395CAV060130 26 year old furnace short cycling.

Glen_S

Premium Member
Joined
May 31, 2008
Messages
41
Location
Alberta
Model Number
395CAV060130
Brand
Age
More than 10 years
Furnace is a Carrier, I think a single stage mid efficiency, even though it says high efficiency on the top cover. It was installed in '92.

About a month ago I noticed my furnace was short cycling, restarting every 3-4 minutes. I went down to look at it and noticed that the large hose that feeds the humidifier I had put on a few years ago had come off, and was dangling with the end down near the upper grated panel that covers the burners. I put the hose back on the humidifier, and it seemed to clear everything up, as I turned up the thermostat a couple of degrees and it ran until the temp was reached.

Everything seemed ok, and I thought that the warm air coming out the humidifier hose must have warmed up the area enough that one of the limit switches was tripped. The a couple of days ago I noticed it was short cycling again, about every 6 minutes the burners would shut off, wait for the blower fan to stop, wait a minute then reignite the burners and repeat the cycle. When I checked it the large hose going into the humidifier had come off again so I thought it was the same issue, and taped back with duct tape this time, but the problem did not go away this time.

After doing some research, I think I can rule out the flame sensor and the burners are not turning off right away. It's not the heat from the humidifier hose because I taped it back on good this time, and I think I narrowed it down to air starvation as I took off the bottom cover, clamped the switch closed and let it run without the bottom cover. It ran well past the 6 minutes this time, so I turned the furnace off and put the cover back on. I noticed also when I held the door up I could really feel it sucking the door in, and from what I read there should just be mild pressure pulling it in.

The filter is new, I have been using 3M filtrete merv 12 electrostatic filters, and have been using these for the last few years. I do have furniture up against two of five the air inlet vents in the house, but they have been there for years and there is a gap of a few inches between the furniture and the vent, and the bottoms of both pieces of furniture are open so I don't really thing they're restricting airflow too much. Both outside fresh air intakes are clear.

As I write this, the bottom (funrace) cover is back on, and I just today closed the flapper that lets warm air from the outlet flow to the humidifier, and left the cover to the humidifier off to act as another air intake. I turned the thermostat up and it ran for about 20 minutes this time before the burners went off, but the set temp had not been reached as it restarted again. The next cycle went for about 10-12 minutes before the burners turned off.

This tells me that it is an air intake issue, but I also read that one of the limit switches may fail slowly over time, even if it reads ok with a multimeter. I have not checked any of the continuity of any limit switches yet. There is a red LED on the main board, yesterday when I took the cover off I noticed it was flashing what seemed to be one short, one long and another short, but when I ran the furnace with the bottom cover off it stayed on red, so I don't know if it's the type of LED that flashes a code or not. There is nothing on the inside of the door diagram explaining anything about codes.

I am wondering if the filters I am using are too restrictive, and if I should switch to a cheaper one that has more flow. Or, could it be my limit switch failing? I'm not sure which one to check, but assume it's the rectangular one that is just above where the burners go in.

I'm basing my assumptions on just what I have researched so far, and hoping someone here with more knowledge on furnaces can chime in.
 

Quehvac

Premium Member
Joined
Mar 2, 2019
Messages
3
Location
OH
Your going to have to watch the furnace with both a temp probe and a multimeter. Stick the temp probe inside your return closest to your filter. As soon as the unit shuts off check for continuity on the limit for the heat exchanger section. If you don't have continuity through it then it's time for s new one.
 

Quehvac

Premium Member
Joined
Mar 2, 2019
Messages
3
Location
OH
Also your heat exchanger may have a crack as well use an inspection camera or remove the blower motor and do a visual check of the heat exchanger checking for cracks and holes around the seams of the heat exchanger
 

Glen_S

Premium Member
Joined
May 31, 2008
Messages
41
Location
Alberta
Also your heat exchanger may have a crack as well use an inspection camera or remove the blower motor and do a visual check of the heat exchanger checking for cracks and holes around the seams of the heat exchanger
Thanks - if I can find a temp probe, what do I look for?

I assume the swtich I check for continuity is the rectangular unit on the left of attached image with red wires going to it?

On another note, since I put in a less expensive (and I assume less restrictive) filter and left the cover off the humidifier so it acts as another return inlet, the furnace has stopped short cycling and now stays on until set temp is reached.

furnace.jpg
 
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