Boiler pump chatter

Help....This is a strange one that has baffled myself and a few HVAC guys.
I have an old American Standard hot water boiler in my house that is used only for feeding baseboard radiators to heat the house. I run the boiler at 150 degrees at the high limit and pressure is set at 12 psi. My problem comes when the water temp gets in the 125-135 degree and above range. At that time my pump starts making a horrible high pitch chatter/squeal that radiates through the pipes in the house....my family has been woken up many times over the years because of it. This does not happen every single heating cycle but seems to happen more often the further we get into the heating season. It also can vary in length....sometimes it chirps quick every 2 seconds, or it can squeal for an entire cycle, until the heat drops below the 130 degree point. I have done the following with no long term results:

Replaced the pump/motor unit with a brand new unit. I replaced a Bell/Gosset with an Armstrong. Still had the issue after a short repreave.

Added isolation valves in all zones with drains so I could flush seperately. This seems to stop the noise for a few days and then it comes back even meaner and louder! Believe me, I have flushed the system more than enough. I was up to a few times a week last year toward the end of the season. I have also opend up the plugs on the boiler a few times and flushed it out also.

The expansion tank is perfect.....I have bled the baseboards of air many times and have never noticed any huge amounts of air in them...just a spit here and there.

The weird thing about this mess is that when you are standing next to the unit you can hear the pump squealing a bit and does not seem that loud. Walk up some stairs to the living area, and it sounds horrible.

Not one person who has looked at this can understand it. I hope that someone out there can!!!!!

Thanks for reading,
Bander1

Where is the expansion tank relative to the pump? To ensure the pump isn't cavitating, first it has to be sized properly and second, the expansion tank should be on the low-pressure side of the pump so that it attempts to maintain pressure and volume so the pump never runs dry. A pump that is attempting to move more volume than the pipe system would allow could have it cavitating (creating low pressure bubbles in the fluid stream).

Note: The hotter the temperature, the easier it is to start cavitation.

I've also had air bubbles get into the pump, so the "air traps" are important since even a tiny 1/4 inch bubble can cause the pump problems.

Remember these pumps use liquid bearings. I.E. The water itself is fed threw holes to float the shaft so that it does not contact the bushing most of the time.

The Squeal is an indication that the bearing is not getting the water it needs due to either low pressure from the pump being too high in the system or A small bubble got into it.

Both responses ahead of this suggest cavitation at the pump impeller and I agree that is the most likely cause of the noise. You hear it more in the house because its causing the pipes to vibrate within the walls or at some metal to metal support points.

Make sure that you don't have restrictions upstream of the pump. Typically that piping is large with respect to the downstream end in order to keep the pressure high at the critical suction end of the pump. If you have a valve in that section that is partially closed it could easily be creating the problem.

If you have a valve downstream of the pump you can partially close it to reduce the flow volume and increase the head pressure on the pump. That may eliminate cavitation right there. The reduced flow volume is unlikely to be a problem with getting heat to the house.

First off, thank you Jadnashua, Billarden, and Souternman for your responses. With all three of you basically agreeing that it is a cavitation issue, I am going to go with that. Your reasoning makes perfect sense to me! I do believe that I have a strange circumstance though....the development that I live in has about a 10 to 1 ratio boiler to forced air heat system and the boiler systems are put together all the same way. No one that I know of has this problem currently. Figures that it is me!
On the output side of my boiler there is a airtrol output fitting that splits out to the expansion tank and the other goes to the 3 zones in the house. Is it possible that this Airtrol has issues? The expansion tank also has an airtrol air fitting on the input. A HVAC guy checked the expansion tank and said it is ok even though it is 47 years old.
This whole thing just leaves me perplexed.
I like the idea of putting a flow control valve on the downstream of the pump to cut down on suction on the input. I just have to figure how to get it in there! Everything is pretty tight and compact before it splits out. Would I be doing the same thing by restricting the flows in all three zones by turning the thumbscrew flow controls that are about 2 feet upstream of the pump? I know there must be a point in a closed loop where downstream and upstream are defined.
Oh, and yes the tank is downstream of the pump.

The tank needs to be on the suction side of the pump...

In my limited knowledge of my own system. How is your make up water getting in? IF you have air in the system then to my way of thinking, your air bleeder isn't working and if you have low water that would produce the same result wouldn't it? Ask this question at Homeheating.com additionally. Nothing against the guys here !! Those guys are all pros at this.

Good Luck

ed....
Is the airtrol fitting my "air bleeder". If not, I do not have one. Thanks for your comments!

jadnashua,

My tank is definitley on the output side. I am going to research your point, thanks! None of the local HVAC guys pointed that out but maybe they were not looking or they did not understand the dynamics of a boiler to begin with.

bander1

The pressure tank should be on the pump inlet side.

If the tank was on the inlet side, then you would be correct in thinking that turning down all zones would be the same as adding a valve after the pump.

The system could function with the tank on the outlet side provided that the system pressure is higher than ~10PSI and the zone valves are open, but why complicate things when the tank should be on the inlet side.

Did the pump direction get reversed at some point?

With these systems, I have found that if the pump is placed on the supply side coming out of the boiler, it will eventually start to squeel and chatter. . I find the pumps to be temperature sensitive after prolonged use. The return side of the boiler would be my option to install the pump. The pump tends to stay cooler longer when its drawing water from the rads back to the boiler.

On the output side of my boiler there is a airtrol output fitting that splits out to the expansion tank and the other goes to the 3 zones in the house. Is it possible that this Airtrol has issues? The expansion tank also has an airtrol air fitting on the input.

Click to expand...

Could you please post some pictures? Airtrol is a product of Bell & Gossett and in my opinion a total piece of junk. I cannot understand why an airtrol would be installed on the boiler outlet, are you sure you are not referring to an air eliminator fitting?

To post pictures you need to first upload the pictures to a photo hosting site such as photobucket.com or villagephotos.com. and then post the public URLs for the pictures (or album) here. More pictures are always better than fewer. Please have CLEAR pictures and have both close up pictures and ones from a far enough distance that we can see how the various parts are interconnected.

I do believe that we are getting somewhere now and I am estatic that there are so many good people willing to help! Only in America! The valve on the output sides nameplate says Airtrol Boiler Fitting. Concerning the pump direction, it is set up as pump output directly into boiler inlet, boiler output to the airtrol boiler fitting, the fitting has 2 outputs...one going to the expansion tank (which has a airtrol fitting also) and the other to the three zones, the returns from all have flow zone controls and the outputs from those tie together into the pump outlet. I will get some pics tomorrow. Does this sound correct? I am positive the water flow is in the same orientation.


Again, thanks to all.

bander1