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Question DetailsAsked on 12/11/2017

Is this sound normal? What is it? Trying to troubleshoot this sound before calling out a tech again...

KCMO area: I purchased a home two years ago. Built in 2005. Has a Carrier 8000 Weathermate furnace. Last year installed a Nest thermostat. Furnace seems to cycle a lot, and makes this weird "pressurized sound" that is very loud upstairs. Usually occurs late at night (after 8pm) and very early AM (3AM, 4AM) upon start-up of the furnace. Did not notice this sound last year. Furnance serviced a month ago, flame sensor was cleaned and new filter installed. You can hear the sounds in the video around 32 sec, 1:04 sec, 1:35 sec and 2:08 seconds. See video link: thank you for your help.

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1 Answer


Could be several things from what I heard - a hard-starting eductor fan, if a high-efficiency furnace a slow-opening flue damper (though looks like a gravity flue from the large diameter metal flue on the wall adjacent to the electrical panel if that is a flue rather than just round ducting) backpressuring the eductor fan, a fan with an obstruction (leves, critter nest, etc), or an electrically controlled damper in the flue ducting opening after the furnace is up and running so the air pressure is backing up then releasing.

Also - check themostat to be sure it is reading correctly and has good batteries in it.

Much rarer, but if the flue pipe (assuming here high-efficiency furnace with plastic flue pipe) is not sloped right, condensation can accumulate in it to the point of causing a water blockage, which the furnace then has to blow out to operate. Furnace eductor fans are not capable of pressuring up much at all, so enough of a low spot to accumulate water that substantially blocks the pipe could not be blown out - so you would be hearing backpressuring, then the unit shuts down very shortly thereafter because the backpressure sensor cuts in and turns the power to the unit off, then after so many seconds auto resets. This can be due to swaybacking in the flue pipe, but most commonly happens because the tech has not provided a drain leg on the flue outside - like ran flue pipe upwards to get required clearance from windows and doors, but did not put a drain hole in the bottom so rainwater, snow melt, and condensate accumulate in it - usually only in cold weather. Or in notably cold weather, frosting/icing up of the flue pipe can do the same thing - because the flue gas is only in the 80's to 100 or so range as it leaves the furnace so cools down a lot by the time it get exhausted outdoors. Ditto to possibility of a squirrel nest or such in it, because mice and squirrels love the nice warm flue pipe on high efficiency furnaces to nest in - if a quite clean burning unit there can still be enough oxygen to support life and not poison them, though I can't say how the baby critter's brains would be.

Basically a new sound like that in an HVAC system is not good and should be looked at as a matter of principle and safety.

Because your furnace is extremely short cycling like that (assuming what I heard was the furnace kicking on and off frequently, not just the blower fan changing from a very quiet low flow to high speed (multi-speed fan), DEFINITELY should be looked at ASAP. If the fan changing speeds is the sound, when unit is firing there would be warm air being blown out the ducts - just at low flow and higher flow when the fan kicks up to high speed. ifshort-cycling, then the burners would be firing for just a very short time, kicking off, then kicking on again within a minutee or so or less, instead of the normal 10-15 minute cycle interval (down to maybe just a few minutes wtihout firing between cycles in very cold westhear where the furnace is barely keeping up with the demand).

Where a furnace is at the limit of its ability to keep up with the heating demand, it may fire almost continuously with very short (just a few minutes or so) break in between - but if that is the case it will be firing almost all the time, not just very short firing bursts with short interval in between - that is short cycling and caused by a thermostat too close to a heat vent so it comes up to temp and shuts it off before the whole house is warmed, damper or eductor not working right, furnace controller problem, rarely thermostat problem.

Personally, I would not trust the unit if short-cycling like that, because unless your NEST thermostat is basically in direct line of airflow (so it responds very quickly to incoming warm air) something must be wrong in the unit to be short cycling like that. My first thought was that an eductor fan or flue damper was not opening, so the pressure sensor is saying that the flue is not working so it is shutting the unit down. Sensor resets, and lets the unit try again - only to shut down 10-20 seconds later again. Not good and possibly dangerous, plus will wear out the furnace in short order with constant restart cycling.

I would consider this sound and especially the short cycling to be cause for either shutting the furnace off till it is fixed (if warm enough out to do that) until it can be fixed on a regular working hours service call - or if cold enough out you really need the heat, I would say justifies a service call ASAP - even if at premium night/weekend rates.

Answered 11 months ago by LCD

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