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Motion Sensing

Motion Sensing For Residential Hot Water Demand Systems

We receive many inquires about using motion sensing to activate residential hot water demand systems.  If you are interested in using motion sensors to start your hot water pump, then here are the details you should know before implementing such a system.
 

How Hot Water Demand Systems Work

 

Hot water demand pumping systems are activated by pushing a button.  Pushing the button starts a pump which pumps the hot water from your water heater to your fixture, and the cooled off hot water in the hot water piping is directed back to the water heater inlet through the cold water piping.

 

When the hot water reaches the pump a temperature sensor senses the arriving hot water and shuts off the pump.  The hot water reached the fixture more quickly than just running the faucet and no water was run down the drain.

 

The water in the cold water lines is now at least in part filled with the cooled off hot water that was left in the hot water piping after the last use.  

 

Presumably when the demand system shuts off and there is hot water at the fixture someone will then use the hot water for something… hand washing, a shower, the dishwasher, or some other purpose.  Some of these uses such has hand washing and showering will use mixed hot and cold water. 

 

These types of uses purge the cooled off water out of the cold water lines and when you finish the hot water use the cold water pipes are once again full of cold water just as though the demand system had never been used.

 

Other uses such as a dishwasher might use only hot water, and in that case the cooled off hot water in the cold water lines just continues to cool.

 

It’s all In the Timing

 

What happens when you push the button to activate your hot water demand system depends on what the temperature of the water inside the pump is.  If the water in the pump is below the lock out temperature of 96 degrees F, then the pump will turn on and begin pumping.  If the water in the pump is over 96 degrees then pump will turn on for ½ second, and then turn off.  This is to alert the user that the water in the hot water piping is still hot from the previous use.

 

Demand pumps turn off when they sense an increase in temperature, not at some specific temperature.  There are several reasons for this. The most significant reason is that since the water is flowing quite rapidly, and there is an inherent delay in measuring the temperature of the water due to the fact that it takes time for heat to transfer from the water and into the temperature sensor.  By the time the actual temperature is measured by the microcontroller, it’s already much hotter than that in the pump.

 

A rapid increase in temperature however, always means the hot water is arriving.  

 

Delay in Use after Activation of the Pump

 

If you activate the system and then wait 10 minutes or so before using it, the hot water in the pipes will have cooled off substantially, and you won’t really have HOT water, it will be luke-warm.  If the water in the pump is below 96 degrees, activating the system at that point will get you the hot water, but it will also fill the cold water line with luke-warm water.

 

If the water temperature is above 96 degrees… let’s say 98 degrees… then the pump is in lock out and it will not run.  You will then have to run warm water down the drain to get HOT water at the fixture, or let it cool for a while longer.

 

Using Motion Detection for Activation

 

As you can see, using a motion detector to activate the hot water demand system would only be advisable if the user was going to use the hot water within a short period of time.  Otherwise the system can just become a nuisance.

 

Another problem with motion activation is that it will try to run the pump even if someone is in the shower.  The pump won’t run for long since hot water must be nearby, but depending on your plumbing layout it could run for several seconds.

 

Tankless Water Heater Operation

 

With tankless water heaters, once the flow stops the water heater shuts off.  So if you wait for a few minutes before using the hot water you will end up with what's known in the industry as a "cold water sandwich".  Your water in the pipes is hot, but since the heater has been off, you will get a slug of cold water which the water heater was unable to heat since it requires up to 20 seconds to bring the cold water up to full temperature.

 

For some people using motion sensing for their demand system will make sense, but for most homes a simple manually operated push button is preferable.

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