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Question DetailsAsked on 1/19/2018

Furnace. Boom noise at start and finish

Furnace fires up a minute or two blower kicks on and it goes boom same thing before the blower shut off

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1 Answer


Almost always a piece of the ducting "tin canning" - being pulled in or pushed out (depending on whether incoming/return side or the blower or the outgoing or "supply" side), then relaxing back to normal position when the unit shuts off. Like the nosie a soda or beer can makes when you crush it - or maybe as a kid (or still) when you pick up a flat-sided metal can you press the side to make it go from slightly convex to slightly concave, making a metallic twang or clunk sound as it snaps into the new position.

If this is a new thing for your existing furnace, is commonly occurring on the return/incoming air side and indicative that the blower is pulling more of a vacuum than before because the air filter is getting clogged, but cann also pop up because house settlement or such has slightly changed the duct stress, causing a duct face to tin-can readily. (That is why better grade metal ducting has the slight x-shaped indentation on the faces, to make it protrude out slightly rather than be balanced between being an "innie" or an "outie".

Rarely cause is a flow dividing damper (flapper valve) in the ducting closing hard - but that usually only occurs at the startup OR at shutdown, or sometimes in between as thermostats fight for air, not both in that case.

Walk around the house while someone turns up the furnace (to make it fire up) till you find the source of the noise - shouldnot take more than a time or three. Probably at least half the time is right in the cabinet sheet metal of the furnace itself, because that is where there is the most pressure differential in effect, so start there. Do not fire the furnace up more than a couple of times per 10 minutes - turn thermostat up to make it fire, then back down so it will shut off (may run for a minutes or three in cool-down mode), then wait a few minutes before doing it again to prevent overheating it.

If you have flexible fabric/plastic ducts (usually round) then ducting is out as a cause, leaving damper in a metal section of the ducting, or the furnace housing itself as the guilty party.

VERY rarely, a loose A/C evaporator coil (loose in its bracket) can flop back and forth as the blower kicks on and off - flapping in the breeze so to speak.

Also VERY rarely but probably only audible in that room, a loose (no screws) register can move enough to make noise when the air flow increases and decreases - but usually just a dull "thud" not a metallic or boom sound.

You said the same boom thing BEFORE the blower shuts off - that would likely be a damper slamming because the rubber edge seal or bumper strip has deteriorated and fallen off. If right AFTER the blower shuts off (or maybe when it goes down to low-speed for cool-down mode on a multi-speed blower), then duct or furnace housing panel tin-canning would be the primary suspect. Normally this occurs very quickly AFTER the fan shuts off (or down to low airflow).

Can also rarely occur just from heating and cooling of the ducts causing tin canning.

if unable to track this down yourself, a Heating and A/C contractor would be the Search the List category to get a tech to track it down and fix it.

If it is occurring in the ducting, you can commonly stop it by putting a piece of blocking (like 2x4) against the offending panel, then using plumbers tape (metal strapping) or metal strip to secure it to the ducting, so that the blocking is pushed in tight against the duct. Technically the blocking should probably, by fire cose, be non-combustible - not sure about the technicalities of that. Needs to be well secured so it does not fall out during heating and cooling movement.

If the furnace housing is making the noise, first make sure it is not due to an unlatched or not fully seated access panel. Otherwise, if a panel-to-panel contact is causing a banging, a small bit of self-adhesive weatherstripping or even small inside-out wad of duct tape in between might act as a bumper. If a panel in the furnace housing is tin-canning, screwing (without puncturing firebox area or any wires) a piece of blocking from it to an adjacent wall to wedge it in place commonly works - be sure it does not block access to the unit or if it has to, is easily removeable for servicing the unit. If doing this in an area of the furnace that gets hot, the blocking should be non-combustible and nothing should penetrate the firebox or exhaust flue in the firebox/heat exchanger area - sheet metal screws OK to fasten it once you get to the ducting past that area.

Answered 2 years ago by LCD

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