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LAARS EBP-175 noise. Please help.
I have LAARS EBP-175 heating/hotwater unit with 3 heating zones. There is a boiling-bubbling noise coming out of the boiler chamber when the heat is on for any of the 3 zones. As the result all my radiators (not cast iron, but those slim aluminum radiators) are very loud with water constantly pulsating back and forth very loudly. I had the plumbing company bleed all the zones, but it is still the same. They are out of ideas right now. It does seem to me that the boiler chamber is not making this noise when there is a request for hot water and it is working on hot water request. It is appears to be happening when heat is on only. Cannot sleep normally at night because of this noise.
Any thoughts and suggestions are greatly appreciated.