I posted about this problem a couple of weeks ago and received some welcome advice. Now, I suspect that I know what the source of the problem is, but I am hoping that someone here can confirm it or else point me in the right direction.
The problem is that our indoor fan blower (York furnace, installed in 2006) has had a habit of coming on at times when it is not supposed to. This happens most often between - A.M. and noon, and between - P.M. and - P.M. Sometimes, it will come on only once on any given day, and sometimes it will come on two or three times. When it does come on, it stays on for anywhere from three to fifteen minutes and then shuts off on its own. Otherwise, the heater functions normally. Usually, this happens a fair amount of time after the heater has been on, and so the problem is not that the furnace is overheating (the system has been tested for overheating as well, so we know that’s not the problem). Our HVAC serviceman, who appears to have given up on pursuing this problem further and has stopped returning my e-mails, has already - in the course of investigating this problem - replaced the circuit board and the gas valve, so both of those parts are new.
Now, in response to an earlier post I made here, it was suggested that I disconnect the thermostat entirely to see what happens. So, on two separate days, I disconnected the thermostat entirely for most of the day but left the power to the furnace turned on. On both days, the fan did NOT come on at all when the thermostat was disconnected. After reconnecting it each time, the fan problem resumed the following morning.
So, would it be safe and accurate to assume that the thermostat is the problem? It has been suggested even before I tried disconnecting it that I try replacing it anyways just to see what would happen, but, after already paying $800 for the circuit board and gas valve replacements, I would like to replace the thermostat only if it seems very likely that it is the source of the problem. It seems logical to me that, if the problem does not occur with the thermostat disconnected, then the thermostat is likely the problem. If the problem is up in the furnace area, then disconnecting the thermostat should do nothing to alter the fan’s behavior, right? I’m not an expert, though, and I am interested in knowing if the conclusion I am drawing is likely the right one or not.
Thanks so much in advance for any insight.