How Does A Hot Water Recirculating Pump Work (Easy Guide)

Want to know exactly how hot water recirculating pumps work?

Hot water pumps can be an effective way to get instant hot water if you are tired of waiting for hot water.

I recently wrote a review on the best hot water recirculating pumps you can read here.

In this guide, I will go over...

  • The differences between full, partial, and under sink hot water recirculating pump systems
  • What is a "sensor valve' and how does it work
  • Why most water pumps are installed with timers
  • And how the cold water supply line is used with a water pump
how does a hot water recirculating pump work

What Is A Hot Water Recirculating Pump?

A hot water recirculating pump is a special type of water pump installed in homes to give you instant or almost-instant hot water to your fixtures.

One of the most common complaints that I hear from new home buyers during my home inspections is the length of time it takes to get hot water.

hot water recirculating pump

A Chief Home Complaint

Sometimes buyers follow me around during the home inspection, and one thing I always check is the mixing valve for the showers.

If the mixing valve is broken, then it can be very costly to open up the shower tile wall to replace it. So I am usually at each shower for at least a few minutes and sometimes longer.

Once in a while I am at a shower waiting for it to get hot, and my client asks me about it. I usually mention it's normal in a lot of homes to wait for hot water and that they may want to get a hot water recirculation pump to give them near-instant hot water.

Read Also: What Are The Best Hot Water Recirculating Pumps?

Main Cause of Slow Hot Water

One of the main reasons for the long wait for hot water is that homes built within the past few decades tend to be much larger --- so there is a increased distance between each fixture and the water heater. 

Most modern homes also have larger diameter water piping than older homes which takes longer for water to be circulated.

How Does A Hot Water Recirculating System Work?

The water pump, usually the size of a softball, is installed either above the water heater (for the whole house) or installed under a sink.

In my experience, the most common installation is above the water heater, but the under sink install has some advantages.

Working in tandem with the pump, there is a one-way water valve installed under farthest sink from the water heater --- it's called a sensor valve.

There are also hot water recirculators that actually have the sensor valve built into the pump, so it can all be installed under the sink.  Here is my review article on the best under sink hot water recirculating pumps.

What Is The Sensor Valve?

How the hot water circulating pump knows when to turn on is because it uses something called a sensor valve. The sensor valve has a thermostatic valve which reads the temperature of the water and then opens or shuts a valve. The sensor valve is installed under the sink that is farthest away from the water heater. 

sensor valve (hot water recirculating pump) (1)

The sensor valve opens or closes based on the temperature of the water which is usually around 85 degrees Fahrenheit. When the water in the pipe drops to that temperature, then the sensor valve will open, and the hot water pump will start running.

The water in the hot water line (which is turned cold or lukewarm) is pushed by the hot water pump. That water is pushed into the cold water supply line back to the water heater.

So just to reiterate, the cold water line, which usually gives you cold water, is used TEMPORARILY to send the water (in the hot water line) back to the water heater. It turns a supply water line into a return line.

When the temperature reaches around 95 degrees Fahrenheit, the sensor valve will close, and the hot water pump will turn off.

It is also very common for homeowners to install timers so that the hot water pump isn't constantly turning on and off even when nobody is home. You can use a timer and program the hot water pump so that it is only active from 6pm to 10pm and 7am to 9am as an example.

Some hot water recirculating pumps have timers built in to the pump --- but others you will have to install separate timers.

Read Also: What Are The Pros & Cons of Hot Water Recirculating Pumps?

How Does An "Under Sink" Hot Water Circulating Pump Work?

Even though most homes have hot water pumps installed above the water heater, you can also install some pumps under a bathroom  or kitchen sink. When you install a hot water pump under the sink, it is basically the same installation as above the water heater. 

The advantages to the under sink installation is that is much easier to install. You don't have to cut copper pipes or do any soldering. All you have to do is connect flexible water connectors.

The hot water pump is screwed onto the cabinet or wall studs, and you likewise install a sensor valve between the water pump and the faucet. With some under sink recirculating pumps, the sensor valve in installed inside of the pump, so a separate installation isn't required.

The only difference (and main disadvantage) with the under sink application is that it will only bring hot water to all of the fixtures BETWEEN the water heater and the sink where the pump is installed. If there are downstream plumbing fixtures from the pump, then those won't get instant hot water.

Read Also: What Are The Best Grundfos Hot Water Recirculators?

How Does A 'Dedicated Return Line' Work?

There are also complete hot water recirculating pump systems, and these are the least common type in my experience. With these systems, it is installed in the same way as the above hot water pump system except for the fact that there is a separate return line back to the water heater --- created a dedicated loop. So instead of using the cold water supply line as a return, it has it's own dedicated return water line. 

With this type of system, the dedicated return water line branches off the hot water line at the farthest fixture from the water heater (behind the wall, not under the sink). The dedicated return water line connects to the bottom of the water heater which is where the circulating pump is installed (rather than at the top).

hot water recirculating pump with dedicated return line (1)

The main benefit is that you get hot water faster than without a dedicated return line because water doesn't have to be kept in the hot water line in order to keep the cold water supply available.

Remember, without a separate return line, the water pump is using the cold water SUPPLY line to send water back to the water pump --- for a brief time. So when the pump stops working, it can't keep using the cold water supply line because you won't be able to have cold water.

But if you have a separate dedicated return line, then the water in the hot water line can be constantly drained which means that when you are ready for hot water through the recirculating pump --- you get hot water much more quickly.

The main disadvantage with the complete recirculating system is the cost to install a dedicated return line. It may be worth it in a very large house, but it would still be costly. Most of these installations are in new construction.

Read Also: How To Install A Hot Water Recirculating Pump?

What's The Bottom Line On Hot Water Recirculation Systems?

Hot water recirculating pumps are a great way to get almost instant hot water to your showers and sinks. In essence, it is a simple water pump that turns on when the water drops to a particular temperature --- usually around 85 degrees. Then, the water pump shuts off when the water hits around 95 degrees.

Hot water recirculation pumps use a special valve with a temperature sensor installed at the farthest sink called a sensor valve. The valve opens and closes which turns the pump on. You can have additional control over the recirculating pump by using a timer or a switch so it will only turn on when you are home.

In addition to hot water recirculators, there are also on demand (single fixture) heaters that you can install for a single shower. Read my guide on all of the ways to get an instant hot water shower.

I hope you enjoyed my article. If you have any questions or comments, please leave it at the bottom of this article in the comment box.

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Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping people understand how homes work. He is a licensed home inspector in two U.S states and owns a residential and commercial inspection company (read his full bio on the About page). To ask Arie a question, please use the comment box at the bottom of the relevant article.

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Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.