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. The below chart is for the Leridian Dynamics 2006SN10 and we will be looking at the High speed line since we are running the pump on demand and therefore want the highest performance from the pump. As the pump will spend most of it’s time off, only running a couple to a few hours a day, this is not a problem for the plumbing.

2006SN10 Flow Curve

From this chart we see the 2006SN10 at 5 GPM is rated at 15.0 ft. of head and will exceed this requirement and would actually be able to provide more than 5 GPM. The chart shows that at 7 GPM the resistance is 10.8 ft. (or 12.6 in type L copper pipe) of head so the 2006SN10 will easily pump water at 7 GPM. The time required to move 2.68 gallons of water at 7 GPM is 23 seconds so depending on your location in the loop this pump will provide on average 11.5 second response time, less for closer fixtures and more for further fixtures.

This is the pump we recommend for recirculation loops up to about 250 ft. in length. The pressure drop in 250 ft. at 5 GPM would be 14.5 ft. so this pump would still flow 5 GPM. There are 6.7 gallons of water in 250 ft of 3/4″ Type M copper pipe so it would take 1 minute and 20 seconds to recirculate the entire loop. So depending on your location in the loop it would take on average roughly 40 seconds to get hot water to your location.

If you have longer plumbing or if you want better performance you can install two 2006SN10 pumps which doubles the power. So at 7 GPM there would be 27 ft of head resistance. At 7 GPM the 2006SN10 has 12.5 ft of head so two of them would provide 25 ft of head so it wouldn’t quite get it to 7 GPM and since there are 6.7 gallons of water in the line we can say it would move it in 1 minute providing on average 30 second wait time in the home. As you can see, because resistance is not linear with flow speed it can take quite a bit of power to get that volume of water moving quickly. Fortunately the norm for recirculation loops tends to be in the 100 ft to 200 ft range.

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.

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 2 – 3 GPM flow in a 100 ft loop 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.