Furnace takes a very long time to reach set temperature

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Old 01-05-08, 03:03 PM
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Furnace takes a very long time to reach set temperature

I have a Rheem classic quiet 80.
The thermostat calls for heat
The igniter comes on and lights the flame
The fan runs at normal speed AND IN ABOUT 5 MINUETS THE FLAME GOES OUT and the fan keeps running. After a while the fan goes off and the igniter comes on and relights the gas.

In the time between the flame going out and then on the fan is pumping cool air.

It acts like its reaching a high limit.
All the led lights on the pc boards indicate all is OK.

Thanks
Kevin
 
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Old 01-05-08, 03:16 PM
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Good description of what is happening. That's a definite help.


You are correct, it is acting like the high limit is shutting the burners off.


So check the usual suspects---

A plugged filter. More than one filter layered on top of each other. Warm air registers closed. Cold air return covered over or plugged. Collaspesed ductwork or anything obstructing the air flow through the furnace.


Use an AC voltmeter to check to see if it's the limit that is opening, or whether it's something else like a pressure switch that's opening, a flame sensor that's dirty, erratic gas pressure and such.
 
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Old 01-05-08, 04:49 PM
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Thanks
the filter and ducts are good.
I think I know where the flame sensors are and Ill clean them

What does the penum sensor do? I got a one new for $10 and changed it already but no joy

Do you have a hint as to where the high limit sensor might be located? And whats the correct terminology on the wire diagram?

Is the limit switch wired direct to the gas valve or does it go through the pc board or a relay?

Thanks
Kevin
 
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Old 01-05-08, 05:47 PM
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Never heardf of a penum sensor. Sounds kinky.


Limit switches are often located above the burner compartment, where they measure the temperature inside the furnace. There may also be a fan switch that is similar that turns the fan on, and there may be a combination fan and limit switch combined in one part.



Your best bet is to refer to the furnace wiring diagram and trace the wiring to the limit switch.

There are lots of ways a limit switch can be wired. Loom on the wiring diagram for the high temperature limit switch and trace the wiring is your best bet.

Burner is on, the limit switch will usually have 24 VAC on both sides on the switch. If the burner shuts off on the limit, you will have 24 VAC on one side of the limit and no voltage on the other side.
 
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Old 01-05-08, 05:54 PM
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oops its plenum.

Ok Ill look around.


Thanks
 
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Old 01-06-08, 12:54 PM
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Originally Posted by rivets49 View Post
Do you have a hint as to where the high limit sensor might be located? And whats the correct terminology on the wire diagram?
Limit.

Wiring diagrams show the color of the wire coming in and the color of the wire coming out, of every device. That ALONE can help you confirm which part is the right one in the case of mulitiple switches in the diagram.
 
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