right recirc pump

peds · April 11

Our house has a dedicated recirculation line on our hot water system. I recently replaced the hot water heater and decided to replace the recirc pump at the same time since it was almost 20 years old.

The old pump was a Grundfos UP 15-18 B5. When I bought the replacement pump, I bought what I thought was an equivalent model (Grundfos UP 15-10 SU7P) but came to find out that the new pump is not designed for a dedicated line system but is instead for a system with a bypass valve. I'm now getting a lot of lukewarm water out of my hot taps.

Is there a way to install an additional check valve, or some other method, that would allow me to continue to use this new pump or do I have to replace it with a different model? Would appreciate some wisdom!

Kaos · April 11

Since these are wear items, this is what I use. Easy to retrofit plus easy to swap out if it fails.

https://www.sharkbite.com/ca/en/push-to-connect/push-to-connect-valves/push-to-connect-control-valves/check-valve

hot_rod · April 12

that pump should work without a crossover valve. But it is a bit smaller size

Any idea how long the loop is?

The new ECM smart pumps like the Alpha, work great on recirc, temperature control, variable speed, less power consumption

peds · April 12

Thanks! I think it's about 60' from the farthest tap back to the tank.

The new smart pumps sound great but to keep from throwing away $$ I guess I would prefer to keep the one I just paid for and installed if I can make it work. But as it is now, it isn't functioning right.

Any ideas what I need to do to make this "comfort system" pump work with my dedicated return? Would installing a check valve between the pump and the tank solve the problem I'm having of lukewarm water and heavy electricity use?

Larry Weingarten · April 12

Hi, A few thoughts come to mind. You're using the new pump without the bypass, right? Just using the dedicated return? Is there a spring check valve in that return line? A test you can do is to close the water supply to your hot system and open a hot tap. Does water flow? If so, there's a crossover from cold to hot and this is part of the problem. Can you take a photo of how the pipes/pump are hooked up to your tank? Ultimately, it is more likely a plumbing issue than a pump issue. I think you're on the right track wanting a check valve to prevent water draws from pulling cold water from the bottom of the tank backwards through the recirc line.

Yours, Larry

peds · April 12

Thanks Larry. The crossover test was positive. With the hot water supply valve off, water does flow through a hot water tap. So I guess that's a part of my problem?

Here are pics of the whole tank and then close ups of the supply lines on

top and the dedicated return line with the recirc pump at the bottom.

Would a check valve between the pump and the tank do the trick or are there other concerns?

Appreciate the help!

PXL_20260412_193824270.jpg

PXL_20260412_193845857.jpg

PXL_20260402_154825055.jpg

Kaos · April 12

Dedicated recirc line needs a check valve in it otherwise you can have cold water from the bottom of the tank flow backwards through the recirc line to the hot water of the fixture. Since your pump doesn't have one, you need to install an in-line one like that Sharkbite I sent earlier.

One interesting tidbit, is if you have multiple recirc lines returning to a single pump, you need a check valve on each line.

Larry Weingarten · April 12

Hi, I'd also like to see a drain valve at the base of the tank. It WILL come in useful. 😉 The crossover and recirc check valve are separate problems to deal with. It could be the old pump masked the crossover problem better.

Yours, Larry

Alan (California Radiant) Forbes · April 13

Did you purge the return line of air? Add a hose bibb and ball valve.

Lyle {pheloa} Carter · April 13

Just a couple of ideas. From your first description, I thought the pump might have been installed backwards , but looking at it in the photos , I believe it is correct with the arrow towards the tank. What type of control system did you have on the old pump? Was it installed to run continuously, If It was that could be why it worked without the check valve.

Once you have the problem resolved, I think you won't need the pump running for six or seven hours. Try the same time period, but, ten minutes off five minutes on. Usually works just as well and saves time and energy.

peds · April 14

Ok, thanks all. Based on these recs, I am going to install a check valve between the pump and the tank (ordered the one Kaos suggested) and I will purge the return line of air when I do the check valve install. When I did the new water heater install, I purged all the house lines but did not do the return line specifically.

A couple of last questions:

It sounds like none of you are worried about the pump that I have even though it is technically spec'd as a pump for a bypass system, not a dedicated return system. I should be able to use it. Am i reading your comments correctly?
Alan, can you explain where I should add a hose bibb and why?
Larry, don't I already have a drain valve at the bottom of the tank? Something else I need to add?

I appreciate all the help!

hot_rod · April 14

The pump doesn't know or care that it has a bypass valve in the system or not. It just circulates the water and needs to be sized to the pressure drop and flow of the piping. That pump, based on your piping description should be fine. It just needs to flow a small amount to overcome the heat loss in the piping.

Insulate as much of the piping as possible as the water heater runs to keep the piping warm.

The pump can tie into the top cold, as it has a dip tube to the bottom, or at the drain port.

A valve helps purge air and flush the tank at the bottom, if you connect there.

Alan (California Radiant) Forbes · April 14

Maybe even add another shutoff valve before the pump in case the pump needs to be serviced.

hot_rod · April 14

here is another option for a check valve. This Caleffi is a serviceable check designed for small circs with a .35 psi “pop” pressure.

The SharkBite has a high 2 psi, 4.62’ head, pop, a bit high for that small circs

Larry Weingarten · 12:01AM

Hi, I'm not seeing a drain at the bottom of the tank, just a drop ear 90. 🤔 You really want a drain there. It would be even better to have a shut off at the tank first as this would allow you to flush air from the recirc line.

Yours, Larry

Kaos · 2:56AM

That looks like an awfully small space for a HPWH. Looks like there is a register on the air handler, so maybe that can provide enough heat. I would still watch that you are not overcooling the space during large draws and run into condensation issues on the air handler.

You have to be careful with recirc and HPWH. Most manufacturers don't allow continuous recirc as it will significantly increase compressor runtime. The best is demand type recirc controls, next best thing is timer with temperature cutoff. I use a cheap plug in programmable thermostat with remote sensor mounted on the return line set to 95F, this halved my standby losses over simple timer.

right recirc pump

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