I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months. Part number HH 12zB and it has 200 degree rating.
All power feeds measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I have
lined out limit switch for short durations ( middle of night and heat
needed), but I consider this a fire hazard over long term, plus unit runs
continually in that mode. With defective limit switch, fan runs
continuously when themostat calls for heat, but naturally the unit doesn't
actuate the pilot, so get cold air only.

Parts are fairly cheap..like less than $10, but more an irritation and
concern when traveling.

Thanks

First, I'm no HVAC tech, but I do maintain mine.
Just a thought;
I wonder what would happen if you put 2 in parallel.

Eric in North TX wrote:
First, I'm no HVAC tech, but I do maintain mine.
Just a thought;
I wonder what would happen if you put 2 in parallel.

Hmmm,
Not a good idea.
They won't work in unison and defeat the purpose.
Can you increase blower speed?

On Thu, 28 Dec 2006 09:57:40 -0600, "JOHN F"
wrote:

I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months. Part number HH 12zB and it has 200 degree rating.
All power feeds measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I have
lined out limit switch for short durations ( middle of night and heat
needed), but I consider this a fire hazard over long term, plus unit runs
continually in that mode. With defective limit switch, fan runs
continuously when themostat calls for heat, but naturally the unit doesn't
actuate the pilot, so get cold air only.

Parts are fairly cheap..like less than $10, but more an irritation and
concern when traveling.

Thanks

So what is happening to them? Did you smash one to bits and look
for clues with the parts? You do know there are different mfgs of the
things, don't you?

"I have lined out limit switch for short durations ( middle of night
and heat
needed), but I consider this a fire hazard over long term, plus unit
runs
continually in that mode."

Can you use the fan switch in your thermostat instead of doing this?

And where are you getting $10 limit switches from? I thought they cost
more like $40?
Link?

I had a problem with mine once, and it turned out to be the crappy push
in terminals on the switch. Replacement models have both the crappy
push in terminals and regular screw terminals (like the back of an
electrical outlet).

JOHN F wrote:
I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months. Part number HH 12zB and it has 200 degree rating.
All power feeds measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I have
lined out limit switch for short durations ( middle of night and heat
needed), but I consider this a fire hazard over long term, plus unit runs
continually in that mode. With defective limit switch, fan runs
continuously when themostat calls for heat, but naturally the unit doesn't
actuate the pilot, so get cold air only.

Parts are fairly cheap..like less than $10, but more an irritation and
concern when traveling.

Thanks

First, you're confusing "limit switch" with "fan switch", both of which
often
occupy the same enclosure.

J

I had a problem with mine once, and it turned out to be the crappy push
in terminals on the switch. Replacement models have both the crappy
push in terminals and regular screw terminals (like the back of an
electrical outlet).

I had the same problem or similar. The push in connectors were
different on the new switch compared to the original.The furnaces
wiring would go into the push in connector but didn't appear to be
very secure, so I soldered on larger gauge pigtails to the original
wiring. I then plugged the proper sized wire into the connectors.

I wonder if his connectors are arcing and failing?

"JOHN F" writes:

I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months.

Hrmm.

Part number HH 12zB and it has 200 degree rating. All power feeds
measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I
have lined out limit switch for short durations ( middle of night
and heat needed), but I consider this a fire hazard over long term,

Wise!

plus unit runs continually in that mode. With defective limit
switch, fan runs continuously when themostat calls for heat, but
naturally the unit doesn't actuate the pilot, so get cold air only.

I'm curious about the root cause of these recurring failures.

It is possible that the limit switches are doing their job and that
there is a real temperature rise issue in your furnace that needs to
be addressed? Clean filters, fan running normal speed, etc?

Best Regards,
--
Todd H.
http://www.toddh.net/

JOHN F wrote:
I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months. Part number HH 12zB and it has 200 degree rating.
All power feeds measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I have
lined out limit switch for short durations ( middle of night and heat
needed), but I consider this a fire hazard over long term, plus unit runs
continually in that mode. With defective limit switch, fan runs
continuously when themostat calls for heat, but naturally the unit doesn't
actuate the pilot, so get cold air only.

Parts are fairly cheap..like less than $10, but more an irritation and
concern when traveling.

Thanks

The "limit" part of a limit switch is a safety device which should never
be exercised in a proper installation, because the furnace should never
reach the temperature to which the limit switch is set.

If yours is activating, you have another problem. As others have
suggested, it could be a fan set to the wrong speed, an incorrect
limit temperature, a clogged filter, and/or incorrectly sized (or
blocked) ducts.

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