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
Water – hot water circulating pump
water
Related Solutions
I suspect you have a set-up like this?
You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?
The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)
In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.
This is just a guess, but you're probably only seeing existing hot water in your house's pipes flow back to the water heater, since there's none of the resistance that the full tank of water can provide.
Related Topic
- Plumbing – Hot water issues in the apartment and plumbers are stumped
- Water – Hard to control water temperature in shower
- Water – which or what type water pump for car washing in remote location
- Water – No Hot water to Shower and Taps furthest away from hot water tank
- Water – Is there such thing as automatic hot cold water mixing valve
Best Answer
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.
Diagram shamelessly stolen from the internet