Jim Thompson wrote:
On Sat, 26 Nov 2005 02:47:37 GMT, Fred Bloggs
wrote:



Winfield Hill wrote:

Jim Thompson wrote...


On 25 Nov 2005 06:12:52 -0800, Winfield Hill
wrote:



Jim Thompson wrote...


BTW: http://www.repairclinic.com

Hmm, repairing my furnace, but the appliance Repair Clinic
folks don't show those in their list.

My forced-air furnace appears to need a new blower-motor
thermostat, which I've discovered the industry calls a
snap disc fan control. Hah, no wonder I couldn't find it
with Google!

I'm off to explore for a plumbing and heating distributor
that'll sell to the general public, perhaps F. W. Webb?

Win, Google on...

switch OR thermostat "fan limit"


OK, except they reserve the word thermostat for the wall-mounted
unit. Anyway, I had quickly gotten the correct name (fan control),
complete with data sheets from two manufacturers, off the web.
My problem was finding a supplier with one in stock. I finally
settled for a snap disc ($12) rated at a higher temperature, and
installed same.



BTDT back when my previous house had a natural gas furnace.

It turns on the fan when the bonnet temperature warms up, preventing
blowing cold air; and likewise "over-runs" until the bonnet cools.

When mine failed I simply jumpered it until I got the part.


Note: they call it a heat exchanger.

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. 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 . Then you want to copy
down manu name and p/n on the control, call any of these people on the
web and get them to cross it. 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.




My old furnace was setup that way, it had a Klixon thermostat disc on
the heat exchanger that controlled the fan, and then a thermal fuse a
bit higher up that would open if it got too hot and shut down the whole
unit. It was built in 1977 so things may have changed, I haven't done a
lot of poking around in my new one but it's substantially more complex.