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Author Topic: HVAC help... furnace blower not kicking on  (Read 2905 times)

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Offline cyberwollf

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HVAC help... furnace blower not kicking on
« on: October 01, 2011, 08:21:03 »
Anybody do HVAC work/help?  Its a 80's Lennox gas furnace.  On a call for heat: the pilot kicks on, ignites, LED on the ignition control is on so induction motor is pulling good vacuum, main burners kick on, but the main blower never kicks on.

I've done a fair amount of googling, I think its the fan limit switch.  I can watch it take little jumps and it gets over 100, but after about 2 mins after the main burners started the limit switch jumps to 150 and everything shuts down... How smooth should the limit switch dial climb, and when should the main blower turn on?

Right now i just set it for heat and after it lights i turn the blower on manually to blow the hot air.

Ideas?
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline kattz

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Re: HVAC help... furnace blower not kicking on
« Reply #1 on: October 01, 2011, 09:26:58 »
If you have a modern furnace, there are fuses on the main board.  Check those.  Most furnaces have a purge blower that is usually a small motor and fan attached to the burner box that comes on first.  This trips a pressure switch once on that turns on the main fan and burners.  Both my fan motor and switch have went bad and I've replaced them separately over the past 13 years.  If you turn the plastic indicator wheel on the back of the purge fan motor and then it takes off, the bushings are going bad but the motor will run for a few weeks. Also, there's a hi-limit thyristor or snap switch on the furnace.  If this goes bad, the furnace will not start.

Most furnaces built in the past 15 years have an LED indicator that will blink a trouble code to tell you what's missing or what's wrong.  You will need one or two jumper wires to bypass the sensors and a meter to troubleshoot.

HTH.

Kev
90g SPS and LPS reef tank, 35g sump, ceramic rock by The Alternative Reef, Neptune Apex w 2 X EB8's, Moonlight module, ATI Sunpower Dimmable 8 X 39W T5's, Octopus Extreme 160 skimmer, PM Kalkwasser Reactor, 2 X Vortech MP40's, Geo 618 Ca reactor


Various thriving montipora, acropora, stylopora, wellsophyllia, blastomussa, hammer, anchor, and frogspawn, lobophyllia, rhizotrychus, pavona, scroll, and pagoda SPS and LPS corals, but no fish because I was too stupid to QT...

Offline cyberwollf

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Re: HVAC help... furnace blower not kicking on
« Reply #2 on: October 01, 2011, 16:45:37 »
If you have a modern furnace, there are fuses on the main board.  Check those.  Most furnaces have a purge blower that is usually a small motor and fan attached to the burner box that comes on first.  This trips a pressure switch once on that turns on the main fan and burners.  Both my fan motor and switch have went bad and I've replaced them separately over the past 13 years.  If you turn the plastic indicator wheel on the back of the purge fan motor and then it takes off, the bushings are going bad but the motor will run for a few weeks. Also, there's a hi-limit thyristor or snap switch on the furnace.  If this goes bad, the furnace will not start.

Most furnaces built in the past 15 years have an LED indicator that will blink a trouble code to tell you what's missing or what's wrong.  You will need one or two jumper wires to bypass the sensors and a meter to troubleshoot.

HTH.

Kev

The little motor is the unduction motor, and my LED is solid so i am getting a good signal from the pressure switch.  Normally there is a ~1 min delay between burners lighting and main blower start.  There is another temp limit switch that is controlled by burner heat to ensure it doesnt blow cold air and keeps blowing warm air even after the burners kick off.  I think this switch is bad, but have no idea how to troubleshoot it
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline reefit

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Re: HVAC help... furnace blower not kicking on
« Reply #3 on: October 01, 2011, 17:16:48 »
does the blower turn on if you move the fan limit buy hand? if it does fan limit is bad if it doesn't blower is bad.

Offline cyberwollf

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Re: HVAC help... furnace blower not kicking on
« Reply #4 on: October 01, 2011, 19:23:50 »
does the blower turn on if you move the fan limit buy hand? if it does fan limit is bad if it doesn't blower is bad.

Nope.  fan limit is pretty firm.  I can spin it like 20 degrees maybe, then it snaps back to its correct temp.

blower should be on between 100-150 right? after 150 it is saftey cutoff?  I dont here any relay clicks or anything when i move it inside or out of the 100-150 zone.. should i?
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline kattz

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Re: HVAC help... furnace blower not kicking on
« Reply #5 on: October 01, 2011, 20:02:25 »
No, you won't hear anything.  Pull the wires off of the sensor in question.  If the furnace works, it's a bad sensor. If that doesn't work, use a piece of wire and jumper the two terminals.  If it still doesn't work, look for the problem elsewhere.
90g SPS and LPS reef tank, 35g sump, ceramic rock by The Alternative Reef, Neptune Apex w 2 X EB8's, Moonlight module, ATI Sunpower Dimmable 8 X 39W T5's, Octopus Extreme 160 skimmer, PM Kalkwasser Reactor, 2 X Vortech MP40's, Geo 618 Ca reactor


Various thriving montipora, acropora, stylopora, wellsophyllia, blastomussa, hammer, anchor, and frogspawn, lobophyllia, rhizotrychus, pavona, scroll, and pagoda SPS and LPS corals, but no fish because I was too stupid to QT...

Offline ohioreef

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Re: HVAC help... furnace blower not kicking on
« Reply #6 on: October 01, 2011, 20:21:25 »
Our furnace doesn't use the fan limit switch to control the blower, it is controlled by the control board. It kicks it on a certain time after main burner fires. The fan limit switch is used to shut down the burner if the furnace overheats.

If it is blowing on manual then you can rule out the blower itself.

When I was having problems with our furnace last year I got a lot of help from the forums at http://www.doityourself.com/forum/.
Gary

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Offline ohioreef

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Re: HVAC help... furnace blower not kicking on
« Reply #7 on: October 01, 2011, 20:22:54 »
Pull the wires off of the sensor in question.  If the furnace works, it's a bad sensor. If that doesn't work, use a piece of wire and jumper the two terminals.  If it still doesn't work, look for the problem elsewhere.

Not necessarily. On my furnace some of the sensors complete the circuit to allow the furnace to run. Pulling a wire shuts down the furnace.
Gary

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Offline reefit

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Re: HVAC help... furnace blower not kicking on
« Reply #8 on: October 01, 2011, 21:16:12 »
Does the blower come on when you turn the thermostat from auto to on? If it comes on it will be the fan limit if it doesn't come on it's most likely the blower motor.

Offline cyberwollf

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Re: HVAC help... furnace blower not kicking on
« Reply #9 on: October 01, 2011, 21:17:30 »
Our furnace doesn't use the fan limit switch to control the blower, it is controlled by the control board. It kicks it on a certain time after main burner fires. The fan limit switch is used to shut down the burner if the furnace overheats.

If it is blowing on manual then you can rule out the blower itself.

When I was having problems with our furnace last year I got a lot of help from the forums at http://www.doityourself.com/forum/.


Thanks for the info Gary.  How were where you able to tell whether it uses the limit switch for blower or just overtemp?

That sounds like what i got, playing with the limit switch i can never get the blower to kick on, but if i let it run 2 mins or so without the blower on, it will jump to the 150 cutoff and it will kill the control board.

The only thing that doesnt add up is, the controll board LED is always solid, it doesnt blink any error messages AND the control board was replaced winter 2008....
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline cyberwollf

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Re: HVAC help... furnace blower not kicking on
« Reply #10 on: October 01, 2011, 21:19:22 »
Does the blower come on when you turn the thermostat from auto to on? If it comes on it will be the fan limit if it doesn't come on it's most likely the blower motor.

Yes it comes on in manual, thats how i have it running now.  how do i determine/troubleshoot whether the control board or limit switch activate the blower?

There is only 1 relay on the blower right?  The thermostat, AC, and furnace all activate the same relay for blower?
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline reefit

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Re: HVAC help... furnace blower not kicking on
« Reply #11 on: October 01, 2011, 22:11:54 »
with out seeing it i would call it bad fan limit how many wires are on the fan limit?

Offline cyberwollf

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Re: HVAC help... furnace blower not kicking on
« Reply #12 on: October 01, 2011, 22:15:52 »
with out seeing it i would call it bad fan limit how many wires are on the fan limit?

Do you do HVAC work? If so are you intrested in comming to check it out for a 6pack or reef related trade ;)
75G Mixed Reef w/ 30G sump/refuge

Electrical Engineers do it on impulse, with faster rise times, with more power, and less resistance at higher frequencies, without shorts, until it Hertz


Offline reefit

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Re: HVAC help... furnace blower not kicking on
« Reply #13 on: October 01, 2011, 22:32:03 »
i do apartment maintenance. i don't have the part you need

Offline ohioreef

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Re: HVAC help... furnace blower not kicking on
« Reply #14 on: October 02, 2011, 14:54:03 »
Can you post a picture of the limit switch? Does the blower wires connect to the switch or to the control board?

Do you have an owner's manual? That should cover how the blower is turned on.

When you say you turn the blower on manually, are you flipping a switch on the limit switch or using the thermostat fan switch?
Gary

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