My 5-yr-old house has a hot water circulation loop with a circulating pump. Until six months ago the pump functioned silently. I have a tankless water heater that has the ability to turn the external circulating pump on every 15 minutes to bring the loop up to temp.

Six months ago the pump started making noise, whirring like a jet engine or a weed-eater that has trouble getting started. I called the plumber and had the pump replaced. A few hours after he left the noise started again. He has now re-replaced the pump with a different, more powerful, model and manufacturer. The noise still starts every time the pump cycles on.

The pump has an internal check valve and there is a separate check valve on the cold water supply line to the water heater.

I have run 1000 experiments to try to figure out the problem. If I turn off the pump and water heater, turn on a hot tap until cold comes out, and turn the pump and heater back on, the pump will run silently, with the loop now back to full temp, for up to 12 hours and then the noise comes back.

I have a switch that can remove power from the pump. If I turn it off over night and then resume normal operation, everything will be quiet for a variable number of hours after it is turned back on in the morning. It seems that the more hot water we use, and so the longer the heater operates, the shorter the quiet time.

I rewired the pump so that it operates continuously, not cycling on and off. Made no difference. After the noise starts, turning the water heater on or off makes no difference. Once the noise starts it continues non-stop.

If, while the pump is loud, I switch it off, count to ten, and turn it back on again, not having done anything else like running water, sometimes the noise goes away, to return maybe a half hour, maybe three hours later. Sometimes shutting it off and counting to ten doesn't work, but then repeating that once or twice usually works, for a temporary fix.

I wondered, could there be a pressure imbalance and/or is the hot water line getting pressurized when the heater runs? Bleeding water from the line while the pump is running loud changes the sound but doesn't stop the noise.

Still I wondered about pressure. I noticed that hot water was bleeding into the cold water side of a Delta @#$%& single-handle faucet. I put a check valve into the faucet's cold water supply line and solved that problem but it had no effect on the pump noise. Could there be an issue with mixing valves in our 5-yr-old Kohler showers? I don't know. Replacing all the guts of 3 showers would run about $450 so I hesitate to experiment like that.

My last thought is that the heat exchanger in the tankless heater may have some internal corrosion, causing tiny spots of overheating, causing tiny bubbles in the hot water, causing air in the loop, which may be collecting inside the pump, causing the noise, as the air accumulates. (Presumably this air is magically dissipated when the pump is off over night or the line is flushed with cold water.) (And somehow turning the pump off and quickly back on cures the problem?) The heat exchanger would be super expensive to replace and the manufacturer would not replace it under warranty on the off-chance that my crackpot theory is right, especially since we have plenty of hot water and no problem when the pump is off.

I have had two different plumbing companies and the water heater distributor at the house and they are out of ideas. Can anyone out there offer any suggestions?

It COULD be cavitation, and if so a "more powerful pump" would just make the problem worse. You may need a "weaker" pump, or a valve between the pump and the heater that you can partially close to slow the water flow down. IF the heater is getting a build up in the coil, that could restrict the water getting to the pump causing it to cavitate due to insufficient water flow through the pump.

"Bleeding water from the line while the pump is running loud changes the sound but doesn't stop the noise".


What type of check valves were used? Spring or swing?

If the noise is pump cavitation from air in the hot water line you should be able to hear the air escaping when you turn on a hot water faucet somewhere, and see the water spit/spray as the air comes out.

Does the installation instructions for the tankless water heater give any information about adding the recirculation system? I'm not sure about the need for any check valves in the recirc loop with a tankless heater. If the recirc pump only comes on when the tankless heater is operating then the recirc pump shouldn't be able to pump water back thru the tankless heater into the cold water line since water is flowing out of the tankless. And the rest of the recirc loop doesn't need a check valve, the water just flows out to each hot water fixture and back to the pump. Am I missing something in my thinking - that's certainly possible.



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You do not need air to cavitate. The lower pressure in the suction side can create steam bubbles in the water which can be destructive to the pump.

Depending on how your home water system is plumbed and if you have an attic to install your system in you may not need a recirculating pump. There is a way of plumbing water systems so that the fixture that is most distant from the water heater will automatically return water through the return line providing the elevation of the return line is at its highest point at that far fixture. The return line going to that furthest fixture must be rising in elevation evenly from the Water Heater to that far fixture. Because hot water rises this can work. If you have far fixtures on opposite side of the home, you will need to tweak the movement of water with ball valves so you can get even flow back to the Water Heater. Someone with experience may add to or modify what I am posting but that is the gist of it. The upside is no recirculating pump to maintain or hear. Another way is suggested here:

Piping without a return line.

A domestic hot water system without a dedicated return line can utilize a "point of use" product that is typically
installed at the fixture furthest from the hot water heater. The cold water supply pipe is used as the return line
to the water heater. These are available at the hardware store or Big Box.



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i have installed many 'gravity fed' recirculating systems and they all work fine.
BUT they rely on a tank of hot water which is naturally hotter at the top that at the bottom .
a tankless heater just sits there full of cold water waiting for a demand to call it on..
adding a small electric tank would accomplish the hot rises and cold sinks connundrum but defeat trhe purpose of instant hot water..