Home furnace troubleshooting

I've got a call in to some HVAC pros, but in the meantime thought the wise folks here might point me in the right direction.

Our three year-old Carrier furnace and central heating system is acting up for the first time since we bought our place a year ago. Last week, we noticed a plasticky, burning odor when running it, but only periodically. I changed the Aprilaire filter, which seemed to help for a few days, but then we smelled an odor again. Checked out the ducts, and there are no holes or obvious problems there. The furnace shows no signs of burned out parts, scorched metal, etc. There's no evidence of rodents in the crawlspace.

Today, we paid more attention to its behavior. When we turn it on from the thermostat, the furnace will sometimes start up right away, but other times won't do anything, or maybe not until some minutes later when it feels like it. When it does get running, it will only sometimes reach and/or hold the set temperature. For example, the thermostat indicates the house is 64 degrees, and I have it set for 68, but it's not running. We also still notice a burning odor from time to time, usually when the temperature reaches around 67+.

If any of you have clues of as to what part(s) might be malfunctioning or need attention, I'd greatly appreciate it.

Thanks!

Is it a condensing furnace?

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Originally Posted by rwsaunders

Is it a condensing furnace?

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Yes, I think so (the sellers left us some paperwork to that effect, but I'll have to check for sure in the morning).

I assume it is forced air so sounds like a fan/blower thing. As in the motor is dying and overheating (smell) and cuts out.

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Originally Posted by rwsaunders

Is it a condensing furnace?

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I’m going to look into these possible issues: High-Efficiency Furnace Combustion Problems

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Originally Posted by sbornia

I’m going to look into these possible issues: High-Efficiency Furnace Combustion Problems

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Sometimes that burning odor is just dust on the heat exchanger which collected after sitting idle during the Summer but the erratic behavior shouldn't be related to dust. I've had birds nest in the intake in the Summer and when Winter kicked in, the furnace would erratically cycle on and off, trying to start in the precombustion stage and never fully igniting. Removing feathers and nest materials and installing a screen solved that problem.

I've also had the condensate drain line clog and the same symptoms appear. When the motor kicks in during preignition, water in the bottom of the fan/blower triggers a switch which shuts the furnace down. Cleaning the drain lines once/year problem solved.

If you're smelling something burning, you're correct in keeping the system off until a tech takes a look. Hopefully it's something as simple as the issues listed above or a bad control board or a faulty low gas pressure relay. A cracked heat exchanger will also cause erratic behavior but that's pretty rare for a three year old unit. You said that you changed the filter and mentioned Aprilaire. Is that your humidifier filter (Aprilaire is a well known humidifier and energy recovery manufacturer) or the main furnace filter?

Thanks!!
There’s an Aprilaire filter attached to the furnace. I’m going to check for nests and leaves, but imagine I’ll still need a tech to look at the condensate drain and electric components. Hopefully I didn’t damage anything by running it again this evening. It’s off for good until I get it sorted out...should be a nice balmy 55 when we wake up!

igniter going out is always a good call, as is dust getting somewhere it shouldn't

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Originally Posted by sbornia

Hopefully I didn’t damage anything by running it again this evening. It’s off for good until I get it sorted out...should be a nice balmy 55 when we wake up!

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I have my thermostat set at 56* - if nighttime lows did not go below 55* my furnace would never run.

(when lows start to get down to less than 15* I'll probably increase the thermostat to a toasty 58*, if only to help keep the pipes that are close to some of the exterior walls from freezing)

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Originally Posted by j44ke

I assume it is forced air so sounds like a fan/blower thing. As in the motor is dying and overheating (smell) and cuts out.

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Winner winner, chicken dinner.

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Originally Posted by sbornia

Winner winner, chicken dinner.

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Excellent. Relatively easy fix then, though still $$.

Before you go off and spend money on a new motor you may want to check to see if some sort of plastic thing didn't drop into your ductwork, so I would go around the house when you notice the smell and see if the smell is stronger in one room vs another; if so remove the grate from the stinkiest room and feel around as far as you can for something that shouldn't be in there, watch out for alligators that live in those ducts...anywho, after you check that one and come up empty go to all of them and do the same thing just for fun.

The other situation could be a melting electrical wire covering which could be caused by someone fooling around inside and moved a wire too close to something hot, did you have anyone looking inside the unit before this happened? There are in some units a lot of wire so look at them all real well, but a good way to check this is when you smell that the next time go to the furnace and put your nose up to it and see if the smell is stronger, if so remove the panel which might turn off the furnace which is ok, and look for evidence of a hot wire which would have darkish gray or black marks on the wire insulation and if you touch it it should be hot, if so see if it's close to something hot, if so move it away; if no hot wire but you smell it strongly at the furnace then it's probably the motor overheating or remote possibility a crack heat exchanger but that's rare for a furnace that's only 3 years old, but maybe.

Again before you attack the motor, check the heat exchanger, if you can see it, and see if something plastic is on the exchanger.

Those Carriers are usually pretty reliable but there was some Carriers that had secondary heat exchanger with a plastic coating that over time would begin to fail and melt, closing off heat exchanger cells, and causing a stink, but I think those models were older than the one you have, but I could be wrong, either that or the previous owner knowing they needed a new furnace to sell the place found an older unused model they got for dirt cheap so the furnace place could get it out of the warehouse.

Be warned, if that plastic smell is indeed a wire with melting insulation, or the motor getting too hot, that could be a dangerous situation that should be addressed ASAP!