"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





"Jay Luke" wrote in message
news:JSmTe.165643$wr.37736@clgrps12...
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.



The whole post now is a bit pointless if you already replaced the fan
control.
As no one will take it back.

But the limits must be set in a certain order, or you will never get it to
work right.

Most have basic instructions in the cover, and that's the proper procedure
you need to use.
And if it came with any printed instructions, read them and do exactly what
it says.

From your original post I still think the original control was working fine,
but might have needed proper adjusting.

But there are also a lot of other things that have to be working right too,
If it was working right but broke/started acting up, at least we all have a
chance at offering a probable fix based on the symptoms.

But when you don't know if it ever was right, after 3 years, the problems
may be caused by countless things.

Bad parts, poor system installation, poor system design, wrong parts
installed, parts installed improperly/not adjusted properly etc.

Only someone there can check/test each part.
none of us can fix your furnace by email

Best we can offer is "reasonable guesses"
But even guesses have to be based on the system being put together right to
start with.


eg, the problem you originally described could just as easily be a
circulating fan turning too fast because of wrong motor or wrong drive belt
pulleys or a dual speed fan stuck on high

And the "technician" should have been able to test what the cause was before
he suggested "trying this" or fiddling with screws.