Furnace Limit sensor (Therm-o-disc type) question

[RandyL...@unlisted.com]

My furnace has both the Fan Limit switch control where I set the
pointers, and a secondary limit sensor which is the Therm-o-disc type.
I recentle replaced the fan limit control because the fan ran all the
time. The new control solved that problem. However, after the burner
begins, the Therm-o-disc shuts off the burner before the fan starts. I
had to set the Fan-on to around 90deg, which I know is too low.

This tells me the Therm-o-disc is shutting off too soon, and a
multimeter confirms it's shutting off. I want to replace it.

I'm a little puzzled about how to select a replacement. It has L135-30
on it. I'm pretty sure the L135 means it opens it's contacts at 135
deg. F. Which seems to be pretty common from what I was looking at on
the web. But what does the -30 mean? I know it turns the burner back
on far above 30 deg (which would be freezing inside the house).

So, what does that -30 mean?

This sensor has the following numbers on it.
200374
T-D-D 60T11
7843
L135-30

Do any of thee other numbers mean anything, aside from factory stick
numbers?

Note, the furnace is a MILLER brand, propane (mobile home) furnace from
around 1979. Aside from this problem of shutting off the burner too
soon, it's still in good shape for it's age. I did replace the blower
motor a few years ago, and that fan-limit switch last year. Otherwise
it's been no trouble.

Thanks
Randy

[mike]

http://www.hvacpartstore.com/limit-switches.html

Did you replace the fan switch with the EXACT same swtich?
Differences in thermal coupling and thermal mass could make a difference.

I'd think you'd want the fan to come on as quickly as possible
without blowing a lot of cold air. 90F sounds like plenty hot
to meet that requirement.

[Tony Hwang]

Hi,
Your fan is now coming on too late triggering the thermo-disc cut off
switch. Did you replace it with identical one? Buy the time fan comes
on, the heat chamber gets too hot.

[RandyL...@unlisted.com]

Yes, the switch is identical.

What temp should it turn ON? If I set it much lower, the OFF position
will be below 70 and if the house temp is 70, the fan will not shut off.

I also dont understand your reasoning. If the fan turns on at 90, and
the limit sensor is 135, that's 45 deg. difference. The fan should be
on much before the sensor gets to 135. That is why I think the sensor
is defective.

This still dont answer my question about the -30.

Thanks

[mike]

I gave you a link.
The answer to your question is in the first paragraph.

[Tony Hwang]

Hi,
Maybe either switch is out of calibration. To trouble-shoot I'd
disconnect the thermo-disc for a while observing the operation. It is
all in the owner's manual. My older furnace had limit switch at some
thing like 170 range.