Whistle while we work

[Mushroom Man]

I have a whistle coming from my boiler return pump. Sometimes it sounds like crickets. Sometimes speeding-up the pump makes the whistling stop. Sometimes it doesn't. Sometimes playing with the isolator valves makes the sound change but it seldom disappears. I think it is air but there is no air coming out any of the bleeder valves. If I nearly close the isolator valve, the back pressure seems to relieve the whistling but I cannot operate with that valve nearly closed for any more than a few seconds. Any ideas what the problem is? BTW: the pump is a Grundfos 15-58

 

[Hydronics]

It is likely cavitation (tiny air bubbles imploding at the pumps impeller due to inadequate pressure on the suction side). Long term, this will damage the impeller and/or volute of the pump. If you have air in system you're more likely to get it. What's your system pressure? -low pressure will also worsen condition. Do you have a long loop through small tube? Try throttling down the valve on the pump discharge.

 

[Chris Hoskin]

I agree, check system pressure - should be 12-15 pounds, but up to 20 is OK too.

 

[Mushroom Man]

I agree that air (cavitation) is the most likely cause and could easily wreck the pump. Fortunately, almost as soon as I wrote the posting, the sound stopped. I really did not do anything. I couldn't fogure out what to do. I was afraid to throttle down the isolator ball valves because I thought restricting flow through the boiler loop might cause it to overheat.

I meant to get a spare pump today but didn't get there on time. Tomorrow! Switching it out is easy in this case but I wonder if my system is plumbed in such a way as to encourage this cavitation. I hope not.

I saw an isolator valve with a drain in one of the postings last night. Now that would be handy.

Thanks for your ideas.

 

[Mushroom Man]

The whistle is back. Worse than ever. I have tried:
1. changing pump speeds
2. throttling the isolator valves
3. altering the Danfoss 4 way valve (boiler tempering)
4. adding water to the system
5. bleeding every conceivable loop

Would loosening the pump flanges (when the water cools off) and bleeding off air trapped there be the next logical step?
Any other ideas?

 

[in hot water]

could be something stuck in the vanes of the impeller. Solder balls, dirt, copper or metal shavings, even teflon tape. You may have to remove the pump from the volute, usually four allen headed bolts. Isolate the pump and relieve the pressure first.

hr

 

[Mushroom Man]

In hot water had a good idea. It wasn't gunk in the impeller though. It was a minor chip in the housing near the rubber gasket surface. It was likely sucking in air and causing the noise. It would have been easy to miss without glasses because the chip was very small. I swapped out the pump and the system is working good. What a relief.

It has been cold (-8 to -12C) and I was afraid of a pump failure with all that noise.

Thank you for the input.