> >> Basically, the thermostat controls the burner and the limit switches
> >> control the blower based on the internal temperature. You want about
> >> 90dF low and 110 high. It lets the burner pre-heat so you don't get
> >> a cold blast, and runs the fan after the burner has shut off so you
> >> don't waste heat up the chimney.
> >
> > I've not checked it with a thermometer, but feeling the air from a
> > register seems to confirm that the fan switch is pretty close.
>
> Am I missing something?
Perhaps ... what kind of furnace do you have ?
> My fan timing is controlled by my thermostat, as in "the digital control
> thing that lives upstairs". Lots of settings in it to compensate for
> various furnace types and personal preferences.
If it's hooked to a typical fossil fuel forced air system, then it doesn't
control the main blower directly. As Wayne noted, this type of furnace has both
a startup and shutdown sequence that should be followed for best efficiency
(plus comfort, safety and long life). The optimum way to do that is with
sensors that measure the bonnet temperature directly. Some newer ones probably
do have that incorporated into an electronic control (but I'll bet you can't
replace it for $40) ... older models had independent switches.
Personally, I think that's a good thing. I don't like the idea that a $40
electronic gadget made in Taiwan has the potential to set my house on fire if it
malfunctions. (Ask yourself what happens if the thermostat starts the burner but
the fan motor doesn't run.) I'd much rather trust that function to multiple
levels of mechanical bi-metal switches. Inaccurate though they may be, they are
extremely reliable. The original ones in my 1958 natural gas dryer still work
fine, too.
> Considering this thing cost $40 at most, dealing with a mechanical one
> that may be inaccurate seems odd...
That whiz-bang thermostat is only as good as it's sensors, which are analog and
may be inaccurate. Call it personal preference, I try not to fix things that
aren't broken.
Randall
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