Heat/fan come on at same time
When the thermostat calls for heat, the fan kicks on immediately, heats home until thermostat says it is warm enough. Heat gas kicks off and fan dies. 30 seconds later, the fan limit control kicks fan on again for about 1 minute and then stops.
I don't think this is a fan limit control problem. What could be happening?
Scott
It is supposed to work that way!
Turns out that the combustion relay is providing power to the ignitor, gas valve, the combustion air fan, AND the low-speed blower fan and the schematics show that it should be that way.
That means that this furnace is NOT waiting for the chamber to heat up and then the limit switch turns on the blower fan. Since the limit control high setting was at 140 or so, it NEVER turns on the fan. Thus when the thermostat stops asking for heat, the combustion relay shuts off and removes power to the low-speed blower fan. Heat remaining in the chamber is causing the limit switch to finally kick back on to cool down the chamber (thus the furnace running, shutting off, and the fan kicking back on 30 seconds later for several minutes)
Here is what I have done and I hope it makes sense:
I LOWERED the high limit switch setting. I turned on the heat and let it run for 3 minutes to get a true HIGH limit reading on the dial, then turned the high limit switch down to just a little below that. Low setting is now at about 90 and high limit is at about 125 or so (who knows how accurate the dial is?? )
So here is what happens now,
Thermostat asks for heat. Combustion relay turns on gas, ignitor, combustion air blower and low-speed blower fan.
Chamber heats up to more than limit switch (~125 degrees) and turns on low-speed blower fan (already running)
Thermostat quits asking for heat, drops gas, ignitor, compustion air blower and low-speed blower fan. However because the limit switch has now turned on, the fan continues to run until chamber cools to 90.
Does this make sense?
I guess I've never seen a furnace that just turns on the blower from the start, I thought they were always supposed to heat up first.
Scott