Rheem oil furnace - burning smell


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Old 02-11-14, 10:34 PM
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Rheem oil furnace - burning smell

I have a 15ish year old Rheem oil furnace and intermittently, the house smells like burning plastic. It seemed to be operating normally but based on internet research, I decided to replace the blower motor. It needed to be replaced anyway as the rear bearing was bad and making noise. This did not fix the issue and it persists. I've had two HVAC techs inspect the unit and they say it's working fine but I could not duplicate the smell while they were here. The smell is strongest coming out of the vent directly above the furnace after it has finished cycling. Some anecdotal information is that the upstairs seems warmer than normal when we smell the smell. Also I have noticed on two occasions that the blower fan shut off about 10 seconds after the burner where normally it runs for around 90 seconds. Is it possible I have some sort of overheating situation? The second tech said the fan limit isn't adjustable and the first tech said if the switch was bad, it would just not work and not be an intermittent problem. I am really at a loss and the smell frightens my wife even though both techs confirmed there is no safety issue with the unit.
 
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Old 02-12-14, 05:28 PM
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As I recall, Rheem uses timers on the fan rather than a temperature control. If my memory is correct, a failing timer could certainly cause the fan to cycle off sooner than it should and cause a burning smell.
 
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Old 02-19-14, 08:42 AM
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Thanks Grady, I managed to find the appropriate replacement fan control and will try installing it tonight to see if that fixes it. If this repair fixes the strong smell issue, I will get the furnace CO checked again to make sure there are no cracks in the heat exchanger.

Once other question, could the fan running short cause permanent damage to the heat exchanger besides actual cracks? Is there a coating or something else that could continue to smell?

The reason I ask is I have been running the fan on the "on" setting since I ordered the part and the bad smell has not returned but my wife swears she occasionally smells "something" but acknowledges it is not the same strong smell we both witnessed. It could be in her head, she is very anxious about the situation
 
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Old 02-19-14, 11:49 AM
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The smell

The fact that you don't get the smell when the fan is running full time could suggest overheating the fire side of the heat exchanger. My experience with a condition like that was the combustion chamber started to fail, and the flame impinged
on the heat exchanger. If i had not replaced the combustion chamber in a timely manor I could see where the flame could have overheated the heat exchanger, and eventually burned through it, causing a furnace replacement. That is why a CO detector is very important with a hot air system.
Sid
 
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Old 02-19-14, 04:46 PM
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The only coating is oil used in the forming of the heat exchanger. It burns off the first time the heater is run. If the heat exchanger is cracked or the chamber is damaged the fiberglass insulation could be scorching.
 
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Old 02-19-14, 08:45 PM
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Thanks guys. The timer replacement was a cinch. Now we wait... the sudden warmer weather in PA won't help discover anything anytime soon as the furnace won't be cycling on as much now...
 
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Old 02-19-14, 10:59 PM
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Well that didn't take long. It was running a cycle seemingly normally. After the burner shut off, the fan was running then I heard the thermostat click and the burner fired back up. After a little while, the fan shut off but the burner continued to run for 15-30 seconds and the smell was evident. Could it be the thermostat?
 
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Old 02-19-14, 11:04 PM
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If you have a a/c coil with a plastic pan,I have seen them burn up.
 
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Old 02-19-14, 11:12 PM
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Well they just kind of melt not burn up.
 
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Old 02-20-14, 01:27 PM
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ok another thing I saw looking at some other threads... I thought I was clever and connected my humidifier (the drip through a mesh filter style) power to one of the relays in the furnace that way I wouldn't need to rely on the temperature sensor of the humidifier, it would only operate when the burner is running. Is it possible that power draw is triggering the burner to re-fire after it had been run?
 
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Old 02-20-14, 03:31 PM
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It is possible the humidifier has overloaded a relay.
Guyold has a very good point about the plastic condensate pan.
 
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Old 02-20-14, 06:43 PM
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After posting that, I disconnected those wires and was hopeful thT was the problem but it just messed up again and caused the smell. This time the fan ran for only about 20 seconds after the burner turned off. Should I replace the relays or thermostat next? It can't be anything else, right?
 
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Old 02-20-14, 07:22 PM
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I would certainly replace the fan relay(s) & anything else which has to do with fan control. I've always hated the way Rheem has their control system. It's the most Rube Goldberg control system on an oil burner I've ever seen.
 
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Old 02-21-14, 02:14 AM
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Thanks so much. I also have a new symptom since installing the fan control. Both yesterday night and tonight we had the unit's fan back in "on" mode as opposed to auto to prevent the smell. Once yesterday and just now the unit ran over temperature. Thermostat set to 69 but house got to 73 before we woke up and noticed it. Changing thermostat from heat to off had no effect. I had to turn the fan from on back to auto and both the fan and burner shut off. I quickly set the unit back to the on position to go back to where we were and the burner did not come back on. I guess I'll get new relays and pray everything works.
 
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Old 02-21-14, 09:57 AM
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This morning the heat stuck on and the house was too warm but to my surprise the thermostat still said 69 on it. I think I might replace it first...
 
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Old 02-21-14, 06:19 PM
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If it isn't too difficult, you might want to pull new thermostat wire when you replace the stat. It is possible a rodent has chewed the wire causing an intermittent short or maybe a nail has gotten into the wire. There might not be anything wrong with the wire but why take the chance?
 
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Old 02-21-14, 06:46 PM
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I found the burning smell

I installed the new stat and when the furnace came on my wife said it smelled all day, not as bad but once installed, I smelled it too. I have not had enough time to decide if the stat has fixed anything but the source of the smell is the insulation from the interior walls of the heat exchanger section coming off and laying on the heat exchanger pipes. I have attached pics showing both sides with this issue. Is there any way to fix this? I assume we could pull out the a/c coil and do it as the only option. Name:  IMG_1152.jpg
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Old 02-21-14, 07:18 PM
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To fix it right is going to be a difficult task. You would have to strip down the furnace, remove the insulation, & glue new insulation it place. Over the years I've had several furnaces or air handlers to have the cabinet insulation come off. A somewhat easier fix is to remove the insulation & glue new insulation on the exterior. It looks like crap but works.
 
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Old 02-21-14, 07:22 PM
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I assume that a full breakdown wouldn't be recommended on a 15 year unit vs a replacement.
 
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Old 02-21-14, 07:29 PM
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I honestly don't remember how difficult it is to break down a Rheem. If it can be done in 4-6 hours by one person, it would certainly be worth the cost of the labor (presuming $75-100/hr) vs the cost of a new furnace.
When I was doing it regularly, it got to the point where I could strip down a certain model of furnace, replace the heat exchanger, put it all back together, & be gone in two hours.
 
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Old 02-25-14, 08:08 PM
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The other thing that's happening is the insulation is laying on the fan limit switch that juts a few inches into the cavity. I wonder if that is what also is making the fan stop prematurely.
 
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Old 02-25-14, 08:13 PM
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I wouldn't think so. If anything, I'd think it would make the fan run longer IF the fan is controlled by temperature which I don't think it is. I think the fan on that furnace is strictly timed. Can you give us a picture of the whole burner compartment?
 
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Old 02-26-14, 01:14 AM
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I will get one. I think this limit switch is just for the burner. Like a safety switch or something.
 
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Old 02-26-14, 05:59 PM
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A furnace with a limit switch opening before the thermostat is satisfied will cause the burner to short cycle but the fan should stay on the whole time. The whole idea is to shut down the burner in order to keep the heat exchanger from getting any hotter than it already is & to keep the fan on in order to cool the overheated heat exchanger.
 
 

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