HE Gas Furnace start up problem when in auto

[exit28]

After reading some of the solutions that have been posted I would really appreciate if you could give me your opinion about the possible cause of the problem with my furnace. We just purchased the house that is about 17 years old and as far as I know this is the original gas furnace.
With the thermostat set to auto and heat selected the furnace cycles through about three times before it stays on. Only if it has the fan selected to stay on all the time does it seem to work properly. I thought that maybe the thermostat was faulty in that it was causing the blower fan to shut off and then on about three times until it stayed on steady. So I replaced the thermostat but it still keeps doing the same thing which means that the problem must be with the furnace itself.
The cycling that occurs with heat set to auto is as follows:

1. The vent blower starts up for about 45 seconds and is quite noisy.
2. Then the burner ignites.
3. After about 30 seconds the main fan starts.
4. Then the burner shuts off 20 seconds later followed by the main fan and the vent blower.

This recycles two more times after which it stays on until the set temperature is reached.

The way I've gotten around this as I mentioned is to set the fan on all the time on the thermostat. This way the furnace doesn't go through the cycling.
I'm worried that the cycling is putting too much stress on the ignitor, vent fan and burner with all of these starts and stops.
Granted, this is a fairly old furnace and maybe I need to consider a replacement but to be honest after the expense of moving it isn't in the budget for at least two years.
Just to give you an idea of the age of the furnace the ignitor control uses a White Rodgers model 50E47-140 module which from searching the manufacturers web site it is listed as obsolete. However to look at the insides of the furnace all of the parts look almost like new except for the dust. I didn't look inside the burner since I don't consider myself expert enough to do that.

[paul52446m]

After reading some of the solutions that have been posted I would really appreciate if you could give me your opinion about the possible cause of the problem with my furnace. We just purchased the house that is about 17 years old and as far as I know this is the original gas furnace.
With the thermostat set to auto and heat selected the furnace cycles through about three times before it stays on. Only if it has the fan selected to stay on all the time does it seem to work properly. I thought that maybe the thermostat was faulty in that it was causing the blower fan to shut off and then on about three times until it stayed on steady. So I replaced the thermostat but it still keeps doing the same thing which means that the problem must be with the furnace itself.
The cycling that occurs with heat set to auto is as follows:

1. The vent blower starts up for about 45 seconds and is quite noisy.
2. Then the burner ignites.
3. After about 30 seconds the main fan starts.
4. Then the burner shuts off 20 seconds later followed by the main fan and the vent blower.

This recycles two more times after which it stays on until the set temperature is reached.

The way I've gotten around this as I mentioned is to set the fan on all the time on the thermostat. This way the furnace doesn't go through the cycling.
I'm worried that the cycling is putting too much stress on the ignitor, vent fan and burner with all of these starts and stops.
Granted, this is a fairly old furnace and maybe I need to consider a replacement but to be honest after the expense of moving it isn't in the budget for at least two years.
Just to give you an idea of the age of the furnace the ignitor control uses a White Rodgers model 50E47-140 module which from searching the manufacturers web site it is listed as obsolete. However to look at the insides of the furnace all of the parts look almost like new except for the dust. I didn't look inside the burner since I don't consider myself expert enough to do that.

http://www.white-rodgers.com/wrdhom/pdfs/04_Cat_pages/04_Cat_pg_068.pdf

It sounds like you a a bad set of points in your module control so when you run your blower it vibrates enough to make it. If yours is a flame rod model, then make sure it is clean. use a little fine sand paper on it.
Another thing that has to be checked is the voltage. The module you have is very fussy about needing 24 volts to it. Check to see if you have 118 volts to furnace. A 24volt transformer will start out at about 27 volts if it has 118 coming to it. If you have something that is drawing to much of your 24 volts,
then you will have low volts, like 22 volts coming to your module control.
They need to have 24 volts coming to them. If your 120 power is down then your 24 volt will go down. If this happens you would have to bank transformers or better yet call your power co. and have them get the power up where it belongs. When line volt drops amp. use goes up and you pay more for your power.
In the short time that the burner is on before the blower comes on, if you had a weak limit switch it could cause the burner to go out, and it will not come back on until the blower goes out. By that time the limit switch has
reset it self. Later Paul

[exit28]

I checked the voltage to the module and it is 24.2 so it should be ok.
Maybe the points in the module aren't working properly as you suggest. I'll see if I can get another module to replace the current one.

Another suggestion I got was that the vent blower fan isn't strong enough to maintain the low pressure required long enough and this is causing the burner to turn off.
I'm not sure about this but it sounds like a possibility.

Rgs,
Leo

[paul52446m]

I checked the voltage to the module and it is 24.2 so it should be ok.
Maybe the points in the module aren't working properly as you suggest. I'll see if I can get another module to replace the current one.

Another suggestion I got was that the vent blower fan isn't strong enough to maintain the low pressure required long enough and this is causing the burner to turn off.
I'm not sure about this but it sounds like a possibility.

Rgs,
Leo

Did you check for that 24 volts on the white wire coming back from the thermostat, when it is on a call for heat. You have a 24 volt on your module and the 24 volts go to the stat and back to the module. later paul