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Old 12-10-2012, 01:15 PM   #16
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Furnace limit switch question (need answer ASAP)


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?
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Old 12-10-2012, 05:06 PM   #17
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Furnace limit switch question (need answer ASAP)


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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Old 12-10-2012, 05:13 PM   #18
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Furnace limit switch question (need answer ASAP)


Lowering will make it worse.
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Old 12-10-2012, 05:21 PM   #19
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Furnace limit switch question (need answer ASAP)


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.
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Old 12-10-2012, 05:21 PM   #20
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Quote:
Originally Posted by yuri View Post
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.


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?
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Old 12-10-2012, 05:25 PM   #21
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Furnace limit switch question (need answer ASAP)


Could be. Should use a manometer to check manifold pressure.
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Old 12-10-2012, 05:35 PM   #22
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Furnace limit switch question (need answer ASAP)


Yes
but it's now time to get a pro in to properly check that gas valve pressure and the rest of your distribution layout.
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Old 12-10-2012, 05:47 PM   #23
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Furnace limit switch question (need answer ASAP)


Will do. And now the last question, until I can get someone here, should I run the furnace without a filter or with a filter and let the limit switch kick it on and off?
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Old 12-10-2012, 06:03 PM   #24
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Furnace limit switch question (need answer ASAP)


Either don't run it, or set the stat temp low, say 60 to 62.
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Old 12-10-2012, 06:07 PM   #25
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Furnace limit switch question (need answer ASAP)


I'd choose running the furnace without a filter (just for now) instead of working that limit over and over again.
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Old 12-11-2012, 04:37 PM   #26
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Furnace limit switch question (need answer ASAP)


The gas pressure was checked today, it was originally at 4.5 inches w.c. It ended up being brought down to 3.5 inches w.c to get the furnace to the proper gas pressure coming in. I closed up the cold air return openings I had opened and ran the furnace without issue to heat the house. Everything seems in order now. Again thank you for all the helpful advice.
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Old 12-11-2012, 05:58 PM   #27
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Thanks for the update.
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