Furnace blower doesn't work with heat

[Fixxxer]

I've got two natural gas furnaces: one for the second floor, in the attic, and one for the first floor, in the crawlspace. Both are a TempStar "High Efficiency Furnace 5000" (model: NTC5075BFA2, Honeywell controller: S8600M). Each floor has its own electronic/programmable Hunter thermostat in which the face can be removed. Yesterday, it was cold enough that I had to turn the heat on for the first time this year. The downstairs furnace came on just fine, and I thought the upstairs one did too, but I noticed that it wasn't blowing any air.

I climbed up into the attic to check it out, and I noticed that the pilot hadn't even ignited. The only thing happening was the vent (exhaust?) fan was coming on. I checked the resistance of the pressure switch, and it appeared to be functioning normally. There are three terminals on the Honeywell gas valve: MV, MV/PV, and PV. When I checked their voltage, MV and MV/PV were both at 0v, and PV was at about 10v.

I'm not really familliar with furnaces, but I believe that I saw four different limit switches in mine. Each one gives a reading of about 24v on each terminal.

After checking all of my connections for corrosion, I closed the furnace back up, and it still would not ignite. So, I went over to the thermostat, removed it's face from the wall (so I could take a look at the wiring), and then put it back. This time, when I turned on the heat, I heard the burners ignite! (Woo Hoo!) Unfortunately, the blower didn't come on...

I figured that there might be something wrong with the thermostat, since removing and re-seating it seemed to fix the ignition problem. So, I took the one from downstairs and plugged it in upstairs... Same thing - burners, but no blower. I switched the thermostats back around and verified that the blower was set to Auto. I then tried switching the blower to ON from the thermostat, and it come on. Next, I tried turning on the A/C, and the blower came on then too. So, it appears that it's only having trouble coming on with the heat. What could be wrong?

Thanks!

[hvacwiz]

circuit board in your furnace seems to be the problem if i am reading correctly.On the call for heat your blower motor turns on with a timer thats part of a reley on furnace circuit board. you have to replace board, reley is solderd on.