I've raised this issue in other forums. I have a high efficiency furnace that has worked flawlessly for the last four years (the house was built in 2000). This year we had to have the circuit board replaced. After that there was a problem with the blower, i.e. it wasn't shutting down after the heating cycle. I had to shut the power off and switch it on at the furnace. We had the limit switch replaced. After a few days the same thing started happening again. We changed filters, checked to make sure all dampers were opening properly, etc. An independent technician on another forum speculated that there was some type of blockage somewhere in the system that was preventing an adequate amount of air from circulating throughout the system. I believe that the problem is a faulty limit switch. The blower is blowing cold air, so that means that the temperature has decreased from where it was when the burners were on, right? So, why isn't the blower shutting down? The furnace was delivering adequate heat during the heating cycle, but the blower wasn't shutting down following the heating cycle. If another heating cycle was triggered, the blower would shut down for a few seconds, and then the furnace would go through the normal startup procedure, and sometimes the blower would shut down following the heating cycle, but most of the time it would just continue blowing cold air following the heating cycle. I have a service contract, so cost is not an issue. Does anyone have any ideas? Could it be a faulty limit switch? Thanks for the info.

The problem started when you replaced the board. Changing the limit switch did not fix the problem. Sounds like a problem on the board. My guess is the relay that powers the blower. Trying fan only or AC may not be a good test if they have their own relay to control the fan. If the blower does stay on, the relay is almost certainly the problem. Your service provider may be reluctant to eat the cost of a second board. Good thing you bought the service contract.

When I had to replace the control board for my Bryant 80%+ furnace, it was $300. Still, that is the only problem 10 years. It may have been lighting. I have since wired in the same varistors as in the surge protectors protecting my computer and other electronics.

Labman: thanks for the insight. The fault indicator on the control board indicates an open limit switch -- do you still think it's the fan relay? I was thinking of purchasing an ohmeter and/or a voltimeter to do some testing on my own, an idea I got from a technician's website. Thanks again for your help.

Eapples <;-)

Quote:

---

Originally Posted by eapples

Labman: thanks for the insight. The fault indicator on the control board indicates an open limit switch -- do you still think it's the fan relay? I was thinking of purchasing an ohmeter and/or a voltimeter to do some testing on my own, an idea I got from a technician's website. Thanks again for your help.

Eapples <;-)

---

... is a good idea and they are quite inexpensive. Labman will respond to the relay question but I can testify to the usefulness of these meters to the extent that I have at least 3 of them and they are never close enough to me when I need one. Home Depot has quite a selection...

Perhaps the place to start is the wiring between the control board and the limit switch. Any bad connection or broken wire would look like an open switch. You could try connecting a piece of wire between the contacts of the limit switch. That would make it look like a closed switch. If that shuts the blower off, the switch is bad. Any component can be bad out of the box.

I agree a multimeter would be a big help. The cheap ones do just as well for most trouble shooting. If you had $25 to spend, a $10 meter and a $15 voltage sensor might be a better buy than a $25 multimeter. I picked up a Sentronics one a year or 2 ago. It became such an essential that when I mislaid it this year, I went out and bought a GB GVD 505A Live Wire. It is adjustable and very sensitive. Just hold it near an insulated live AC wire, and tip glows red. Turn the sensitivity up to far, and a wire a foot away lights it up. No need to find an uncovered contact, or leads to tangle. Any hardware or home center should have them.

Labman and Flickit: thanks for the information and encouragement to proceed.

Eapples <;-)

Roll out switches

I have a Bryant Furnace (Model# 373LAV). The burners ignite for about 10-15 seconds and then go out. Do you know what the problem is? Is it the limit switch?

ljaskols: Does the blower go on? If it doesn't, then maybe the flame sensor is faulty. The limit switch, if it was faulty wouldn't allow the unit to ignite at all. If it does ignite as yours does, and the blower turns on and runs for a while, something in the system may be tripping the limit switch (blockage in the system, plugged filter etc.). Hope this helps.

I hope they aren't still having trouble a year later...

Where is limit switch on Coleman Presidential II located?

Look on the wiring diagram located on your unit.

Goodman Furnace model PGB048075-1
Is it possible that a bad high limit switch that causes the fan to constantly blow whenever power is applied. The coil was clog and air couldn't get through. I cleaned the coil and now I want to turn the furnace back on and see if it trips the limit switch now. Could you tell me how to turn the unit back on, cause our thermostat inside is not on unless the unit is on.

The problem sounds like welded contacts on the control board on the heat relay. After so many contacts they arc and finally weld together or at least stick. Thump on the heat relay will sometimes break them loose but relay or control board has to be replaced. Relay about $4 and some soldering or $80 for new board.

Also be sure to fix before going to AC because if the heat contact is stuck and the cool contact is closed you are trying to run furnace on high blower speed and med or low speed and will cause blower to overheat and you may be buying a new 3 speed motor at $ 175+ plus it will do it again if not fixed...

You can tell if this is the case by AC on Blower running and you remove the cool or high speed wire on the board and the blower still runs then it is getting power from the welded bad contact.