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Question DetailsAsked on 3/6/2017

I have a gravity furnace (1948) and when heating often there's a loud BANG coming from the furnace. Any suggestions

The BANG happens I'd say 2 or 3 times an hour and if I have guests over, it really jolts them. Besides replacing this antique, for the time-being is there any stop-gap measures? I have full access to the heater, the return, etc. The sound is specifically coming from the heating area.

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4 Answers

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Answered 1 year ago by Member Services

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Have someone kick the thermostat up while you are by the furnace, so you are there when it starts up. Watch for any major burst of light or blowout of dust from underneath or such which might indicate the gas is building up before it ignites - causing an explosion, If that is the case, usually if you remove the front cover and look around you will see sooting and scorching of paint on the housing and such. If that is the case, obviosulyk the unit needs servicing by the Heating and AC tech.


Sometimes, when it kicks on (or when the fan kicks up to high power) the suction from the blower will pull in the sheet metal on the side of the furnace or at the filter housing, making a booming or metallic drum sound. Commonly, putting a non-combustible concrete block or metal brace against the offending panel (with a bit of self-stick foam padding to prevent metal-to-metal noise) so it holds the panel inwards all the time will stop that. Make easily removeable for maintenance, especially if it is a front removeable panel doing this. If front panels moving, sometimes people jsut put a piece or two or duct tape to hold it tight so it can't move as airflow changes.


Most common cause, and commonly loudest after the furnace kicks off at the end of a firing cycle, is thermal ior pressure adjustment, where a piece of the ducting (which may or may not be part of the furnace housing or may be in the metal ducting (usually in the larger ones near the furnace) pops inward or outward - may have to listen around a few times to tie down which one is doing it. Solution is putting an additional support bar on the ducting, with the support bar sheet-metal screwed to the duct, or sometimes with a self-adhesive foam pad (weatherstripping) between the duct and the support piece to hold it constantly inward so it cannot pop in and out with pressure or temperature change.


Commonly, this latter happens in ducting which is cold - in crawlspace or cooler garage, because it is undergoing a far greater temperture change than normal.


One other cause, though not normally the "bang" you are describing - is an airflow damper (some furnaces have them at the air inlet) dropping shut as the furnace blower shuts down, hitting the metal sealing edge as it closes. Probably not legal because it is modifying a rated appliance, but thin foam rubber weatherstrip tape on one of the sealing surfaces stops the noise. Make sure it is well adhered so it cannot come loose and get pulled into the blower motor and jam it up - might be best to use contact cement or similone adhesive rather than trust the self-adhesive on the tape.


If this noise occurs just as the furnace blower is coming on or shutting down, I would be looking at furnace or air filter housing panels or damper moving because of the change of airflow. If occurring some minutes after furnace shutdown, more likely in the ducting itself - some largist duct panel dimpling in and out, or rarely (though tends to be more of a thumb than a bang) expansion and contraction of the ducting causing noise as it hangs up in a hanger or opening through a wall and then lets loose.


Oh - for tracking it down - sometimes you can just go along and press in and pull out (with a piece of tape) on panels and the flat surfaces of larger ducts to see which readily move or buckle in and out - might help trace down which ones to listen at when furnace is firing. And of course, the more people doing the listening, the quicker you track it down.


Oh - one other thing - if you have a solenoid or motor-controlled damper in the ducting to control the airflow to two different zones, sometimes those control dampers close noisely - insulation tape is solution there too, the type with glossy plastic surface so the damper does not stick top it when it sits closed.

Answered 1 year ago by LCD

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Reading through my response after it posted - the "dimpling" of the sheet metal making the noise can be from the sheet metal sides moving inward on the return side of the furnace or in the filter housing or the "lower half" or inlet side of the furnace housing, or outwards on the pressure or supply side of the ducting - makes same sound regardless of which way it moves, and restricting it so it can't move is the normal solution - usually easier to do by pressing the side inward from the outside.


Technically anything you use to restrain it should be fireproof, not wood - so sometimes cinder block if a ground-level furnace housing panel (easy to move for maintenance), or commonly suspension strut or perforated steel angle stock (aka "slotted angle") or (normally only on commercial jobs) unistrut would be the normal means of doing this. Residential jobs commonly a number of wraps of duct tape or some self-adhesive weatherstripping is commonly used to prevent metal-to-metal scraping, commercially (where it will likely be inspected) a wad of fiberglass or rock wool insulation is commonly used. I have also seen plumbers tape (the metal perforated strapping, not "tape" used for this, with some wadded insulation blocking between it and the ducting to keep it under pressure.

Answered 1 year ago by LCD

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OK - ghosts from my childhood - we had a 1952 American furnace which did that after it was about 12 years old and again at about 20 some - I remember it had a felt damper seal on the furnace damper (fifth paragraph in my first response) - which wore down to nothing over time. We contact cemented another strip of felt or strip of batting or such over the residual one to provide a soft landing for the damper plate. Obviously make sure the gas valve is down to pilot when doing this so it can't fire up with your hand in there and maybe burn you or ignite the glue before it sets. I would let it sit for an hour or two before firing it back up to let the glue set if flammable glue, and of course make sure the glue in only on the "down" side of the felt or batting strip - that it does not weep through and glue the damper shut. First time it fires up be sure the damper open correctly, because that old a furnace would probably not have pressure sensor to shut it off if it failed to open.


One safety thing - that old a furnace, hope you have had it inspected periodically for possible heat exchanger cracking - though probably cast iron so a lot more likely to be in good shape than modern aluminum ones 1/7 its age. I have seen over 150 year old furnaces with original heat exchangers in them that look as good as they were when a year or two old (slight rust but no cracks or rust flaking), so on old cast iron heat exchangers age is not indication of condition.


Though I would hazard the efficiency of that unit is probably about 40-50% or so (likely less if true gravity unit - no blower fan), so one might consider upgrading from an economic viewpoint perhaps, albeit probably at a reduction in operating reliability.

Answered 1 year ago by LCD




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