Winfield Hill wrote:
Fred Bloggs wrote...

Winfield Hill wrote:

With the new part in place I learned my original one was fine.

The symptom is first that the blower starts and stops repeatedly
for a few minutes after the burner starts running, before it's
hot enough to go continuously (which can mean the fan thermostat
temp is too high), and second the blower fan blips on four times
in five minutes after the burner stops running, (which can mean
the fan thermostat temp is too low - it should cycle only once).
Obviously it cannot both be too high and too low, but I reasoned
it might have lost its hysteresis. However, after observing the
same symptom with a new thermostat that obviously can't be it.

Now I think the fan blower thermostat is fine, and perhaps the
problem is the heat-exchanger is simply too dirty, despite the
furnace being "cleaned" just a few months ago. The dirt on the
control assembly makes me wonder if this part of the furnace
hasn't been cleaned in years, perhaps has never been cleaned?
But aren't they supposed to check the heat exchanger for leaks?

I'm also eying those nice Honeywell thermostats, er, excuse me,
controls, with independent adjustable high and low temp on/off
setpoints. I'm also thinking of instrumenting the whole mess,
duct and flue temps, etc., to evaluate its performance.

It is unlikely the fan control consists of a single snap disc.
It sounds like you replaced the safety cutout, a part that kills
the furnace at excessively high temperatures, usually 60oF above
normal, and is not really designed to be exercised on a regular
basis.


No, that's the "limit control" (next to the fan control), which
I assume was OK and didn't replace. Indeed, I've never observed
the burner to stop except when the thermostat is satisfied.

Sorry fellows, I did indeed replace the "fan control" - with an
adjustable version, and as a result observed the expected start-up
changes as I adjusted its temperature.


You want to look for a fan relay control center type of module,
it will have a dial for cut-in and cut-out temperatures- just
follow the wires from the blower. See the link for some examples:
http://arnoldservice.com/fan_blower_controls.htm


Hah, I've been admiring those components and others like them on
the web. My poor HVAC system (stupid cheap builders!) has none
of that stuff (plus, what I see and described matches the simple
schematic in the HVAC installation manual). It doesn't sport any
of the nice features offered by the various items on those pages.
No separate blower fan ON and OFF setpoints, no air-handler time
delay, etc., etc. :(


It is also possible that your blower has not switched to low
speed- for heating in an AC/Heat combo unit- and this will cause
the control to short cycle in some cases.


That's something I hadn't thought of, although the blower sounds
like it did last month before the new problem, etc., IIRC. But
that's certainly worth checking.



You can clip a voltmeter across the snap disc and verify that is or is
not opening the circuit causing the blips- and if so then you have a
heat exchange problem. If not then the next thing is to clip the
voltmeter on the HEAT ON 24VAC coming back from the thermostat (on the
wall)- this can be jittering but the gas burner valve is too slow of a
response to notice. If you don't have the Fluke type that catches MINs
and MAXs then you can buy a cheap 24VAC panel lamp at home improvement
store- and clip across the circuit to COM- working from where tstat
comes in forward to output side of fan control. You should be able to
see the lamp blip.