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Subject Author Date
Forced Hot air limit switch JOHN F 12-28-2006
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Posted by DK on December 28, 2006, 12:08 pm


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

>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?






Posted by on December 28, 2006, 2:20 pm



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


Posted by Todd H. on December 28, 2006, 3:21 pm



> 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/

Posted by CJT on December 28, 2006, 5:15 pm


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.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

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