When temp is below freezing furnace will not ignite. Fan will turn on but there's no combustion. eventually it kicks on but shuts down after a few min. We only have this problem in the really cold temps.

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.

does the main flame every ignite? what brand and model heater is it?

Do not run contentiously during heat mode. the fan runs at a slower speed with heat and heat fires first then the fan cycles on during heat cycle then when heat satisfies the thermostat the heat will cycle off and fan will run for a couple more minutes and then cycle off. So set stat on fan cycle to auto.

The fan relay is stuck in the on position. When the temperature drops the heater does not ignite because the fan is already running and ignition is locked out after the heat cycle starts an the fan turns on

Yes, you most certainly can. It depends on how handy you are and to what extent you are willing to go to make this happen. A lot will depend on how comfortable you are with electrical wiring and understanding HVAC controls and their function.

First though, this is the typical way a wood fired forced air furnace is designed to operate. There is a control transformer and relay to control either a combustion air fan (that you do not have) OR a motorized damper for combustion air that you also do not have. The relay closes on a call for heat from the thermostat and opens the damper from its minimum position to allow combustion air into the firebox. As long as there is solid fuels (wood) in the heat chamber, the fresh supply of combustion air will allow the fire to burn or begin to burn very quickly.

As the temperature in the wood furnace plenum increases, the automatic fan control will start the fan and it will run as long as the bonnet temperature is higher than the fan'on' setpoint on the fan control... Now the house warms up and the thermostat satisfies which allows the combustion air damper to spring back to the minimum position, causing the hot fire to die down due to lack of combustion air.

The now smoldering fire will not generate enough heat to keep the fan running too long, so the automatic fan control shuts the fan off until the house cools approximately 2 degrees below thermostat setpoint. The thermostat calls for heat and opens the damper and on goes the cycle....

An important item to remember is that on the automatic fan control, there is a 'limit switch' function. The motorized combustion air damper or the combustion air fan must be wired through the limit switch in case the bonnet temperature exceeds 200 degrees F. This function will then allow the damper to go to the minimum position or the combustion air fan to go off, causing the fire to cool down due to lack of adequate combustion air.

You don't want the thermostat controlling the circulating fan directly, because of two things: 1). If the fire is out, there is no heat to circulate but the thermostat wouldn't know that and it would start and run the fan continuously until a fire is built. and ....
2). You would still be running up and down the stairs monitoring the fires burn rate by opening and closing the lower door manually.

ALL the components you need are readily available from WW Grainger....basically : a fan/limit control
a thermostat
a 110v x 24v 40 va control transformer
a 24v spdt relay
20 ga. two conductor thermostat wire
24v actuator and linkages to mount to the existing combustion air damper. A bit of creativity is required here. I would not recommend retro-fitting a combustion air fan.
14-2 wg romex for wiring the fan control

Again, it depends to what extent you are willing to go to achieve the end result. Will it be exactly like a new system?? No, but it will operate a lot better than that which you describe.

Hi,
It appears to be the fan limiter switch. It comes on at a certain temp when heating up and shuts off on the high end of its setting. Once the burners are shut down, the fan continues for a few minutes to cool down the burners. The limiter switch is adjustable but it sounds to me to be defective and should be replaced. Good luck and stay warm, :-)

Check the draft inducer air switch ( vacuum diaphragm switch) If the switch is opening when the burner starts, you might have a blockage on air piping coming into the furnace. Also prime the condensate trap on the condensate drain from the combustion fan. Also check the gas pressure, if your gas pressure is dropping, the gas pressure switch will shut it down. If it's too high the moment it comes on could raise the pressure in the furnace dropping out the combustion fan proving switch.

could be the thermostat sticking of the fan control afc not working sticking off on shut down thermostat sounds more like it

Sounds like a problem with the circuit board.

It should have a diagnostic set of leds on the board, with the code list on the door and the control board itself. If it get to the pilot stage and the shuts down its most likely the gas valve. You can check for continuity from mv to c on valve, you should have it. Then with a valtage test from mv to a ground or common as you here the next click of a relay on the board after the pilot has had a few seconds.