How does the sensor in a hot water circulation system inform the pump to turn off? [closed]

Question

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?

Answer 1

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.

Answer 2

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.