Furnace limit switch question (need answer ASAP)

Americanhvacparts

It is NOT safe to run w/o the limit switch. The reason it failed is because of a lack of airflow due to dirty filters, dirty underside of the ac coil, furnace fan running too slow or too small or dirty ducts or too many registers closed. You should get all those checked by a Pro to find out why it is overheating.

yuri said:

Americanhvacparts

It is NOT safe to run w/o the limit switch but as long as you sit beside it or do not leave it unattended ( ie. leave the house ) you can run it for no more than 10-15 minutes at a time to warm the house up and then shut it off and do that once an hour. The reason it failed is because of a lack of airflow due to dirty filters, dirty underside of the ac coil, furnace fan running too slow or too small or dirty ducts or too many registers closed. You should get all those checked by a Pro to find out why it is overheating.

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I did change the filter. It got pretty dirty thanks to basement renovations. I'm hoping that was the problem. Is it possible that the sensor simply failed? I mean if it shut the furnace off due to overheating or whatever reason shouldn't it reset once the power is cycles on and off. I don't think it's doing this since when plugged in the furnace throws the error light immediately.

It is a bi metal snap disc warp switch and has been cycling on'off for a long time and finally broke. I have changed hordes of them and that is a common problem with that furnace.

The common problem being that the switch needs replacing or something else with this furnace?

I placed a thermometer where the switch was, it's at about 140 f. The switch is stamped 180f-40. Does this mean it would kick the heat off once the temp. Reached 180? If so should the furnace regularly get that hot or not?

Thanks!

It is about 1/2" away from the heat exchanger and if the furnace runs too hot will drastically shorten the life of the heat exchanger. 170 or 180F limit switches are very common on all newer furnaces and the tolerances are so low that proper airflow is a must. The manufacturers design furnaces for efficiency and unfortunately air flow is pretty critical nowadays. Limit switch failure is a very common issue nowadays due to the previous reasons I stated. You can do a temp rise test by sticking a cooking thermometer about a foot downstream from the top of the plenum and out of sight of the heat exchanger and subtract the return air temp or house temp. Then get the rating from the model # sticker. Usually 35-65F rise allowed. After that it cycles/rides the limit control.

yuri said:

It is about 1/2" away from the heat exchanger and if the furnace runs too hot will drastically shorten the life of the heat exchanger. 170 or 180F limit switches are very common on all newer furnaces and the tolerances are so low that proper airflow is a must. The manufactures design furnaces for efficiency and unfortunately air flow is pretty critical nowadays. Limit switch failure is a very common issue nowadays due to the previous reasons I stated. You can do a temp rise test by sticking a cooking thermometer about a foot downstream from the top of the plenum and out of sight of the heat exchanger and subtract the return air temp or house temp. Then get the rating from the model # sticker. Usually 35-65F rise allowed. After that it cycles/rides the limit control.

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I put a thermometer where the limit switch was. It went up to about 140. Is this normal or is this high? What the average temperature it should be?

You don't test it there as it gives you a false reading. Have to do the duct test as that is how the manufacturers design those units for static pressure drop etc. There is also radiant heat from the fire and heat exchanger plus actual temperature near the limit switch so a thermometer there is the wrong test. Radiant heat can give you wrong readings also.

yuri said:

You don't test it there as it gives you a false reading. Have to do the duct test as that is how the manufacturers design those units for static pressure drop etc. There is also radiant heat from the fire and heat exchanger plus actual temperature near the limit switch so a thermometer there is the wrong test. Radiant heat can give you wrong readings also.

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I will give this test a try the next time I run the furnace. there's an opening on the supply duct right above the furnace. can I test there? I'm not sure what you mean when you talk about the temperature rise though....

The manufacturers allow a certain amount of increase in temperature to occur when the furnace runs. Air enters at say 70F and gets heated and should not leave at higher than the rating ie: 135F (36-65F from the rating sticker). If it is higher than 135 F then the air is moving too slow and the furnace runs too hot. Just like a plugged radiator in your car. The test needs to be done in the horizontal duct about a foot away from the top of the plenum or the thermometer will literally see the heat ex and read the radiated temp from it as well as the air temp and give you a too high false reading.

yuri said:

The manufacturers allow a certain amount of increase in temperature to occur when the furnace runs. Air enters at say 70F and gets heated and should not leave at higher than the rating ie: 135F (36-65F from the rating sticker). If it is higher than 135 F then the air is moving too slow and the furnace runs too hot. Just like a plugged radiator in your car. The test needs to be done in the horizontal duct about a foot away from the top of the plenum or the thermometer will literally see the heat ex and read the radiated temp from it as well as the air temp and give you a too high false reading.

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This is starting to make sense. Thanks for all the help I really appreciate it!

So I replaced the limit switch today. The furnace is running so that was the problem, however every 10 - 15 minutes of run time, the furnace throws a code 33 error and shuts the flame down for a few minutes, then kicks back on again. No vents seem obstructed either intake or exhaust. The only other thing I can think of is I did raise the furnace up about 1/4 inch to put rubber pads under it as it was vibrating a lot. That being said I don't even know whether or not it was throwing the error code before that or not.

What should I be checking?

Thanks!

The reason it failed is because of a lack of airflow due to dirty filters, dirty underside of the ac coil, furnace fan running too slow or too small or dirty ducts or too many registers closed. You should get all those checked by a Pro to find out why it is overheating. Also get him to check the gas pressure to make sure it is not overfiring.

yuri said:

The reason it failed is because of a lack of airflow due to dirty filters, dirty underside of the ac coil, furnace fan running too slow or too small or dirty ducts or too many registers closed. You should get all those checked by a Pro to find out why it is overheating. Also get him to check the gas pressure to make sure it is not overfiring.

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I don't have an air conditioner, the filter is new, no ducts are closed and none seem obstructed....

This morning I cleaned the squirrel cage which had a decent amount of buildup on it. Put everything back together and within 5 minutes the furnace threw the same error. I checked the manual and apparently this model doesn't have the option to adjust fan speed. Again all my ducts are working fine and are all open, and the cold air returns are sucking a lot of air. Any other suggestions before I call someone in?

I just ran the furnace without a filter and with the cover off. It ran fine and didn't throw a code. I checked the wiring and the fan is running on high speed. Will lowering the speed make the problem better or worse? I'm going to get the least restrictive filter I can find, but for now I plan on running it without a filter.

I'm guessing that my only long term solution is to add an additional cold air return?

Lowering the speed slows it down and makes it worse. I would add another cold return and add a simple register on the hot air plenum or duct as you may need to release pressure on the ductwork as well. Probably it has been cycling on the limit since day 1 and that happens a lot. The gas pressure should be checked by a Pro as well and I am not going to describe a DIYer manometer as setting gas pressure should be done by a Pro IMO.

Lowering will make it worse.

What kind of filter are you using? Some of the high dollar ones restrict air flow too much, causing a pressure drop which equals lower/restricted air flow. Use a cheap "rock catcher" filter, see if that works.

Lowering the speed will increase the outlet air temp but if the filter is right than it's plausible.

yuri said:

Lowering the speed slows it down and makes it worse. I would add another cold return and add a simple register on the hot air plenum or duct as you may need to release pressure on the ductwork as well. Probably it has been cycling on the limit since day 1 and that happens a lot. The gas pressure should be checked by a Pro as well and I am not going to describe a DIYer manometer as setting gas pressure should be done by a Pro IMO.

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First of all thanks for all your advice and explanations. I wouldn't have gotten where I am now without that. Since my last post, I opened up two 12'' by 8'' covers right on the return above where the filter is. I also installed the cheapest filter I could find, which was also 5'' too narrow. Even like that it thew the error code within about 10 minutes.

Although I'm definitely not an expert when it comes to this, I clocked my meter today and based on my calculations the furnace is running at 20000 btu/hr above what it should. The sticker in the furnace says 88000 btu/hr as the input, and based on my calculations it's taking in about 105000 btu/hr worth of gas. While an extra hot air register may help I'm sure, is it possible that the problem is mainly because of the gas pressure being off?