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sorry for the long read, but I have a few questions in regards to my furnace.

I have an older system from the early 2000's (Honeywell PC8900 and a Lennox g50uh(x) furnace) in my newish house. It seems to be working but possibly poorly. I have it set to heat at 72 degrees (fan is set to auto) however, when the furnace kicks on, it seems like it blows "cool" air for quite some time (several minutes) before the air gets warm. House seems warm enough, some rooms warmer than others but it seems to be working. Is this strange? I have checked, and the burners light fine. I read through the manual and the only relatable section says the blower might be on for 20 seconds before the burner is lit. However, it also says that after 3 or 4 tries it goes into a sort of "safe" mode which does not appear to happen. I have zero fault lights/codes on my furnace either. With the lack of going into safe mode and a lack of fault codes, I am assuming the furnace is working as it should.

The thermostat however, is a different story. A couple of months ago It started displaying a "Humidity" sensor error. Per the manual, it says that the sensor is built into the thermostat control panel and will basically have to be replaced; unfortunately it is discontinued and eBay thermostats of the same model are twice the price of a new modern panel. I checked my wiring on the furnace and there does not appear to be anything connected to the "Humidity controls" so I want to think that my "Humidity" sensor error is not problematic (or leading to other issues) seeing as how I have no components to be controlled. Am I correct about this, or would my furnace still kick on hot or cold if the humidity gets too low or cold?

I might just go ahead and replace the thermostat with something modern but I have a few wiring questions. On the back of my thermostat panel, I only have four (4) wires. They are more-or-less labeled 1, 2, 3 and 4. These wires lead to a separate box (mounted to my furnace, but part of the thermostat components) which have the standard wire inputs (C, Y, W, R, etc) as well as inputs for remote sensors. The wires from the furnace go to this box. Obviously, the numbered wires at my thermostat panel won't carry over to a new thermostat as is. Can I just disconnect those wires at the separate box and then wire those to the C, y, w, r, etc wires on the furnace? Most likely, I will just run new 18/5 wire from the furnace to the thermostat panel wall seeing as how I only have 4 wires currently.

Common sense tells me this will be fine, but when I run a google search for replacing my currently thermostat, it comes up with a lot of people retrofitting all sorts of things to make the wires compatible. If anyone cares to chime in that would be great. Thanks!

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  • If you jumper R to W at the interface box, does the heat come on straight away or does the furnace still blow cold air foir 20s? – ThreePhaseEel Jan 17 at 15:57
  • Would jumping the two contacts cause the heat to turn on right away once I turn on the furnace? – ewrjontan Jan 18 at 7:18
  • Yes, that is correct – ThreePhaseEel Jan 18 at 12:12

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