Cavitating boiler circulator - seeking advice

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Shleprock

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I have an 1860's stone home with a gas fired boiler force feeding cast iron radiators on three floors. There are two zones controlling cast iron radiators and one zone controlling a PEX loop installed in joist bays under my kitchen floor. I am trying to solve cavitation on the main boiler circulator whenever heat is called for only for the PEX loop. This is a common condition as the kitchen has no cellar underneath and the area is less insulated..so the one zone that is almost always on is this floor loop. The hydronic PEX loop has its own circulator (Grunfos 1/25 hp which is how it is controlled via thermostat), but the main circulator also runs when this loop is on only...and cavitates...and there is no zone valve on this loop. Is the restricted flow (the other zones being closed) and the taco 1/25 hp forcing 180F through 1/2" PEX causing the cavitation? My concern with lowering the cut on temp of the PEX loop is possible freezing in the line...Just looking for some suggestions..thanks!
 

Dana

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Some actual model numbers of things like pumps & boilers and pictures of the near boiler/near-pump plumbing might provide more insight.

The description is a bit confusing. Is this plumbed primary/secondary, with the one zone pumped direct by the primary pump or ...???

And the "...PEX loop..." is a radiant floor or something? (If yes, how many feet of half-inch PEX?)

Are you sure it's cavitation you're hearing, not micro-boil "kettling" on the boiler due to low flow?

Have you calculated the pumping head and likely flow rate? A 1/25 HP pump can usually handle quite a bit of head without cavitation. But if it's pumping against a manifold of zone valves that are all closed the pumping head is infinite- the control scheme needs to be modified to prevent that from happening. Putting the radiant floor pump on a separate single-zone relay that operates independently of the pump driving the zone-valved zones should work, if that's what's going on.

A couple of things can mitigate cavitation on the pump. The easiest to try is to raise the system pressure, as long as it's kept well under the pressure relief valve on the boiler. (Be sure to adjust the pre-charge on the expansion tank if/when you do this.)

If plumbing on the input side of the cavitating pump isn't dead-straight the turbulence caused by the ell or tee can aggravate the condition. Placing the expansion tank a foot or so of straight pipe prior to the pump can also reduce the condition.
 

Shleprock

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Some actual model numbers of things like pumps & boilers and pictures of the near boiler/near-pump plumbing might provide more insight.

The description is a bit confusing. Is this plumbed primary/secondary, with the one zone pumped direct by the primary pump or ...???

And the "...PEX loop..." is a radiant floor or something? (If yes, how many feet of half-inch PEX?)

Are you sure it's cavitation you're hearing, not micro-boil "kettling" on the boiler due to low flow?

Have you calculated the pumping head and likely flow rate? A 1/25 HP pump can usually handle quite a bit of head without cavitation. But if it's pumping against a manifold of zone valves that are all closed the pumping head is infinite- the control scheme needs to be modified to prevent that from happening. Putting the radiant floor pump on a separate single-zone relay that operates independently of the pump driving the zone-valved zones should work, if that's what's going on.

A couple of things can mitigate cavitation on the pump. The easiest to try is to raise the system pressure, as long as it's kept well under the pressure relief valve on the boiler. (Be sure to adjust the pre-charge on the expansion tank if/when you do this.)

If plumbing on the input side of the cavitating pump isn't dead-straight the turbulence caused by the ell or tee can aggravate the condition. Placing the expansion tank a foot or so of straight pipe prior to the pump can also reduce the condition.


Thanks Dana - I have some answers to your questions but not all The boiler is actually kettling recently, but the circulator has always cavitated as described since I bought the place...I will take some pics and fill in the details..appreciate your help!
Tony
 
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