There are many different recirculation pumps on the market and it can be quite confusing to know which one to choose for your recirculation system or even if the pump that you currently have installed is adequate, so we’re here to help with our hot water recirculation pump recommendations.

Because our Smart controller is designed to run the pump only when you demand hot water, having a high-flow pump is important to provide adequate responsiveness and comfort. It is for this reason that we recommend a pump that will flow 15 – 20 GPM. But pump speed isn’t the only thing to consider.

Why Circulation Pump “Head” Matters

Pumps also have a rating called “head.” Head is a measure of pressure that the pump can generate and is defined as how high the pump can push a column of water against gravity. It can be translated to PSI via the formula 2.31 ft. of head = 1PSI. This means that a 1” square column of water that is 2.31 ft. tall will weigh 1 lb.

But, you may ask, “because a recirculation loop starts and ends at the same place there is no head, so what does head matter?” Head is what allows the pump to overcome frictional losses in the piping. Frictional losses increase as the water flow speed increases.

Below is a chart that shows frictional losses for a couple of different-sized pipes generally used in recirculation loops. These charts were generated using the Online Hazens-Williams Calculator at the Engineering Toolbox.com with a roughness coefficient of 144.

As you can see ½” copper piping has pretty high frictional losses as the speed of the fluid increases. This is why in older homes there can be issues with the shower getting hot or cold when someone else in the home turns on a hot water faucet or flushes a toilet. Modern plumbing typically uses ¾” piping so as to significantly reduce the pressure drop created by the additional flow of a 1 GPM faucet.

Additionally, if you have a tankless water heater, the water heater itself can present a large pressure drop to the recirculation pump. The tankless water heater manufacturers can provide a pressure drop graph for their models that show the pressure drop generated by varying flow values and can be pretty significant for the high recovery style of condensing water heaters. Here is a chart for a Navien model NPE-240A2.

As you can see the chart is supplied in pressure drop in PSI per water flow in GPM. To convert PSI to Ft of Head you multiply by 2.31 ft/psi. So at 4 gpm there is roughly 11 PSI of pressure drop and when multiplied by 2.31 ft/psi results in 25 ft of head. This has to be added to the pressure drop generate by flow through the piping itself which is what this article was written to address. The article titled “Ruminations on Tankless water heaters” discusses this issue as well as other issues related to tankless water heaters.

Hot Water Recirculation Pump Selection

Getting back to the issue at hand, pump selection. The recirculation loop is generally ¾” copper piping with ½” lines T’ing off for the plumbing fixtures. So if you have a 100 ft. ¾” copper pipe loop and you want to recirculate the water in 30 seconds you need to know how much water is in the piping.

This depends somewhat on the schedule of copper piping used as you can see in the charts. Schedule M has thinner walls and is less expensive and it therefore used if Schedule L is not specified. We will perform the calculations for Schedule M ¾” which has an inner diameter of 0.811” and a 1 ft. length has a volume of 6.20 cubic inches which translates to 0.0268 gallons so 100 ft. contains 2.68 gallons. To move 2.68 gallons of water in 30 seconds is approximately 5 GPM and the pressure drop in 100 ft. of ¾” copper piping is 5.8 ft. So a pump that has this much head at a flow rate of 5 GPM will satisfy this requirement.

To know this we need to look at the pump curve. This curve tells us how much head the pump has based on the flow rate. Curve D in the chart below is for the AquaMotion AM5 pump.

From this chart we see the AquaMotion AM5-SF1L at 5 GPM is rated at 8.0 ft. of head and will exceed this requirement and would actually be able to provide more than 5 GPM. The chart shows that at 6 GPM the resistance is 8.11 ft. of head so the AM5 won’t quite be able to provide this level of flow but we will use it as it is a close approximation. The time required to move 2.68 gallons of water at 6 GPM is 27 seconds so depending on your location in the loop this pump will provide on average 13.5 second response time, less for closer fixtures and more for further fixtures.

This is the pump we typically recommend for recirculation loops up to about 150 ft. in length. The pressure drop in 150 ft. at 5 GPM would be 8.7 ft. so this pump wouldn’t quite flow at 5 GPM but would certainly flow at 4 GPM. There are 4 gallons of water in 150 ft of 3/4″ Type M copper pipe so it would take 1 minute to recirculate the entire loop. So depending on your location in the loop it would take on average roughly 30 seconds to get hot water to your location.

For Better On-Demand Performance

For even better on-demand performance we offer the  AquaMotion AMR-S3F1L who’s curve is shown below.

Looking up the flow rate for the AMR on its high setting we see that at 5.8 ft. the flow rate is roughly 12 GPM so if this pump were installed in a 100 ft. loop it would recirculate the loop in 13 seconds so you would get on average 6.5 second response time.

For a 150 ft. loop, it would flow about 6.5 GPM and recirculate the loop in about 37 seconds providing 18.5 second response time on average. For a 200 ft. loop, it would flow about 5.75 GPM so it would recirculate the entire loop in 56 seconds providing 28 second response time on average. And for a 300 ft. loop, it would flow about 4.25 GPM so it would recirculate the entire loop in 1 minute and 54 seconds providing 57 second response time on average.

For loops longer than this it is recommended to increase the pipe diameter to 1” to reduce the resistance and provide better performance or to create multiple loops and use multiple pumps to serve different sections of the home. 1” Schedule M pipe has an inner diameter of 1.055” and a 1 ft. length has a volume of 10.49 cubic inches which translates to 0.0454 gallons so 100 ft. contains 4.54 gallons so 400 ft. contains 18.16 gallons.

At 7.25 GPM there is 12.8 ft. of head drop in 400 ft. of 1″ piping and the AMR has about 13 ft of head at that speed. To recirculate 18.16 gallons at 7 GPM would take roughly 2 minutes and 36 seconds providing 1 min and 18 second response time on average. As I mention, it may be better to break this long of a loop up into multiple loops with multiple pumps and controllers in order to reduce wait times and provide more comfort for the home owner.

Stepping up a size in pipe diameter makes a big difference in flow capacity.

How Fast Will You Get Hot Water?

Now, these are ideal calculations and there are many factors that impact the performance of the pump and the time to recirculate hot water. Some of the factors include:

  • Pump start-up time. Water has momentum and doesn’t start moving immediately so this will delay the flow somewhat.
  • The number of elbows in the recirculation loop. Elbows provide additional resistance to flow than what is in a straight pipe.
  • Type of pipe. PEX typically will provide faster flow as it is slipperier than copper pipe and typically has larger radius curves.
  • Heat absorption by the piping. The initial hot water will be cooled off as it heats up the piping as it recirculates; there is still the short section of ½” piping from the recirculation loop to the fixture itself that needs to heat up, this is typically less than 6 ft, but at 6 ft. it will add a second or two.

So if you are concerned about the speed at which the water gets hot I would error on the side of the faster pump with more head. And because the pump runs on demand and only until the loop is hot you don’t have the issue of pipe erosion that plagued early recirculation loops that ran 24/7. Typically with our control, the pump will be running 2 – 3 hours a day.

Will Your Current Pump Work with an On-Demand Controller?

If you already have a pump installed and are wondering if it is adequate to use on-demand, look on the pump for the rated wattage. If it is less than 50 watts, it will probably not provide fast on-demand performance. Specifically, we have come across some very small 1/40 HP (20-watt) pumps that only provide 3 – 5 GPM flow and they don’t provide fast delivery in an on-demand environment as they can take up to 5 minutes to heat up the loop. If activating the pump and waiting a few minutes is an acceptable trade-off for conserving water and energy without running your pump 24/7 , the smaller pump will be adequate for that purpose.

A simple empirical test you can perform to determine the performance of your pump would be to unplug the pump  at night before you go to bed. Then, first thing in the morning before anyone uses hot water, get a timer with a second hand and plug the pump in. Hold the return line with your hand and time how long it takes for the pipe to start getting warm. Anything longer than 60 – 90 seconds will probably not provide adequate performance if you anticipate seemingly “instant hot” on-demand hot water. 

So as you can see, pump selection is as much an art as a science. If you have any questions or want to discuss your installation, please contact our customer support team, we  are here to help.