THE LIGHTS COME ON BUT NO HEAT COMES OUT

This was my 1st thought after a crack. The fact the indoor blower affects it is a give away. I would get a carbon monoxide detector, if it goes off you have some work to do.

Ok so here is a sequence of the heater lighting and the solution for your problem...
1. Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution:You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks.Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. Please see "limits, rollout switches & furnace control boards" further down on this page. The furnace's control board might not be supplying the voltage to the ignitor. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning. Please see the pictures below to help you identify a flame sensor.
Thanks and I hope this helps. Feedback apprec.

Well these simple heaters are good but can be quite annoying. This sounds as though you have lost the fan motor. You should hear a roar after ignition and being that all you have is a blue flame and little to no heat this tells me you lost the fan motor.

If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

Check air filter for blockage, clean off flame rod found in the flame pattern of the last burner that ignites, check high limit switch with multimeter to see if good (found above the burner box). Make sure room air blower turns on within 90 seconds of first flame.

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).

The flame sensor is not sensing flame.

You can try cleaning it, but will probably have to replace it.

When the sensor does not detect flame after ignition, it will shut down the gas flow.

air gas mix racio to high on air.

This sounds like your gas valve is weak. An orange flame has to much gas and not enough air mixture. Have you trie to ajust the air mixture. If it won't ajust then you will need to replace the valve.

I hope this helps.

Remove the front panel. Tape the door switch to the closed position. If the thermostat is calling for heat the furnace should do the following: a light on the circuit board will flash and then the exhaust blower will start. A few seconds later (10-15) you should hear clicking at at the circuit board and an orange glow should be seen thru the small viewing port. A few seconds later the gas valve should open and the orange light will turn blue. If the blue flame only lasts a few seconds and then goes out, their may either be a blockage not allowing the flame to travel to all the burners and not allowing a flame sensor to sence flame, shutting down the unit. After a few cycles of not starting, the circuit board will flash a series of codes that you can read if you look on the back of the front cover. It will give you a starting point where we can research and come up witha more difinitive answer. Let us know what you find for codes.