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The furnace send signal to the fan in about 30 seconds, now this fan is set in auto from my termostat, my problem is the fan is starting slowly and some time it don't start, but if I use the option manual to turn on the fan in the termostat runs perfect and faster. can please any body explain me what's the problem?

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Depending on the model of the furnace, the blower is either a 2 or 3 speed. When the furnace operates in heating mode with the thermostat in the auto position, the furnace starts the fan when the furnace reaches a specific temperature or after a set time delay, but it operates the fan in low or med low speed on heat mode, when you put the thermostat fan switch in the "ON" position, the furnace operates the fan in "HI" speed, the same speed used for cooling mode. You may have a blower moter that is starting to short. I would recommend having a professional replace the motor before now before it becomes an emergency in the middle of the night during a cold spell.

Posted on Nov 04, 2009

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My good man furnace model gmh80903bnca will not get to temp fan shuts off and burners keep burning then kicks off on high limit then starts process over but never gets to temp set at 655


If you have a switch that can manually turn on the fan, try it to verify the fan works. This is what I think is happening. The normal operation should be thermostat calls for heat, inducer fan comes on, pressure switch closes, igniter glows, main burner ignites and then in 30 or so seconds the fan comes on and stays on until the thermostat is satisfied. Once the thermostat is satisfied the burner shuts off and the fan runs for approx 120 seconds then shuts off. I think the control board that automatically turns your fan on is faulty. On most modern furnaces if the auto fan control fails and the furnace gets to hot and limits, when the limit opens and shuts off the burner it also energizes the fan motor to cool the heat exchanger.

Jan 13, 2018 | Heating & Cooling

1 Answer

Intertherm Furnace Model #mmhb070abhr06. Burner lights up but shuts down after about 10-30 seconds then recycles over & over again when fan is set to auto (because fan does not cycle on when set to...


Sounds to me like it is going off on a weak high limit switch. With the fan on it will prolong the event but will not stop it. I would check all of the high limit switches. It would most likely be one close to the heat exchanger.

Apr 06, 2014 | Intertherm 56000 Btu Gas Furnace

1 Answer

My Aveo starts up normally and slowly heats up to normal operating temp. But the cooling fan never turns on and eventually the temp guage climbs from the halfway mark to the top mark. I checked the fuses...


Most systems work this way: temp sensor on engine sends a signal to the computer which in turn sends signal to blower relay that then sends signal to the fan.
Any problem along the way, and the fan won't come on.

May 14, 2010 | Cars & Trucks

1 Answer

Can you tell me the order in which each componet of a gas furnace comes on to start the furance?


Yes there is a sequence:
0:00 Dial for fan/limit switch is at 70 deg position. Furnace is cold. I turn T-stat to 80 deg to start furnace.
1:00 Burner comes on and dial begins to move slowly up.
2:30 Dial at 140 deg and fan motor comes on. Dial never goes above 150 deg.
3:00 Burner turns off. Dial slowly moves back towards 90 deg.
4:45 Burner turns on and dial is at 110 deg.
4:50 Fan turns off with dial still at 110 deg.
5:30 Burner off. Dial at 120 deg.
6:15 Fan on. Dial at 140 deg. Burner is still off.
7:00 Burner on. Dial at 120 deg.
7:40 Burner off. Dial still at 120 deg.
8:30 Fan off with dial at 110 deg., and it quickly snaps back to the 80 deg. position.
8:30 plus, cycle continues.....

Mar 25, 2010 | Heating & Cooling

1 Answer

My furnace will not shut off on auto. the fan will not run when switched to on. what should I check?


Some new furnaces have a feature that when it has an issue, it starts the room air blower until you reset the thermostat or kill power to the furnace. Not knowing what you have for a system I can offer you only how a furnace works and what you could look for "generically".

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

Jan 18, 2010 | Honeywell Heating & Cooling

1 Answer

The furnace is blowing warm air even if the thermostat low.


The only thing that tells the unit to come on and off is the thermostat so I would definatly say it is it. When you turn the system back on, does the a/c run normally? If it does, then it's the problem with the heating side of the tstat. Also, does the fan come on when you put it to on and then turn of when you switch it back to auto? If it does, then the system is working fine, its the t-stat. Let me know if you have any more problems.

Mar 23, 2009 | Intertherm P3RA-048K Air Conditioner

1 Answer

Furnace starts properly, then shuts down, led flashes 8 times


The first thing to look at is the flame sense rod.
Shut off power to the furnace, remove, clean sensor with steel wool or sandpaper then reinstall the sensor. Reapply power and it should run properly.

Jan 10, 2009 | Goodman GMS90703BXA Heater

1 Answer

Humidifier won't turn on with furnace fan set to Auto.


There is a devise called a current sensing relay (a little black box that hangs on a blower motor wire that senses current from that wire) If you have such a devise it should hang from the white (common) wire. Loop the wire through twice.

Nov 23, 2008 | Aprilaire 600 Humidifier

1 Answer

Goodman U-30 Furnace


observe the sequence of operation. stat calls for heat and sends a signal, to furnace. check for 24 volts to common and white wire at furnace.first the forced draft motor starts and should have 120 volts to the plug it attaches to. after forced draft motor starts the pressure or vacuum switch closes should show no 24 volt when checked across terminals. shortly after the ignitor glows and shortly after click the main gas valve opens and main burners fire. shortly after the indoor fan comes on and hot air blows into the ductwork. follow the chain of events to see what doesnt happen and it is a clue to whats broke. maybe start by switching fan switch on stat to on. if fan comes on you have high and low voltage thereby eliminating the stat power and transformer as a problem. if you can see the ignitor look for it to be cracked . look close dont touch any part of it .it is powered by 120 volts when energized from board. dont forget when you take the door off there is no power to the furnace so be prepared to tape the safety switch closed when you are diagnosing.

Oct 24, 2008 | Heating & Cooling

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