The furnace heats up and fan works when in the fan on position. When I put it in auto the thermostat says heat is on but fan never turns on and it does not heat the house.

My heil high efficiency furnace fan works in the on position but pilot doesn't ignite and heat the house. When I tried in cool, the same thing. Can runs in on but not in auto. Any thoughts?

Is this oil or gas?

Fan-On puts the unit on in cooling-speed.

There is some type of Fan/Limit Control for the heat.

It has a probe usually right above the heat exchanger that senses the temperature of the air and will energize the blower after it runs and heats up. This control will also shut off the burner if reaches it's Limit (and keeps power to the blower energized).

The older mechanical type has a silver cover (Honeywell and White Rogers are common manufacturers) and a dial-type mechanism with three set points: Off, On, Limit which are adjustable. This is usually located inside the jacket-cover towards the top of the unit. There will be typically a sliver BX wire-connector(s) going to it. If you remove the cover you can actually see the dial slowly turning as it heats-up.

You must wait for the sensor to reach temperature for the blower to come on. This could take several minutes depending on the size of the unit.

New models use a fan-timer board and have only a limit sensor (you will see two wires going in towards the top of the inside of the jacket to a small square-looking removable probe). If you provide a model number I can provide you with more help.

We tried to start our heater and it won't go on. The green light in on on the little box. The model isa SU9541. At least that's the only number I saw. The heater was put in about 3 years ago. We put the thermostat up to 83 degrees but it still won't go on.

One simple question first, does your thermostat have batteries that may need replacement?

You said that heat and ac doesn't work, but you put it to fan-on and the blower runs, right?

I would pull the thermostat off the sub-base, take a jumper-wire (any copper wire, paperclip, etc. will work). Connect the RH (or just R if there is no RH) to W. The burner should come on, and after a short period the blower should run. If this works then the thermostat is bad.