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BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22426
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have Trane furnace about 13 years old, the upper limit

Customer Question

Hi
I have Trane furnace about 13 years old, the upper limit switch fails about once a year. I have installed 5 different switches in the past at $50 each. Just had another fail, have bought several places on the web.
Tried swt1259, swt1278, now have a swt1260 in it. The swt 1258 is recommended by trane.
They all seem to be working after I take them out and tap on them.
Do you have good place to buy one that will work for more than a year?
***** *****
Submitted: 1 year ago.
Category: HVAC
Expert:  BillyHvac replied 1 year ago.

Hello,

this is indicative of a different issue. It indicates we may have too high of a temp rise. With the unit running can you tell me what the temperature is at the return air grill and also at the register closest to furnace?

Customer: replied 1 year ago.
I measured last year when it failed and was about 140 deg
Expert:  BillyHvac replied 1 year ago.

Thank you....by chance are you using a high efficiency filter?

Customer: replied 1 year ago.
don't think it is a high, let me look at it
Customer: replied 1 year ago.
it is a cheepy, just changed last month, energyAir from lowes.
Expert:  BillyHvac replied 1 year ago.

the end result is you are running too hot...and actually wearing out the limit. It is cycling on limit as most furnaces never trip limit at all. Only if blower fails or filter plugged.

Basically the furnace is running hotter than designed and the safety limit (which there for rare trips) is being activated on a regular basis.

High efficent filters can cause this, blocked registers/returns, duct too small etc.......we can look at increasing the blower speed if you are game.

Customer: replied 1 year ago.
cleaned the blower wheel two years back, not dirty, but if you have easy way to up the speed, go for it.
jim
Expert:  BillyHvac replied 1 year ago.

Thank you...coming from blower motor to circuit board...there are commonly red, black, yellow, blue etc (some colors can be different.)

Then at the board where theese wires attach will be terminals lik COOL, HEAT, LO or HI...

can you tell me what color wire from motor is attached to what terminal?

Customer: replied 1 year ago.
give me a few min.
Expert:  BillyHvac replied 1 year ago.

standing by

Customer: replied 1 year ago.
black connected to Cold term
Blue Heat
Red Park
Yellow Park
Expert:  BillyHvac replied 1 year ago.

well someone already had this issue or could have even been done on start up.

Black is highest speed for ac

Blue is med high

yellow med lo

red is lowest...

red is factory set at heat. Black at cool.

then yellow and blue on Park which are dummy terminals to put unused wires.

Someone has already bumped your speed from red to to blue bypassing even yellow. The overheat has been an issue for sometime. You can buy the limits regularly but the type and brand of limit will not matter.

You need more registers, more return or something to get the temp down...if you have ac...check the bottom of the ac coil at furnace and see if plugged

Customer: replied 1 year ago.
are you saying to swap the red and blue?or is it correct now?
I have AC, cleaned the coils above the heater last summer, not too bad.
Expert:  BillyHvac replied 1 year ago.

Ok you want to leave black on cool.

Leave blue on Heat...What I am saying is that someone has already recognized the overtemp issue and already raised the blower as high as it will go.

Not only is this tripping the limit but is also costing extra in gas as the unit is cycling..

You will need to have some duct changes or the issue will continue.

Customer: replied 1 year ago.
all was ok for the first 11 years and then the ignighters started go bad, changed three times before finding a good one. since then the limit switch has been the problem. 5 times. Nothing elese has changed but I will open more vents.
Have the 200 deg switch in there now and will re check the temp on output.
Customer: replied 1 year ago.
not to buy another Trane
Jim
Expert:  BillyHvac replied 1 year ago.

same reason ignitors go bad...they are wearing out.

A normal cycle would be 1 glow one igniton blower runs, shuts off....yours is glow, ignition blower, trip, glow ignition, blow trip. Your parts are not bad.....they are getting 10 years use in one year. Whether ac was added, duct changed, blocked etc....

And the fact that the unit is already on blue wire shows this issue existed for a while...or...the red would still be on heat where factory put it. This has been going on and an attempted fix by changing speed.....changing speed, ignitors, limits, 130 plus register temp...all point to running too hot

Expert:  BillyHvac replied 1 year ago.

I would just say get "this type part"...but it will not change the issue.

I really wish you the best...

Billy

Customer: replied 1 year ago.
AC and heater all installed in apriol of 2002 by Plumbline service.
Thanks anyway.
Jim
Customer: replied 1 year ago.
They installed a Honeywell humidifier at that time too, may too much heat going back into cold air duck, maybe I will close off some of 6 inch pipe that goes into the cold air return.
Expert:  BillyHvac replied 1 year ago.

Anything you can do to lower that output by 20 degrees...

This remains available for follow up 30 days after the service is rated.