Heater not starting as before

[bigtbasser]

My forced air heater isn't starting up like it did last winter.  The progression is different.  Upon starting it up after this long summer the main blower motor comes on right away.  The glow plug lights up and the gas fires up. The unit works fine but I know something is wrong.   Before,  the small fan motor came on and when the gas lit and heated up, the blower would come on.  Any ideas on what is turning on the blower motor first instead of the small unit.  Thanks, Terry

[hvacdrd]

Were there any changes made since last winter such as a new tstat or has any work been done on the unit?

[bigtbasser]

I'm changing out the thermostat this morning.  Over the summer the battery connections became corroded and I took it apart and cleaned it up.  Maybe I knocked something haywire.  Hopefully that is it.  Terry

[hvacdrd]

I wasn't suggesting a new tstat, I just wondered if any changes were made. Let us know what happens with the new thermostat.

[Guest ahammer48]

Hello

It sounds like ur fan limit switch has gone bad. Depending on the age of the furnace and make, most older furnaces had a fan limit switch which would allow the furnace to heat up 1st and then allow the fan to run. This would stop cold, luke warm air from being blown into the house. Its secondary purpose was to continue to run the fan after the gas had shut off to cool down the heat exchanger.

In the newer furnaces they've done away with this temperature switch and now its just a timed program contained in the logic board.

U will need to take the cover off the furnace to get to the control section and also to expose the gas train (manifold). U are looking for a approx 4" x 4" box. Normally has a silver cover with "Honeywell" stamped on it. May have a red knob sticking out of it. Thats the limit switch.

Some manufactuers have moved away from the big Honeywell control and gone to 2 smaller, cheaper thermostats to do the job of this control.

Please supply the make, model & serial number and approx age of UR unit. You'll find the M & S number on the units nameplate. That would help alot.

[Rrey199]

 If it's an Hot surface ignition system, it's probably a newer model. If it has a integrated circuit board, try disconnecting the wire from the "G" terminal to see if the problem goes away.

As I understand, the blower comes on immediately with a call for heat, and stays off when there is no call.

[bigtbasser]

Wasn't able to run down the problem due to being out of town.  Just got back and took apart my thermostat.  Inside there is a small sliding button that I must have moved when replacing the batteries.  I moved the switch to the other side and the unit is working as usual, I think.  Thanks, Terry

[hvacdrd]

that would be the switch for an electric furnace - it energizes the fan circuit immediately with a call for heat...splainz everything- good find.