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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8327
Experience:  Furnace, A/C and heat pump specialist.
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My furnace has a history of faulting. Does the blinking

Customer Question

My Coleman furnace has a history of faulting. Does the blinking light reveals a combustion air switch failed years ago I had the same problem and I purchased a brand-new combustion air switch but it did not fix the problem. The problem seems to go away on its own so I left the old combustion air switch in and kept the new one for emergency purposes. But like I said the new combustion air switch never actually fix the problem. Well every year I seem to have to do something to make the furnace continue to work usually in the fall it will fall on the three blinking light fault which is combustion air switch. This year neither of the switches is going to repair the problem and the problem is every morning I wake up and the furnace did not kick in and tells me I need a combustion air switch. Can you help me figure out why the brand-new combustion air switch does not work does not fix it but yet I keep getting a fault code telling me the combustion air switches failed will not close
Submitted: 11 months ago.
Category: HVAC
Expert:  airheatman replied 11 months ago.

Steve you could have an issue with debris or condensation building up in the little rubber hose if it has one.Another common issue is a restriction in vent such as a spider web.You could also just have a failing board that is not sensing switch closed.

First step in troubleshooting this is going to be to get a manometer and check to see if the motor is actually drawing sufficient vacuum to close the switch.

Customer: replied 11 months ago.
OK so when I take an ohms meter reading across the switch with the fault code stating that the switch will not close. The ohms measures anywhere from 5 to 25 it varies it's not a steady value. I measured before I turn the furnace back on and it measures about 400 homes both switches measure around the same both switches have a variant ohms 5 to 25. The rubber tube is not blocked. When I use my own suction the closest switch the ohms measured 5 ohms
Expert:  airheatman replied 11 months ago.

With unt off we should not read any ohms.Switch should be open

Expert:  airheatman replied 11 months ago.

If we get an ohms reading switch is closed. Does it go to zero when of?

Customer: replied 11 months ago.
OK let me rephrase my last statement. The power was turned off by the switch. But when the two wires going to the switch are connected I get 400 ohms. But when I pull the two wires off the switch it reads open. It never read 0 ohms, 5 ohms is the lowest. So the two wires even with the power off are closing the switch to within 400 homes which is strange
Customer: replied 11 months ago.
Does this sound like a board problem do you is there anyway to check the board ?
Customer: replied 11 months ago.
The 400 ohms is coming from the 24 V AC transformer which powers the combustion air switch. Measured with the power off
Expert:  airheatman replied 11 months ago.

Ok that will not tell us anything about the switch

Expert:  airheatman replied 11 months ago.

I suspect board being that it is itermittent but would want to verify that vacuum before condemning.I we can verify sufficient vacuum we will know it is board not recognizing closed.

Customer: replied 11 months ago.
OK it will take me a couple of days to get the Manno meter in. What value are we looking for in the differential of pressure? And how much do I owe for helping me today.
Customer: replied 11 months ago.
Look I am definitely getting ohms across the switch when it is in a fault condition. According to you the switch is working fine because any ohms means it is closed. Which brings me to this question why are we shorting out a 24 V AC supply ?
Customer: replied 11 months ago.
I was told when I first contacted just answer that would cost me $37 for this help I had to pay five dollars just to get my first question. Since this will not be fixed today how much do I OU today ? And how do you get paid I am only off the rise five dollars at this point. Steve
Customer: replied 11 months ago.
That last sentence was garbled. Here it is again.
I only authorized a $5 charge so far.
Expert:  airheatman replied 11 months ago.

I do not have anything to do with fees. I am only sent he question and then compensated if you rate answer 3 or higher. I usually suggest to wait until we are finished to rate.Since I am going out of town I am going to opt out so another expert can take over when you are ready to continue.

Customer: replied 11 months ago.
Can you tell me what value on the Manno meter I should expect. I have no problem paying if we get this thing fixed it be a big help
Expert:  airheatman replied 11 months ago.

It will be on a label on the pressure switch.See example below