Question

Dear HVAC wisdom God:

I have a forced air heater-A/C combination in the attic of my house. It is about 7 to 9 years old. After about three years the honeywell gas control valve quit working... so I replaced that and it was fine. In general I notice that during the heating cycle-- that the exhaust fan motor would spin up, then you could hear the flame-up cycle come on and then the blower fan would come on. But a few month's ago I noticed that it would go thru the cycle and then after just a few minutes and long before it reached the thermostat setting, that the flame would go out and the fan turn off... then in a few moments it would spin up the exhaust fan, the flame would come on, but the fan would not come on and the flame would go off, but the exhaust fan would run for 30 minutes before the flame would come back on. If I turned off the digital thermostat and wait for a few minutes and then turn it back on, it would run normally form a few hours... but I knew a new malfunction had developed...

Now a few days ago I turn it on A/C and the compressor came on outside, but blower fan did not come on. So I turned it off. I set the switch to "Fan On" and the fan would not come on. I set it back to auto and turned on the heat, but the fan would not come on.

Soooo - I pulled out the blower motor thinking that might be it... I did notice first that when the thermostat fan was set to on, that there was a very clear solid click that came from the computer black box as I call it -- mounted on the blower fan door. Normally, with that click, I would expect at least the fan would come on. But it did not, so thinking it was the blower motor, I pulled it out... took it out of the housing and direct connected it and it ran just fine. But, the shaft was clearly slightly bent, so I decided now was a good time to change it anyway... so I did and tested it successfully -- with the new capacitor intact before I put the blower unit back into the plenum cavity. I'm sounding like a gyneocologist, sorry...

Now it did not work, which I did not expect it to, but at least the new motor is in. I looked at the schematic and it showed a relay... but in looking at the actual wiring there is no relay... but there is a 24 vac transformer - put it on the v-meter and it was fine... 24-25 volts AC constant. So I am thinking that even though the schematic shows the relay in line upstream from the black box, that actually, by the time it left the factory they probably put in a black box with the relay built in... What do you think. Now there is still a strong click when the thermostat switch is set to ON, but there is a humming sound that comes from the black box that doesn't sound right to me... what's your take on all of this??? What do you suggest I do next?

Honeywell black box Model:ST9141A1002    Part #406650

Bob