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Furnace shuts off prematurely



 
 
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  #1  
Old October 10th 05, 12:22 AM
Rod Dahl
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Default Furnace shuts off prematurely

I have a 4 year old Lennox gas forced air furnace. When I get up in the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off. The
digital thermostat still displays the icon calling for heat, but the furnace
stays off for 5 or 10 minutes and then starts up again and finishes heating
the house to 72. After that, it maintains the temperature perfectly.
It seems the thermostat is working properly because it shows that it's
calling for heat. Why would the furnace shut off prematurely like that?
Thanks


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  #2  
Old October 10th 05, 12:30 AM
Rich256
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Default Furnace shuts off prematurely


"Rod Dahl" wrote in message
...
I have a 4 year old Lennox gas forced air furnace. When I get up in the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off.

The
digital thermostat still displays the icon calling for heat, but the

furnace
stays off for 5 or 10 minutes and then starts up again and finishes

heating
the house to 72. After that, it maintains the temperature perfectly.
It seems the thermostat is working properly because it shows that it's
calling for heat. Why would the furnace shut off prematurely like that?
Thanks


Does the furnace fan continue to blow or does is the burner still lit in the
furnace?

There are switches that turn the fan or the burner on and off if the
temperature in the plenum is too hot or cold.





  #3  
Old October 10th 05, 12:42 AM
RBM
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Default Furnace shuts off prematurely

It reached its high limit temperature, shut down the flame until the blower
cooled it down, then restarted. Once the room temperature was raised it was
able to satisfy the thermostat before reaching its high temperature limit in
future cycles



"Rod Dahl" wrote in message
...
I have a 4 year old Lennox gas forced air furnace. When I get up in the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off. The
digital thermostat still displays the icon calling for heat, but the
furnace stays off for 5 or 10 minutes and then starts up again and finishes
heating the house to 72. After that, it maintains the temperature
perfectly. It seems the thermostat is working properly because it shows
that it's calling for heat. Why would the furnace shut off prematurely
like that?
Thanks



  #4  
Old October 10th 05, 01:54 AM
hvactech2
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Posts: n/a
Default Furnace shuts off prematurely

On Sun, 9 Oct 2005 17:22:24 -0500, "Rod Dahl"
wrote:

I have a 4 year old Lennox gas forced air furnace. When I get up in the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off. The
digital thermostat still displays the icon calling for heat, but the furnace
stays off for 5 or 10 minutes and then starts up again and finishes heating
the house to 72. After that, it maintains the temperature perfectly.
It seems the thermostat is working properly because it shows that it's
calling for heat. Why would the furnace shut off prematurely like that?
Thanks


Is the furnace shutting off or just the fan?
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  #5  
Old October 10th 05, 03:25 AM
HeatMan
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Default Furnace shuts off prematurely


"hvactech2" wrote in message
...
On Sun, 9 Oct 2005 17:22:24 -0500, "Rod Dahl"
wrote:

I have a 4 year old Lennox gas forced air furnace. When I get up in

the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off.

The
digital thermostat still displays the icon calling for heat, but the

furnace
stays off for 5 or 10 minutes and then starts up again and finishes

heating
the house to 72. After that, it maintains the temperature perfectly.
It seems the thermostat is working properly because it shows that it's
calling for heat. Why would the furnace shut off prematurely like

that?
Thanks


Is the furnace shutting off or just the fan?


Is your filter clean?


  #6  
Old October 10th 05, 04:14 AM
[email protected]
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Default Furnace shuts off prematurely

Is there any chance it is the digital thermostat trying to gradually
raise the temperature? Some digital thermostats do try to cycle off
and on to avoid overshooting the setpoint. The only way to know if the
thermostat were actually calling for heat would be to hook up a volt
meter and watch when the circuit were opening and closing.

  #7  
Old October 10th 05, 09:23 AM
PaPaPeng
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Default Furnace shuts off prematurely

On Sun, 9 Oct 2005 17:22:24 -0500, "Rod Dahl"
wrote:

I have a 4 year old Lennox gas forced air furnace. When I get up in the
morning and turn the thermostat up from say 68 degrees to 72 degrees, the
furnace fires up and heats the house up to 70 degrees and shuts off. The
digital thermostat still displays the icon calling for heat, but the furnace
stays off for 5 or 10 minutes and then starts up again and finishes heating
the house to 72. After that, it maintains the temperature perfectly.
It seems the thermostat is working properly because it shows that it's
calling for heat. Why would the furnace shut off prematurely like that?
Thanks



Its a common occurence. The heat exchanger heated up faster than the
fan could kick in cool it to below the upper shut off temperature. By
the restart all the parts would have warmed up for the sensors to work
correctly.

  #8  
Old October 10th 05, 04:49 PM
Rod Dahl
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Default Furnace shuts off prematurely

The burner and fan shut off just as it would normally. I keep the filters
clean...it has both a wire mesh type filter and an electronic air cleaner
built into the return.
I suspect that you're right about it reaching the high set point and cycling
off as a safety feature. So then , is there something that can be done
or adjusted so that it will reach the thermostat setting in one cycle or is
this the way it has to be? It's not a big problem, I've just always
been curious about it.
Thanks to all who responded.


  #9  
Old October 10th 05, 06:21 PM
Rich256
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Default Furnace shuts off prematurely


"Rod Dahl" wrote in message
...
The burner and fan shut off just as it would normally. I keep the

filters
clean...it has both a wire mesh type filter and an electronic air cleaner
built into the return.
I suspect that you're right about it reaching the high set point and

cycling
off as a safety feature. So then , is there something that can be done
or adjusted so that it will reach the thermostat setting in one cycle or

is
this the way it has to be? It's not a big problem, I've just always
been curious about it.
Thanks to all who responded.



If it reached the high safety point then the fan should continue to run. I
think there are often two switch settings. One as to when the burner turns
off and another for when the fan will turn on and off.

For example there may be a fan on switch setting of 125F and a fan off of
100F. The high temperature limit setting might be about 180F.


  #10  
Old October 11th 05, 06:58 AM
Rod Dahl
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Default Furnace shuts off prematurely

When this happens, everything shuts off.....burner and fan. It just sits
there, thinks it over for a few minutes and fires up again. The first few
times it happened, I thought it had reached the temp I'd set.....until I
checked and realized it was a couple of degrees short of the target temp.


 




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