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I have a newer home about year old, ever since we moved in we have had this problem. its a two bath home single level with the master at the far end of the garage were the hot water tank is in the garage. We have a Grundfos UP10-16A pump just for the master bathroom. The plumber installed the pump at the hot water tank return the water into the bottom of the tank were the drain was I looked at the installation direction on line at Grundfos and this one way the show it to be done. The problem is the two lav's sinks in the master work great instant hot and stay hot but the shower which has a Moen shower mixer valve, comes on gets hot fairly quickly but then gets cold and then gets hot again so your adjusting temperature the whole time you are trying to shower. I have rebuilt the shower valve with new cartridge and limit set and handle stop and etc.... but that didn't help like I said its only year old. The pump has three settings Auto, Temperature, or 100% I have tried all of them makes no difference. Any Suggestions

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  • Sounds like your pump's control loop can't keep up with the shower's flow rate. CHeapest experiment: install a flow restrictor in the shower head and see what happens.
    – Carl Witthoft
    Jan 31, 2017 at 18:54
  • This a new house shower head has a built in flow restrictor as all of the newer fixture do.
    – RGK
    Jan 31, 2017 at 19:38
  • Are you sure someone didn't remove the flow restrictor? Lots of people do this. One builder I talked to said he won't do it himself, but he will "tell" people how to do it . . . hummm. . . One plumber told me that without instructions to the contrary his SOP is to remove the flow restrictors so he won't get call-backs. Can't you set the flow rate on the shower? Set it at a low level and see if the temp stays more constant.
    – Jim Stewart
    Jan 31, 2017 at 20:52

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I have heard of issues like this with Moen "pressure balance" valves, in circumstances where there is a pressure difference between the hot and cold supply (call Moen, they will tell you about it too). You said the valve was fairly new so it is likely a pressure-balancing unit, and this sound like a pressure-balance feature malfunction. Turn the pump OFF and test the system to see if the problem goes away.

If the test proves the pump is the cause then it may be oversized for the small single circuit on which it has been installed, maintaining an increased pressure on the hot-water side even with the water running, whereas optimal would be for the pressure to drop to normal system pressure when the valve is opened. As a matter of fact, removing any restriction to flow, as suggested already by others, might help (though, counterintuitively, for a totally different reason).

Grundfos re-circulators are powerful pumps, putting one on a small loop is not commonly done. More often a thermosiphon circulating loop is established, without a pump, and they work quite well. You could convert yours just by yanking out the pump and connecting the pipes up (make sure there is a check valve on the return line near the boiler, as mentioned by @d.george.

enter image description here

Diagram shamelessly stolen from the internet

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Did the plumber install check valves at the point of return, (where the return pipe starts to go back towards the pump and tank)? If not the water could flow from the bottom of the tank, back through the pump and affect the water temperature at the shower. Is your house piped with 1/2" or 3/4" water line? I would run the pump for a few minutes, then turn off the pump , and shut off a valve to isolate the pump line and go and take a shower. If the temperature in the shower remains constant as it should then the problem is the pump line and check valves will have to be installed. ( one note: the pump should have been installed with valves to isolate it for when you need to repair the pump.)

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  • I have that same set-up as illustrated in the picture from jimmy fix-it. I installed mine 18 years ago when my house was built. I have a B&G pump part # 103257LF which has a maximum head of 8 feet and a maximum flow rate of 7 GPM. The Grundfos pump used has a head of 5 feet and a flow rate of 3.5 GPM, which is much smaller than mine. I also orficed my pumps discharge to 1/4 inch and have 3 return pipes manifolded together. I do not have the back flow problem. To get the thermosiphon to work you may need to add a valve at the bottom of the return line to start the flow with hot water.
    – d.george
    Feb 1, 2017 at 10:41
  • One more thing : the thermosiphon probably will not work unless you have a long cooling leg in that line using finned tubing similar to hot water baseboard elements since the water must cool on it's way back so it becomes heavier as it approaches the tank.You can use the pump to start the flow, turn the pump off and the water will still flow thru the pump. I know all this because I had the same problem and corrected this by trial and error.
    – d.george
    Feb 1, 2017 at 10:52

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