The cleaner was also a technician, so he also did basic maintenance and a 21
point inspection on the motor; I just threw the question out there to get
his thoughts after he had finished.

And yes, after the ducts were "cleaned", I wondered if the whole procedure
was even necessary. (but agreed, lets avoid that issue for now)

I ran the furnace through a few cycles with the existing limit switch, and
it seems the limit switch is indeed faulty. When the burner begins the
heating (prior to the blower kicking in), the limit switch dial slowly
starts to rotate to reflect the increasing temperature in the plenum. But
the On/Off settings on the limit switch's dial don't rotate with the dial.
So the "Off" setting may be set to 90, but the dial turns past the setting.
Hard to explain, but the end result was that the furnace didn't stop running
when it hit 90.

No work had been done to the heating system before this behaviour started,
and as long as i've lived in the house (about three years) the furnace has
always run this way.

"Amun" wrote in message
.. .

"Jay Luke" wrote in message
news:7wISe.224715$HI.196868@edtnps84...
Since I bought the house three years ago, the furnace has always operated
as
follows:

- kicks in and blows heat for 30-45 seconds
- fan stops for 10-15 seconds
- repeats this cycle for 30 to 40 minutes, then the fan and the
heating
stop
- 60 to 90 minutes later, starts all over

When I had my furnace vents professionaly cleaned the other day, I asked
the
technician what his thoughts were, and he stated that this behaviour is
due
to a faulty limit switch on the furnace. He quoted a price of $300 plus
tax
to install a new one.

After researching online and verifying for myself that this was rapid
cycling, I went down to the local hardware store and found a new
replacement
limiter switch for $40, so I purchased it. Installing should be
straightforward since the existing switch is exposed and already wired,
so
I
just need to set the start/stop limits on the switch.

How are the off/on temperature settings determined? The default settings
on
the switch (out-of-box) a
Off: 95
On: 135
Limit 210

I can't refer to the current settings on the existing switch because the
tech had moved them around before leaving.

Anyone have any knowledge or advice on this?

Cheers in advance - jay




Forgive me for asking.
But why would you trust a duct cleaner to work on your furnace ???

(avoiding the issue altogether that duct cleaning is often a waste of
money)

From what you said the old fan limit switch was working fine but might
have
simply needed a slight adjustment.

It would help to know if the blower ever worked differently, and just
"changed" recently.

Or if any work has been done to the heating system before this problem
occurred.


AMUN