high temp limit switch causes A/C to fail? - http://www.hvacmechanic.com/ Forums
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Replies:
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| yuri
|  "Re(1):high temp limit switch causes A/C to fa"
, posted Wed 15 Aug 18:30:  
The limit switch will keep the blower running if it opens. I would get some attic ventilation fans installed. You are overworking that poor A/C if any attic air enters the air handler. Make sure the ducts are properly insulated and sealed up there. Get the evap coil drain lines checked also before they cause an overflow problem.
Yuri
[this message was edited by yuri on Wed 15 Aug 18:31] |
| | jcummings
| "Re(2):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 19:03
Yuri,
Thanks for your reply. Do you know if the limit switch will also prevent the outside unit from coming on?
The issue is probably that I had the A/C turned off during the day. I guess it got too hot and tried to cool itself by turning on the blower. I understand why this is necessary for the furnace, but it seems like a poor design for the A/C. Since the house was also hot (95 degrees or so), there was no hope for recovery until the outside temperature dropped. Typically, how hot does it have to be to trip the switch?
John
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| | yuri
| "Re(3):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 19:14:
I have seen them as low as 170 F so it may have got that high in an enclosed space. I have never run into your problem in Canada. Not so hot and few attic units here. It will energize the blower and block power to the gas valve. It probably thinks you are in heat mode and will block the A/c mode and cooling fan relay from energizing. A very unusual occurence but something they may have not designed for. Freon is talking about an old style combo fan/limit switch which you don't have. One of our techs is from Houston and works with Carrier/Payne so he may see this later and help out. Hopefully he hasn't got baked in some attic.
Yuri
[this message was edited by yuri on Wed 15 Aug 19:21] |
| Freon
| "Re(1):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 16:13
You need to check the type of limit control you have. Look for a 3x5" metal (aluminum) box on the front of the furnace. See if the cover comes off easily (by squeezing it). If you see a circular dial(calibrated in degrees) inside with 3 metal pointers, your problem is easy to fix.
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| | jcummings
| "Re(2):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 18:58
Thanks for the response. Unfortunately, I don't see a 3x5 box on the furnace. When you say "front of the furnace", is that the same side as the controller circuit card?
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| | static
| "Re(3):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 19:28:
The limit switch in this furnace will be a tan, brown or white fiberglass square, meas. about 2 x 4 inches, it will be
above the burner rack (if the furnace were upright). Behind the fiberglass square sits the limit switch itself, it is snap-disc thermal switch. No need to pull it out, it's rating will be at the bottom corner of the fiberglass square. Depending on the model of the furnace, it will be anywhere between 160 and 250 degrees for the temperature where it opens.
It is not a poor design at all. It is a SAFETY and it does exactly what it was meant to do.
On this particular brand of furnace, when this switch opens, it removes control voltage to everything. Your A/C will not be allowd to run until the switch resets.
These switches do sometimes weaken and fail (trip at a lower temperature than it's rating or stick open) especially if the furnace is over-fired or not getting enough return air.
[this message was edited by static on Wed 15 Aug 19:31] |
| | jcummings
| "Re(4):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 20:04
I may have found it. The square says "L140-40F" and there's a note that says the switch is set for 140 degrees. I think it's possible it reached 140 degrees in the attic yesterday.
But shouldn't the blower cool it down? Even if the return-air temperature is 95 degrees, it seems like that should be cool enough to close the switch and allow the A/C to come back on. Because the A/C didn't come back on, the blower was just making the house hotter.
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| | static
| "Re(5):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 20:32
"L140-40F"
That is indeed your main Limit Switch. What that number means is that the switch opens at 140 degrees and recloses at 100 degrees.
If it takes 100 degrees to close it, and the airstream is 95 degrees.....is there exactly 95 degree air going over the switch, or is there a possibility that the air could be a few degrees warmer? Not much wiggle room there.
Those switches are only rated for so many trips over their "lifetime". If the furnace has been "rocking" on this switch then it may now be out of spec or even stuck.
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| | jcummings
| "Re(6):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 20:48
I think that explains what happened then.
This goes back to my point about it being a poor design. For the furnace, it works perfectly to shut off the burner if things get too hot. But, it means that I can't turn off my A/C during the summer because I risk not being able to turn it back on until the outside temperature has sufficiently dropped. It's too bad the switch doesn't know that I'm in A/C mode.
Yesterday was the hottest day we've had in Dallas since I've been in this house, and it's the first time I've observed this behavior. Can the switch be replaced for one that has a higher re-close value, such as 120 degrees?
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| | static
| "Re(7):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 21:31
quote:
Can the switch be replaced for one that has a higher re-close value, such as 120 degrees?
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Not sure about that.
I don't know if it matters to you but swapping the switch for one of any other temperature ratings than the factory original will void the UL rating of the furnace as well as any warranty you may have on the furnace.
The problem with doing this is that it may not allow the heat exchanger enough time to cool off appropriately in a limit trip event. I wouldn't feel good about recommending this.
If this is the first time you've had this problem, the switch may just have gotten weak and could be replaced with one of the same specs. They are cheap.
Otherwise the Honeywell fan/limit combo switch that Freon mentioned would do what you are asking, if it will fit in your furnace. There may also be a Camstat switch with an adjustable reclose temperature.
I would recommend to consult with an on-site tech to evaluate your furnace and situation before making this sort of modification.
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| | jcummings
| "Re(8):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 21:42
Thank you very much for the advice.
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| | Freon
| "Re(3):high temp limit switch causes A/C to fa" , posted Wed 15 Aug 19:24
Yes, same side as the controls, usually. Older dial limit switched had the problem you are describing. The newer systems were more a timing based shut-off so the ambient temperature didn't matter. If you don't have a dial type control, you may have a flakey IC. Model number might help others to help you.
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