Hi!

My downstairs HVAC has started acting weird. It is a forced air heating, with outdoor AC unit.
When the heat turns on, there is no forced air coming out of the vents. Instead the blower fan runs very slowly. By changing the thermostat fan to "on" from "auto" does not make any difference. If I later turn off heating at the thermostat and set fan to "auto", fan keeps on running at low speed.

I don't think this is a thermostat issue. I tried shorting the cables coming to the thermostat to turn on fan or heat but it did not have any effect.

This afternoon, I tried these experiments:
1. In the attic, where the blower and furnace are located, I turned off the blower switch. The fan stops (as expected). After that, I turned back the switch and set thermostat to turn on the fan. The fan turned on immediately at normal speed. I then set thermostat fan setting to "auto" and fan turned off immediately.

2. Later, when I set the thermostat to "heating" mode and increase the set temperature, the old behavior returns. Blower fan turns on at very low speed and no hot air comes from vents. When I reduced thermostat temperature, fan keeps on running at slow peed but does not turn off.

I have also tried changing thermostat battery and furnace filter without ant effect.

What do you guys think? Bad motor, capacitor etc.

Thanks.

Hi jbh and welcome, we have a really helpful HVAC guy here but you can help by posting make and model of your system. Good luck

It is a York furnace, model LY8S100B16UH11B.As I went to check the model number, I saw the diagnostic LED flashing 11 times.Did some more experiments. After I turn on the furnace switch:1. If I turn on the air-conditioner, then LED is flashing green and system behaves normally.2. If I just turn on the fan, the LED is flashing green and system behaves normally.3. When I turn on the heat, then initially the blower turns on at normal speed, and air comes out of vent.But LED starts flashing 4 times.If I leave it in that state, blower slows down but keeps running. Then the LED is flashing 11 times. The only way to recoverfrom this state is to power-cycle the system.I am trying to dig up the manual (at home or online). However, the cryptic message on the furnace door says that4 flashes of LED: limit switch open?11 flashes of LED: limit switch open, blower fan failure.One more data point: I haven't changed the furnace filter for over 4 years. It is located high in the ceiling (over stairs) and there is a 20 feet drop next to it. Can dirty filter cause these issues?Thanks.

Hi again jbh, a dirty filter could very well be your problem I would have that changed before doing any other repairs, just be careful, at that height a fall could be fatal. Grady usually see all questions of this nature hopefully he'll chime in with his expertise. Change that filter and let us know what it does.

I changed the filter. After that, I needed to reset the limit switch. There were 4 limit switches, but only one of them was accessible easily. Luckily, after resetting that, heat is working again.

That's great, thanks for the update.