 2022 at 17:11
  • I have a cement wall in between the farthest bathroom and the water heater. WiFi signals do not get through that wall. I presume the sensor is not using WiFi to communicate with the pump, but how does it communicate with the pump to inform it that it is closed? I don’t want to install one and then find it doesn’t work because of the cement wall.
    – irrational
    May 7, 2022 at 17:49
  • 1
    which valve and pump are you using? ... it would make sense for the sensor to be in the pump itself on the return loop side ... running the pump until warm water returns, would ensure that hot water has reached the far end
    – jsotola
    May 7, 2022 at 18:02
  • What model is your controller? A thermocouple is a millivolt junction that produces a signal these can be the control or a thermistor a resistor that varies with temperature, in some systems a bimetallic switch is used and others are totally open loop and go on a timer only. But most do have one of the 3 previous temp controls to stop the pump when temp is reached.
    – Ed Beal
    May 7, 2022 at 18:48
  • 2
    I’m voting to close this question because its not about home improvement Dec 30, 2023 at 0:15

2 Answers 2


I presume that you are referencing a system that does not have a dedicated return line. These "newer" systems use a "sensor valve" (not mentioning brands, this is often referred to as the comfort valve) at the far end of the system which connects the hot and cold water piping together and opens when the temperature-sensing element inside the valve cools off.

If the sensor valve opens when the pump (which is installed on the hot water piping; usually adjacent to the water heater but not necessarily) is running, the cool hot water is pumped into the cold water piping until the water heats up and the sensor valve closes. This creates a circulation loop.

Typically, the sensor valve does not communicate with the pump. The pump either runs continually or on a timer which the user programs to operate during times of likely hot water use. I have heard of hacks wherein people who don't want the pump running all the time use timers with auto-shutoff features (e.g., 15 mins.) and activate those when they want immediate hot water, either by manual activation or by "Alexa" type control.

  • I would disagree with a dedicated return or the cold water line return there s a timer or sensor that controls the pump, if not the cost for such a system would cost more as the pump would not know when to stop running and then cost more than any advantage of a recirculating system.
    – Ed Beal
    May 8, 2022 at 0:10
  • Hi @EdBeal, yes on systems with a return line there is typically a thermocouple on the return line that operates the pump (often also controlled by a timer). On these new systems with no return line there is not; the pump runs continuously or on a timer. Some have a "sensor valve" equipped with RF signal for controlling the pump, but most don't. The pumps are small and are designed to run continuously. May 8, 2022 at 1:24
  • @ jimmy I don’t know how many of these you have done but I have done several dozen and the pump is at the remote location no RF!
    – Ed Beal
    May 8, 2022 at 19:56
  • @EdBeal you may not be familiar with the latest/greatest thing since sliced bread. I have not installed any but have looked into them for my home. youtu.be/rNmxo6VGhLo and grundfos.com May 10, 2022 at 2:03

There are essentially two systems to consider here, retrofit and dedicated return. A retrofit system pumps water from the hot water supply at your favorite location (ie primary bathroom sink) back into the cold water supply which results in a loop from hot water source near a fixture back to the water heater via the cold water line (cool hack if you ask me). A dedicated return system returns water from the hot water source back to the water heater via dedicated pipe (or circuit of pipe).

In my experience the pump which drives a dedicated return operates on a timer independent of temperature. The pump will run for five minutes and rest for fifteen during the hours that matter - in my home that meant run that pattern from 5:50am - 8pm. The pump was not aware of the water temperature, only it's runs schedule. On the other hand the hot water heater always monitors its own temperature and heats the water it holds as needed. I actually had a tankless water heater with a dedicated return so my pump drove water through the tankless and the tankless only activated if the water was below the threshold - this was not recommended by the manufacturer of my unit but it worked fine for years.

The other version of this I've seen from a control perspective is where a pump duty cycle of configurable duration (perhaps five minutes) is triggered by a button press at the remote location (ie bathroom sink) - this way when you wake up and go to the bathroom you hit the button and a few minutes later water is good to go.

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