harmonics in hydro-air system

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Posted by Dave on March 08, 2001 at 15:14:33:

We have moved into a new house with a gas-fired 300K BTU boiler that powers both a hydro-air (air over hot water coils) and an indirect hot water heater. There are 2 air handlers-one in the basement and the other in an unheated attic). The system used to make a terrible, very loud howling sound just before the boiler shut off, and I was told it was due to a low flow state and that the water was "flashing". The plumber came out today and did some repiping as well as replacing the main circulating pump with one that generates much more flow (30 gal/min). He set the operating temp of the boiler at 180deg and the "high Fire" temp at 165. While the sound is MUCH improved, there still is a slight whine that can be heard through the house. I looked at the temp gauge and it rises steadily without any whine. Then, when it reaches about 190 deg, the whine starts and continues until there is a "click" (maybe the flame turning off?) then the temp begins to decrease and the boiler shuts off after about 20 seconds. I turned down the operating temp to 170 and the high fire to 155 and it does not whine with these temps. My questions are: does this level of noise seem ok as far as the operation of the boiler, and will leaving the operating temp at 170 allow sufficient heating? (My second air handler is in an unheated attic on the third floor, so the hot water must make it to there before there is any heat on the second floor). The fan on the air handler up tere shuts off when the water temp drops below 155, but it is adjustable. Should I decrease that some also? Thanks for any input you may be able to give me.

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