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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8679
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Lennox furnace and AC in Phoenix. When AC is on there is a

Customer Question

Lennox furnace and AC in Phoenix. When AC is on there is a sucking sound at the air return filters. This is a 2012 model 140ACX and furnace is ML180. This just started this summer.
What could cause this and how do you fix. The noise generated is not acceptable.
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.
Hello,If the installer used plastic flexible duct in the return air line, the plastic liner inside can get sucked flat and block the air flow, causing that sound, and leading to very poor performance. If you access the duct, by removing the return air grill and then reach inside with a screw driver about 2 feet and probe for hard metal... if all you can find is soft plastic air duct, that is the problem. This can also happen when fiber glass duct line, comes loose. To look for that open the furnace blower door, and and use a flash light to look inside the return air plenum for such a situation. Look that over, let me know what you can find, we can go from there............ We are on the honor system here. Please remember to rate my service before you leave today. It is how I earn a living, and does not increase your charges. If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost.
Customer: replied 2 years ago.
It seems like fan is really trying to get more air into system. Is that caused by the collapsing duct?
Customer: replied 2 years ago.
Sorry. Number is9529341435
Expert:  Phil replied 2 years ago.
Hello again, Yes, a collapsed duct will restrict the air flow, resulting in much higher air velocity past the restriction... that is almost certainly what you are hearing............We are on the honor system here. Please remember to rate my service before you leave today. It is how I earn a living, and does not increase your charges. If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost.
Customer: replied 2 years ago.
I will have to get serviceman. Tks