Matt, I don't know of any DIY sources, mostly I suspect because of the
liability issues involved, but also because furnaces are usually pretty
simple.
> The first problem is that the fan doesn't run correctly when the
> thermostat
> is on automatic fan. It doesn't seem to start when it should and
> shuts off
> even while the burner is running. I suspect a bad sensor.
> There only seems to 3 sensors in the furnace-
> One for the pilot light (connected to the gas regulator)
> Another placed above the burners in the heat exchanger area (also
> connected to the gas regulator)
> And the third is placed even higher on the furnace, still in the heat
> exchanger/exhaust area. This is the one I suspect. I don't have a meter
> to check this sensor and just pulling the 2 connections and trying them
> both open and shorted seems to make no difference. Would this be a
> thermistor type sensor that would create a voltage with
> sufficient heat, or
> is it just a switch that would short or open?
Hard to say for sure, but the conventional setup is a switch, that closes
with increasing temperature. However, it's normally mounted into the heated
air outlet near the exchanger, the idea is to not start the fan until it
will blow warm air; and not shut it down until all the useful heat has been
extracted from the exchanger. This is normally the only control on the fan,
if you short across the switch the fan should run, regardless of the
thermostat setting. There may be a relay involved too, might be worth
tracing the wiring from the fan motor.
> The other main problem is that the furnace seems to run extremely
> hot, the
> metal towards the front top by the hot air output is extremely hot to the
> touch. I mean it will burn you. This doesn't seem right and since this
> portion of the furnace is only a couple of inches from the walls can't be
> safe. Could this be a cracked heat exchanger?
This is just a WAG mind you, but based on both of these complaints and your
comments about general quality; I suspect someone has wired the fan into the
overtemp limit switch ! A typical fan switch will close at something like
110-135F, and open at 80-100F. It's important that it have some hysteresis,
so the fan doesn't 'false start'. Could just be a bad switch tho.
Other possibilities include inadequate airflow, caused by ducts too small or
crushed, clogged filters, perhaps even the blower motor running backwards.
Don't laugh, I've seen this even on professionally installed systems. A
cracked exchanger normally wouldn't cause this, since the outside of the
exchanger is under positive pressure the air should leak into the combustion
chamber, rather than out of it.
I would definitely put a cheap DMM on the Christmas list for yourself. Even
a $10 cheapie from Harbor Freight is far better than nothing !
Randall
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