I have a hot water recirc system as shown in the diagram. It is a “smart” system with a temperature sensor just downstream of the hot water outlet from the tank and a 2nd temperature sensor just downstream of the last faucet, on the return line. When a flow meter just downstream of the hot water outlet detects flow, the pump is activated, and an electronic controller shuts the pump off when the difference between the temperature readings from the two sensors is less than 5°. If the 5° differential is not achieved within 15 minutes, the controller shuts the pump off and has to be reset before it will activate the pump again.

I recently flushed my hot water tank for the first time since the recirc pump was installed.

After refilling the tank I went to each faucet in the house and bled the air.

I think there is still air in the return line. I think this because the on-demand recirc system is not operating correctly. The pump doesn't shut off; it just keeps running for 15 minutes and is then shut off by the controller. Also, I can feel that the copper portion of the return line on both sides of the pump doesn't get warm, and the pump flanges also remain cool. Finally, both when the pump starts up and stops, I can hear unusual noises in the pipe connected to the pump, which suggest to me there is air in there.

Also, if I let the pump run for a little while and then open a faucet, I get mild spitting at the faucet again. This suggests to me that there is air in the return line that is moving through the pump into the tank and back into the supply lines when the pump is running. But if I unplug the pump, there is no spitting (once I rebleed).

There doesn't seem to be a purge valve installed. So how do I bleed the return line of any air that might be there?

Thanks for any help someone can provide!

On the horizonal section of where your return line ties back into your cold feed to water heater install a ball valve. Remove elbow next to it and replace with a tee and hose bib. You can turn off ball valve and bleed thru hose bib.

Also by having the 2nd ball valve you can service the pump without having to shut down the whole system.

Thanks for the quick response. I wish I had had the foresight to ask the plumber who installed the pump system to install a valve that could be used for bleeding the air. Or maybe that something the plumber should've thought of himself, since he's the expert!

As far as installing a second isolation valve downstream of the pump for the purposes of pump servicing is concerned, doesn't the check valve make that unnecessary? If I close the ball valve on the return line below the only flow into the pump would have to come from above, but the check valve prevents that, right? (I may well be misunderstanding something here.)

In theory the check valve should hold, in reality they don't. I would never trust a check valve to hold so I can service something/pump.

Most check valves I see on recir systems are stuck open from the hot/hard water deposits.

You need the second ball so you can shut it off and then bleed the return line thru the new hose bib that is above the existing check valve.

Got it! Your answer about the possible unreliability of check valves shows the difference between somebody who only knows the theory (me) and somebody who has had experience (you), which they say is the best teacher.

Thanks for taking the time to reply to my question on Memorial Day.



Edited 1 times.

"flow meter just downstream of the hot water outlet detects flow, the pump is activated, "

that is the stupidest "smart" system i have ever heard..

why would the pump come on when you open a faucet?
the act of opening a faucet makes the water flow!
why pump water that is already moving?

as far as air locked pump...
loosen the flange on the top of the pump to create a little leak.
the leak should expel the air above and inside the pump.

just add a hose bib in between the pump discharge & check valve