Limit Switch / Short Cycling

Old 12-11-13, 01:44 PM
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Limit Switch / Short Cycling

Hello. I believe I have a difficult question about my furnace. Thank you in advance if you can offer any advice or solution. And I apologize in advance for the wordy description but this has been going on for a month and I want to give all the details.

It has so far stumped 3 HVAC techs, and takes a bit of explaining, so please bear with me.

I have a Heil 8DNL downdraft furnace, manufactured 2005, 75,000 BTUs in my 1100sqft house. It is short cycling with a 4 blink error code (open limit switch). Burners run for 2 minutes, limit switch pops, blower cools it down in about 40 seconds, and burners start back up. I do not know how long it has been doing this (maybe all 3 years I’ve lived here). The house has always been warm, so I never paid attention to the furnace. Until 4 weeks ago…

The furnace was always made a loud squealing noise and shook when blowing air. My first winter in the house (3 years ago) I asked a HVAC tech if the noise/shaking was normal, and he said yes and I took his word for it (big mistake). Until 4 weeks ago, when I decided to research the noise/shaking for myself, and through the internet I learned that it was in fact not normal. During this research period I also started paying attention to the furnace, and that is when I noticed the 4 blink error code and short cycling (this is before I touched anything).

I continued my research and learned that the noise/shaking and open limit switch could be related, ie. A blower issue that could be causing the furnace to overheat. So I removed the blower wheel, cleaned it out really well (it was very dirty and also had dry caulk in it), and realigned it. It had slid to the front of the motor stem and was hitting the side of the housing, which was causing the noise/shaking. I reinstalled the blower and no more noise, no more shaking. I was very happy with that result, however, the short cycling and open limit switch issue persisted.

So I got back to researching and tried a few other things.
-Replaced the filter – no help
-Ran with no filter – no help
-Cleaned the A coil – no help
-Set blower speed to high – no help
-Felt the blower motor – was not hot
-Replaced the run capacitor – small help (after installing new cap, it took about 5 minutes for the limit switch to pop the first time, then each time after that it popped every 2 min again)

At this point I called a HVAC tech to take a look at it. He told me he was 95% sure that I had a bad limit switch. He said he would charge me $200 to replace, or I could do it myself for about $20. So I ordered the limit switch from ($40) and installed it myself. No help - still popping the new limit switch.

Tech came back and told me my supply ducts were way too small. There are eight 4” ducts running from the supply plenum to the vents, and 1 additional 4” duct that runs to nowhere (capped at the end). He theorized that the previous owner upgraded to a bigger furnace, but didn’t upgrade the ducts.

He also discovered that if you remove the door to the blower housing everything works - no short cycling. So he decided the issue was a lack of return air and told me to install a grill on the return plenum. In my naiveté I said, “OK, thanks.” I was researching how to do this and learned that I absolutely must NOT do this. The furnace is directly next to my natural draft water heater, and a return grill would suck in the exhaust from the water heater into my air supply. So I am done with that tech/company.

But in my mind, the fact that the furnace runs without the blower door on tells me the return air is the issue. I ran an experiment: I turned off the water heater, took off the blower door but covered up all but 36sqin of the opening. And with only 36sqin of extra return air the furnace ran beautifully. Ran it for 50 minutes and the limit switch never popped. So I thought I had a solution – add more return.

Got to researching my return situation and found I only have one return grill on the other side of the house. It’s a 30”x10” grill, connected to a 12” round duct in the attic, which connects to sheet metal covering joists (14.5”x7.25” rectangular duct), which runs into the very top of the return plenum. I thought a good solution would be to add return grills into each of the three bedroom, giving me more return air to resolve the limit switch issue and better airflow in the bedrooms.

I started with a test by cutting an 8” tap directly on top of the return plenum. So the existing return duct is in no way restricted this extra ~50sqin that I added. Remember 36sqin kept the furnace running beautifully so I thought this would be more than enough. Wrong! It did not help at all. Limit switched popped 2 min after the burners came on.

This made no sense to me, but I decided to try to add more return with another test. The furnace is adjacent to a bedroom wall. So I cut a second 8” return tap from the bedroom, through the wall, into a lower part of the return plenum. It was a lot of work, but I sealed it off well to make sure no air from the utility room would be sucked into the return, only from the bedroom.

The result was that it helped a little. The burners now run for 3 minutes instead of 2 before the limit switch pops. But I am totally dumbfounded… I tested 36sqin of return by removing blower door and it works. I add 100sqin into the plenum and it doesn’t work.

Can anyone explain this???

I gave up my experiments and called another HVAC guy. He wanted to replace the limit switch! After arguing with him for 10 minutes he agreed the problem wasn’t the limit switch. The furnace is getting too hot and the limit switch is doing its job. He said I might have a clogged heat exchanger, but seems unlikely because the pressure switch isn’t popping. Also there is no flame rollout so a crack in the exchanger is unlikely.

Eventually I told him what the previous tech said about the ductwork, and he changed his tune. He completely agreed that the supply was too small and said it would cost $6000 to replace the ducts.

I understand that the supply ducts may be restrictive, but I do not need a perfect system. I just need a safe system. How can I justify spending $6000 when adding 36sqin to the blower cabinet fixes the issue. When I asked this tech to explain how the 36sqin worked but the 100sqin that I added didn’t, he refused to answer the question and insisted on supply ducts.

I called another HVAC tech who is a friend of a friend and he said he didn’t understand why the 100sqin of extra return didn’t help.

So what I’m looking for is a reason why my extra return did not fix the issue. If the issue is the supply, why would removing the blower door help?

Forgot the mention… in the crawlspace the supply plenum had an access panel. I took it off to look in and found that part of the plenum is being blocked by a piece of sheet metal. The plenum is only 15”x11” and the sheet metal sticks out about 5”, making the effective plenum only 10”x11”. I have no idea why that sheet metal is there, but I can’t reach it to get it out or investigate further. I agree that 110sqin of supply does sound way too small for a 75000BTU furnace, but why does removing the blower door help, but not my extra return ducts?

Sorry again this is so long, and thank you for any advice.
Old 12-11-13, 04:20 PM
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the motor speed needs to be Lo or Med LO nothing else might think a hi speed fan will cool the heat X it won't.slow air heats up faster then hi speed air.that return is a mystery... how was the original was it sucking big time air back to the unit?removing the fan access door is just a quick test you sure this is a downflow furnace,and not a up flow furnace stupid question but had to ask...sorry do you have A/C coil on this furnace? ...did the tech read what temp it trips at?if it takes an effort to get the fan access door off when just the fan is running you have a major return air problem it should pop up/off easy.where is the nearest R/A duct grill? should take a sheet of newspaper and hold it up on the grill
Old 12-11-13, 07:06 PM
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Thanks for the response. I also tried the lower blower speeds bit it didn't help. It actually made it worse. I set it back to medium high.

Yes I'm sure it's downflow. I read the manual and the blower is on top, a coil is on the bottom. Haha

Tech didn't take temps but the limit switch is 210.

Blower door sticks pretty hard. It will stay up there without screws.

Ask of the return grills are sticking hard enough to hold an evermore. The closest will hold heavy cardboard.

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