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I have a conundrum that has been quite vexing, and any advice would be appreciated. For a number of months, our indoor fan blower has been coming on at seemingly-random times, blowing room temperature air through the vents in our house. It usually comes on 2-3 times per day, for anywhere from 3-10 minutes each time, and the times at which it comes on can vary considerably (though late morning and mid-evening seem to be more common). When it does come on, it is usually several hours after the heater has last been on (and the heater is currently operating as it should when heat is called for). Turning the thermostat switch to “Off” does not shut this random fan blower action off, and there are in fact times when the fan comes on when the switch is already set to “Off”. Now, here is where it gets interesting. We have a York furnace in the attic (which came with our new Lennar home in 2006), and, the first time an HVAC specialist came to investigate this problem, he suggested that the circuit board was faulty (it was showing some error codes which indicated a needed replacement). So, in December 2012, he replaced the circuit board. The problem continued, however. So, last week, he came out again and found an error code indicating that a flame was being sensed - but no gas. Theorizing that the gas valve was responsible (and there was evidence of water having gotten into both it and the circuit board at some point in the past due to insufficient drainage), he replaced the gas valve. Both repairs may have been necessary, but the two combined ran us almost $800 - and neither solved the problem of the fan’s erratic operation. The air filter has been replaced multiple times through this process, and I have also replaced the batteries in the thermostat (though not the thermostat itself). I did notice when I was replacing the batteries a few weeks ago that the fan came on while I had the batteries removed. So, my question is, what could be responsible for this? Could the thermostat itself be at fault? It is a programmable model, but the HVAC specialist checked and cleared all programs. If the thermostat is not the likely culprit, what could be? And how serious is this problem? My wife has asked me if this is really the sort of thing that I should worry about at all, given that we have already replaced the circuit board and gas valve. Thanks!
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furnace wiring
could be the furnace wiring to the thermostat. something's not jiving with the colors.
you'll need the wiring diagram of the furnace burner and fan relay and the inter connections of the thermostat.
disconnect the whole thermostat and see if the fan kicks on.
if it does then something is mis-wired at the furnace. also check out the fan plenum switch.
there is a switch you pull out so the fan runs continuously. the plenum switch has a dial that rotates and makes up a set of contacts that turn on the fan. this too might be faulty. -
Thanks for the suggestions. I'll try disconnecting the thermostat first when I have a chance. Just now, it came on again and remained on for 18 minutes. What I have noticed is that this problem, while it can occur at other times, seems to occur most frequently between 10:00 A.M. and noon and betwen 9:00 P.M. and 10:00 P.M. I have wondered if this is indicative of some sort of error occurring in the thermostat's programming circuits, but, as I said in my first post, the fan kicked on while I had the batteries out on one occasion and so I'm not sure what to think.Comment
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???
The thermostat operates off a small computer chip inside. a conventional thermostat uses a mercury switch that tilts to make up a set of contacts. A solid state thermostat [programmable one] uses a memory chip and a reed relay or a triac chip which has a gating circuit to turn it on and off.
with the battery removed its relying on its stored RAM memory to function.Comment
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here the answer
Pull the wires off the thermostat to see if the thermostat is the problem.
If it still happens, then the problem will be upstairs at the unit.Comment
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