ringing noise from recirc pump after water shut off for a while

[Niccolo]

I'm curious to try to understand something I've observed a few times. I have a tank water heater with a dedicated hot water recirculation loop with a pump installed next to the water heater that pulls water from the loop and back into the bottom of the heater. Twice recently, I've had a plumber at the house working on other issues, and he's turned off the main house water shut off valve (but left on the hot water recirculation pump).

Later, once everything was fully operational again (including various faucets run for a while to purge air that entered the system), I've noticed the recirculation pump making a significant ringing noise. So I shut it off, and by the morning everything was back to normal.

With a full water heater tank and the recirculation pump still running, how was the plumber able to work on hot water faucets--which he did--without hot water spewing out, despite the main house water valve being shut off?

I guess the ringing sound was the recirculation pump running dry or semi-dry. I suppose even once everything was fully operational again, some air remained in the hot water recirculation loop and/or the water heater refilling had implications for the pump's operation. Obviously, in future it makes sense to shut off the recirculation pump when the main house water is shut off (if the plumber doesn't think to do that himself).

 

[WorthFlorida]

Could be the pump needs pressure behind the water for water flow. They are fairly small motors and if you have a second story it may not have enough power for the 8' head.

 

[Sponsor]

 

[Niccolo]

Could be the pump needs pressure behind the water for water flow. They are fairly small motors and if you have a second story it may not have enough power for the 8' head.

The plumber worked on a ground floor faucet; the water heater is also at ground level. I'm realizing he must have first drained enough hot water out of the tank, via the faucet he worked on and nearby ones, that the system was essentially dry.

 

[Fitter30]

Hot water recirculation uses a circulator and not a pump. Here's a article explains the difference. Your circulator just ran and might have over heated and went off on thermal overload.

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