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?
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
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.
Does the furnace fan continue to blow or does is the burner still lit in the
There are switches that turn the fan or the burner on and off if the
temperature in the plenum is too hot or cold.
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
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.
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.
Without being there it is not possible to troubleshoot this. it does
sound as though the thermostat is shutting it off though. what brand
is the thermostat?
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The thermostat is a digital Honeywell. It has a little icon on the
display that shows when it's calling for heat. When the scenario that I
described happens, the temp on the display is typically a couple of degrees
short of the desired temperature and that 'calling for heat' icon is
displayed. After it fires up the second time and reaches the desired
temperature, the furnace shuts off normally and that icon goes blank as
well. After that point, the furnace maintains the proper temperature
throughout the day.
Unless one can measure the voltage out of the thermostat it is difficult to
tell. It is probably the "anticipator". When bringing up the temperature
several degrees they turn off prematurely waiting to see if it gets there.
I might have expected that the icon would turn off too but not necessarily.
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