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Old 02-13-2009, 04:01 PM   #1
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Burner does not ignite but main blower motor starts


I have a Comfortmaker HVAC unit @10 years old. Occasionally the fume extractor fan starts, the pilot light ignites then the main burner does not come on (the honeywell gas relay does not click) but then the main blower motor starts & remains on all night. Any ideas? I am thinking that the controller has an issue as this issue is intermittent. If it was the pilot thermocouple it would be all the time right? I plan to clean the thermocouple tonight & see if that helps.

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Old 02-13-2009, 04:07 PM   #2
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Burner does not ignite but main blower motor starts


Fuel starvation.. replace the filter after checking that the line is not clogged.

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Old 02-13-2009, 04:55 PM   #3
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Burner does not ignite but main blower motor starts


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Fuel starvation.. replace the filter after checking that the line is not clogged.

Huh?
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Old 02-13-2009, 04:59 PM   #4
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Burner does not ignite but main blower motor starts


that fume extractor "Inducer Draft Fan" IDF.should be the first thing to start on a call for heat...and should stay on till the heat satisfies the space setting.pull the stat off the subbase and jump R to W and see if you get a continous run...might be the stat?when that IDF runs it proofs a pressure switch mounted above the main fan in the electrical section to OK the main gas valve telling it thatthe IDF is running and it can have the main burner to come on...do the jump....check the IDF run check the tubes running from the pressure switch for water or dirt.with the furnace powered OFF if you lightly blow into the tube connected to the pressure switch you should hear a click every time you blow into the tube.
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Old 02-14-2009, 04:28 AM   #5
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Burner does not ignite but main blower motor starts


There are a couple possibilities, might be as simple as a dirty flame sensor, I wouldn't suspect pressure switches, because if they weren't closing you would not get a pilot. Certain units, the main blower will come on to alert you to a problem after the ignition trial period has passed with no oper.
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Old 02-14-2009, 09:28 PM   #6
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Burner does not ignite but main blower motor starts


Thanks for the help Guys. (not sure about the filter change on a natural gas furnace though?)
My buddy at work mentioned to clean the thermocouple with a fine sandpaper so I did that last night. I had already checked out the pressure valve & that works fine. The issue hasn't happened for over a week now anyway so I'll just wait & see if it happens again. I would have thought that the system would shutdown if there was no main burner ignite, but as you say perhaps that is the way they indicate trouble. My electricity bill seems to have doubled last month, I hope having the main blower running all night (upto 7 hours) twice is enough to add $130 of electricity???
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Old 02-14-2009, 10:24 PM   #7
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Burner does not ignite but main blower motor starts


No, the blower doesn't add that much to the electric bill if it runs 24/7 for the month.

But, a leaking electric water heater can.
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Old 02-15-2009, 11:01 AM   #8
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Burner does not ignite but main blower motor starts


Hate to be negative but it does. The average d/d motor uses 3.2 amps x 120 volts = 360 watts. Say 500 watts = 1/2 kilowatt. I pay 8 cents/kwhour. That fan costs about 4 cents/hr to run at 25 hrs/day = $1 a day approximately = $30/month and $365/yr. That is why the ECM motors are SO much better. They use 80 watts. If your elec rate is over 10 cents/kwhour then it is even worse.

Last edited by yuri; 02-15-2009 at 11:07 AM.
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Old 02-15-2009, 07:02 PM   #9
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Burner does not ignite but main blower motor starts


Yuri.

A. There is only 24 hours in a day.

B. Even by you adding an hour, it still doesn't come out to 130 bucks for 30 days. Let alone 130 bucks for running it 7 hours straight, 2 times.

360 watts an hour x 24 hours = 8.64KWH a day x .08 =.69.12 cents a day, times 30 days = $20.76

You also have to subtract the time it would run normally.
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Old 02-15-2009, 08:06 PM   #10
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Burner does not ignite but main blower motor starts


I was rounding things off. There are taxes to pay on the electricity etc. We use these figures to show people why ECM motors are worthwhile. They use 80 watts on low speed. As the air filter plugs up etc etc the 3.2 amps can climb also.

Cheers
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Old 02-15-2009, 09:46 PM   #11
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Burner does not ignite but main blower motor starts


Actually, PSC motor uses less electric as the filter clogs.

And a VS motors uses more.

The higher the static pressure a VS motor works against, the more current it draws.

Don't know what brand you use.

But, Trane has a good blower data sheet, that shows the wattage at higher statics. Which can be used to show customers how important it is to keep the air filter clean with a VS blower.

I don't handle Trane, but I have one or 2 of their sheets for VS blower.
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Old 02-16-2009, 12:09 AM   #12
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Burner does not ignite but main blower motor starts


Quote:
Originally Posted by biggles View Post
that fume extractor "Inducer Draft Fan" IDF.should be the first thing to start on a call for heat...and should stay on till the heat satisfies the space setting.pull the stat off the subbase and jump R to W and see if you get a continous run...might be the stat?.
It's not the stat. The furnace cycles normally. to the point that the gas valve doesn't open. If you read the post he is saying the blower runs continuosly, which is normal for a timed on blower control board.
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Originally Posted by biggles View Post
when that IDF runs it proofs a pressure switch mounted above the main fan in the electrical section to OK the main gas valve telling it thatthe IDF is running and it can have the main burner to come on...do the jump....check the IDF run check the tubes running from the pressure switch for water or dirt.with the furnace powered OFF if you lightly blow into the tube connected to the pressure switch you should hear a click every time you blow into the tube.
The press. switch is really a vacuum switch. Blowing through it will not close the switch. you would have to suck on the tube to close the switch.
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Old 02-24-2009, 02:47 AM   #13
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Burner does not ignite but main blower motor starts


Ok I have some more info. don't worry about the elec. bill we believe that was a different set of specific issues that month that drove the bill up.

So the issue happened again tonight (3am!!). The difference tonight is that it is bitterly cold outside (0degF). I have checked the pressure switch & that checks out fine as the exhaust fan comes on, the pressure switch clicks, this then starts the ignite sequence for the pilot. Then I get no main burner ignite but the blower motor kicks on. But I wondered if the cold might be a thermocouple not being able to get hot enough from the pilot with very cold input air to the burner box? So I warmed the thermocouple with a lighter for 30 secs then the main burners ignited without issue. Therefore I am presuming it's a thermocouple issue right?? I have cleaned the thermocouple with sandpaper so it's not that. Could I measure the thermostat output to see if it's that. Or can you trick the system in some way to see if the thermostat is bad i.e. ground the wire or something? Thanks

Last edited by slaptop; 02-24-2009 at 02:56 AM.
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Old 02-24-2009, 04:32 AM   #14
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Burner does not ignite but main blower motor starts


Good morning. I had the exact problem on my neighbor's furnace. The pressure switch would work, but would not stay closed. They can "wear out" and leak air past the bellows during the cycle. You can measure the voltage across the terminals of the switch during the cycle and see it change if it's bad. I replaced the switch and fixed the problem. Hope this helps.
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Old 02-24-2009, 06:06 AM   #15
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Burner does not ignite but main blower motor starts


Your t-stat is o.k., sounds like the pilot orfice is dirty, is the flame touching the flame sensor, that's why it would light after you held a lighter on it to warm it up, and your blower fan is timed on, it has nothing to do with the burners not igniting.

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