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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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

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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.
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wrote:

Is the furnace shutting off or just the fan? +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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Is your filter clean?
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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.
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wrote:

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.
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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.
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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.
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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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wrote:

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? +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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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.
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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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I would guess that is did get to the target temperature. Are you certain that the thermostat is still asking for heat or simply going by what the temperature reads?
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Rod Dahl wrote:

False emergency shut down like over temp. ? Tony
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If it was a limit shutdown the fan would still run. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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