A few weeks ago, I noticed that the temperature in the home (as reported by the thermostat) often does not come up to what we've set it to. Typically, we set it to 22C (71.6F), but it often only makes it as high as 21C.
I have a heat pump (York YZE series) with a natural gas furnace (York YP9C) as the backup. However, during winter months I typically set the thermostat to "EM Heat" in order to use only the gas furnace.
When I first noticed and tried diagnosing the problem, the temperature was set to 24C, but could only seem to make it up to 21C. As a test, I decided to try switching to the regular heat mode and then checked outside to make sure the heat pump had come on... it had. After a while the temperature managed to get up to 22C, but couldn't seem to make it any higher. A little while later, I also noticed a fairly loud rumbling coming from the furnace so went down to check it out. The 3/8 liquid line was quite hot to the touch, only a bit cooler than the 5/8 gas line. As I was growing increasingly concerned that this was quite abnormal, I decided to switch back to EM only (at least the furnace didn't sound like it would tear itself apart that way) and called the technician the next day.
When he came out, he spent 3 hours fiddling about and ultimately decided that the filter needed replacing and that I should remove the thin dust register filters I had installed throughout the house. The filter we use claims to be good for a year, but I nevertheless changed it and clearer the registers and opened them all fully (some were partial to help adjust temp in diff rooms). As I expected, nothing has changed.
In discussing with the technician, he felt that the symptoms suggested that not enough air was flowing through the system and so was leading to not moving enough heat out of the furnace. This made sense enough to me, but when I asked about the blower or perhaps the blower control system being the problem he dismissed the idea... something about "those boards rarely fail like that". It almost seemed like it was too hard, so he couldn't be bothered (or perhaps didn't know how) to troubleshoot that type of issue.
I'm now at the point where I'd like to run some additional tests myself to try and narrow down the problem before getting someone to come out again. Anyone have any thoughts? Oh yeah, could the thermostat be at fault in some way? It isn't all that clear to me how the thermostat controls the fan.