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.


--
Thanks,
- Win

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.

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.

Yep. Sounds like Win hasn't ever had to repair/maintain his furnace
before.

The fan limit switch/thermostat certainly isn't the snap disc.

And he doesn't seem to understand "bonnet" ;-)

Fan speed usually is a relay that switches motor windings and should
be controlled by the "heat/cool" flip-switch on the (wall) thermostat.
Maybe that's defective or mis-wired as well?

He speaks of "dirty". Does he understand that the burners lift right
out for cleaning. Sometimes even the heat exchange baffles do also.

If he has dirty ductwork he has a very hazardous condition on his
hands.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

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.

James Sweet wrote:
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.

KLIXON!!!!!! That's when TI had the market cornered on bimetal snap
discs for every imaginable application- you could even call them for a
special run of *one* if necessary and at no extra charge.

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.


--
Thanks,
- Win

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.