Alright, I installed a new furnace a few months ago and have begun to notice an issue with the blowers. When the furnace first starts up, the exhaust blower should turn on to expel gas, then the flames are lit, then the main blower turns on. My issue is that the main and exhaust blower turn on at the same time. The house still gets heated and the furnace runs it's normal cycle, but I first get a blast of cold air. I haven't done anything to the circuits on the furnace. Could this be an issue with the thermostat calling for this to happen?
Why is the furnace blower coming on so early in the ignition process
blowerfurnacehvac
Related Solutions
I finally found some time yesterday to take a closer look at the furnace.
Gas Pressure
I started by measuring the incoming gas pressure using a gas pressure gauge. I found it to be 6 WC (inch water column), which is well within the acceptable range specified on the furnace.
Dirty Gap
Next I took a closer look at the spark gap itself, and watched closely as the spark was generated. I even recorded the spark with my video camera, which allowed me to zoom in fairly close. This is when I noticed a bit of carbon buildup on the electrodes, and wondered if it could be that simple...
Cleaning the Gap
I grabbed a bit of 220 grit sandpaper, and cleaned the electrodes up a bit. steel wool is probably the tool of choice for this job, but I didn't happen to have any on hand at the moment (not yet unpacked).
Success!
After testing the pilot ignition a few times, the problem seems to be fixed. We had a cold night last night, and the furnace fired up fine all night. As I write this, the furnace has just cycled successfully yet again.
Update
I found this YouTube video from Grayfurnaceman that describes the problem exactly, and says that the only solution is to replace the assembly. I'll be replacing the pilot assembly once the weather warms up, so we'll see if that solves the problem.
There are a couple of things that it might be: It could be the flame sensor. I have seen them where they look clean but are dirty enough that the DC voltage being generated is not enough for the circuit board to keep the gas valve open. I would pull it out and clean it with some scotch brite or a brillow pad. be care full not to damage the porcelain base. It is generally fairly easy to remove, one or maybe two 1/4 screws holding it in place. Gently pull it out and clean it with the pad like you would if you where sanding a wooden dowel. The 2nd thing it might be (less likely) could be a roll out switch. If your air filter is dirty it would prevent enough air flow across the heat exchanger to keep it from over heating causing the roll out safety switch to open. If this is neglected for a long period of time it can cause the ROS to weaken and begin to trip at lower than designed temperature. ps. there are typically LED lights on a the circuit board that will blink error codes to help in diagnostics. It is best to capture that info and will usually point you in the right direction.
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Best Answer
Most furnaces use a preset, adjustable, time delay circuit to control the start of the blower motor. Some higher end models may use a temperature sensor to determine when the blower should come on, but this is a less common setup. It's possible that something in the furnace is broken and/or configured incorrectly, but it's more likely that the thermostat has been installed incorrectly.
A thermostat in its most basic form, is simply one or more thermally activated switches. In the olden days, it was likely a mercury switch, that only controlled a heater. Most more modern thermostats control heat, A/C, and the blower fan. In your case; since you're having trouble with the fan, we'll focus mostly on the fan settings.
A typical thermostat will only have two settings for the fan,
ON, orAUTO. In theONposition, the fan will run continuously. In theAUTOposition, control of the fan is handled by the furnace instead of the thermostat. Since your fan does not run continuously, I'll assume you have the fan control on the thermostat set toAUTO.If you accidentally connected the
FANwire to theHEATterminal and/or theHEATwire, every time the thermostat called for heat the fan would immediately turn on. Since this is the easiest, cheapest fix. I'd start by making sure the thermostat is wired properly.