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-   -   Furnace limit switch question (need answer ASAP) (http://www.diychatroom.com/f17/furnace-limit-switch-question-need-answer-asap-165984/)

Lemkie 12-09-2012 08:20 AM

Furnace limit switch question (need answer ASAP)
 
I have a Bryant furnace. Woke up today and it's not heating. The blower was running constantly. I bypassed the limit switch and it worked fine. I cleaned the switch and reconnected it and the furnace immediately showed the error code. I take this to mean the switch is faulty but I'd like to confirm that.

If that is the case, how long can I bypass the switch for. I'd like to run the furnace for a bit just to heat the house back up, but not if it'll cause a problem.

Other than this the furnace has run fine since I bought the house (3 years ago).

I'm trying to find somewhere to buy the switch but seeing as how it's Sunday all the shops are closed.

Thank you!

yuri 12-09-2012 08:51 AM

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.

Lemkie 12-09-2012 08:59 AM

Quote:

Originally Posted by yuri (Post 1069437)
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.

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.

yuri 12-09-2012 09:05 AM

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.

Lemkie 12-09-2012 09:16 AM

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!

yuri 12-09-2012 09:21 AM

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.

Lemkie 12-09-2012 09:23 AM

Quote:

Originally Posted by yuri (Post 1069471)
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.

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?

yuri 12-09-2012 09:26 AM

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.

Lemkie 12-09-2012 09:32 AM

Quote:

Originally Posted by yuri (Post 1069478)
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.

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....

yuri 12-09-2012 09:52 AM

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.

Lemkie 12-09-2012 09:53 AM

Quote:

Originally Posted by yuri (Post 1069506)
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.

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

Lemkie 12-10-2012 08:41 AM

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!

yuri 12-10-2012 08:48 AM

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.

Lemkie 12-10-2012 08:55 AM

Quote:

Originally Posted by yuri (Post 1070144)
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.


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

Lemkie 12-10-2012 10:28 AM

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?


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