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Question DetailsAsked on 8/11/2017

Brand new gas furnace emits barely audible hum when unit is not running - Is this normal?

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1 Answer


Watching out where you touch - some transformers are exposed like this -

Transformers (to convert household 110/120 volt power to the 12/24/36 volts most HVAC systems use for control circuitry) commonly hum - not a problem. (May be mounted on frame of furnace, or rarely on the main control board itself.)

Also - if furnace has been operating recently, some have two-stage blower fan which steps down to low speed (so a slight hum) during the cool-down phase to extract the last heat out of the heat exchanger - if that is the case you would have a slight airflow out of the registers.

Ditto, if you have a high efficiency unit (direct vents outside through plastic pipe through the wall rather than through metal duct to the roof) it has an eductor fan in the exhaust flue which extracts the exhaust gas - commonly runs till the heat exchanger cools down, so maybe 5 minutes or so after the unit shuts down. Mounted in the exhaust flue piping commonly a few feet from the unit.

If you meant a boiler for a steam or hydronic (hot water) heating system, they have a pump in the front of the unit which looks about like this:

Sometimes these turn on and off with demand, in other setups they run continuously and hum a bit - commonly audible not just at the boiler but also in the piping and walls, especially if you put your ear to the wall.

Also - if it has a standing pilot flame, that will cause a slight sound, though more commonly would be described as a hiss or slight whoosh than a hum. Units with a thermopile rather than a thermocouple for igniting the pilot flame sometimes also hum a bit.

If these did not put your mind at ease, call the installing company - hopefully with the knowledge in hand of exactly where on the unit the nosie is coming from in the unit.

Answered 3 years ago by LCD

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