HVAC Questions? Ask an HVAC Expert for Answers ASAP
Hello there! I'm Norman but you can call me Norm. Let's see what I can do to get you some answers today. I'm ready to get started whenever you are.
Let me first ask, do you happen to have a multi-meter that is used to measure electrical voltage? If so, have you ever used it?
When you are able to come online I will be standing by. Please reply and I'll jump back in just as soon as I can. If you are capable we will need to conduct a few test to try and pinpoint the source of the problems. - Norm
Yes I have a meter. I'm an electronics technician by trade.
Nice. Are you working with a laptop, tablet, or smart phone which you can take to the furnace with you?
Great. Is your thermostat battery powered?
Perfect. First go to the furnace and pull all of the doors off. Then go back to the thermostat and set it to HEAT and AUTO, make sure the temp is bumped up higher than the indoor temperature.
Once you do that return to the furnace and sit infront of it.
Locate the furnace control board.
In one corner of the board you should see the wires going to the blower. COOL, HEAT, SPARE-1, SPARE-2 and a White Common wire. (Com.)
I'm currently not at the location, but I've done a lot of troubleshooting and can probably answer your questions.
My question. When you encounter this issue are you measuring 120v from Com to HEAT on the board?
You should read the 120v right before the high limit trips. This would be after the furnace has fired and flame has proven.
Yes. When the furnace turns on, the low speed combustion fan comes on for 30 seconds, then the ignitor heats up and the gas valve opens and the flame is lit. The failure mode is that after a couple of minutes the over temp sensor trips and the flame shuts off. Once it's gone throught this cycle and I reset the blower temp sensor, I can go through the cycle again and the blower will kick in after about 45 seconds of flame and the furnace heats the house.
Do you get 120v to the blower every single time the furnace fires? From Com. to HEAT?
So when it's heading towards a high limit trip you can't get a reading?
not at the blower motor.
And every time you read 120v the blower starts?
Ok, so then we know it's not the blower motor or capacitor. That's good.
Ok. Have you cleaned the flame sensor with steel wool or sandpaper?
Is the flame sensor above the burners in the heat exchanger, if so, I have removed it and cleaned it. Not with steel wool or sandpaper though.
Yes, it hangs down in the flame. You also want to make sure it's positioned correctly in the furnace so it's catching the flame. Now if you were not getting good flame it shouldn't run long enough to trip out the high limit. However, I always like to be thorough. If it were the flame sensor you'd likely find the furnace with a 34 error.
To me it sounds like you are on the right track. If the furnace fires the same every time but the blower is working intermittently, I'd first suspect a failing ignition module (control board).
Can you give me more detail on the location of the sensor and where to look for a code?
It's going to have a single white wire going to it from the boards wiring harness. It hangs down in front of the burners and looks like this.
Or it could be straight.
I have a picture of the furnace I could email you if that would help.
Here is a video that directly addresses this and how to clean.
Although a crucial part of the furnace operation and a component that must prove before the blower is powered, I still would think the board may be our suspect.
I have not removed or cleaned that sensor, but would that not turn off the ignitor if it didn't sense flame?
The ignitor would start, the flame would fire and if the flame didn't prove it would shut the furnace down and try again. That's why I don't suspect this is your main problem.
It would likely try three times then lock out.
I would agree.
This is your board. I'm going to look to see if I can find one cheaper - CLICK HERE
Here is another - CLICK HERE
Is there a sensor that dectects the heat then trips the relay to start the blower?
And the cheapest yet - CLICK HERE
Once the flame sensor proves the flame the board closes the blower relay which is built into the board.
Here's one more like, cheaper still - CLICK HERE
So you think the flame sensor works to keep the gas valve open but doesn't work to trigger the blower?
It does a little of both, the flame must stay proven to keep the gas valve open, If the flame is good for a period of time the blower will be energized. However if it were the sensor you'd likely see the furnace lock out with the 34 error first.
Take a look at this Service Manual
GO to page 12
There is a flow chart. If you print this and take it to the furnace next time you are there you can see and test each stage of the furnace ignition sequence.
This is also a great video which I recommend watching. This will give you each step of the electrical process when the furnace fires and runs - CLICK HERE
I see you have gone offline. Please remember that you can come back to this chat and ask more questions as you need. I'm here to help! I'll be online most of the day if you need assistance.
When I went to look at the manual, my computer locked up and I had to reboot.
Ah. Sorry to hear that. Have you tried the link again? If it doesn't work I can upload the manual.
I was able to print it once I got back on-line.
Good deal. It will help you.
Again, I feel confident it's the board though.
Just one more question. Once the flame is detected, is it a timed event to turn on the blower or a temperature evnt that turns it on?
Once the flame is proven (monitored and converted to milliamps) for a predetermined period of time it triggers the blower. So it's both.
OK, that's what I thought. Thanks, XXXXX XXXXX very helpful.
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Circuit board fixed it, thanks again.