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Today at 6:15 am, my furnace had no heat. My programmable white rodgers thermostat was set for 72 but the house temperature was 68 on automatic. I set the thermostat to on from automatic and the blower motor on the furnace started up. A few minutes later I felt heat come out of the vent.
I later switched the thermostat back to automatic and it is now working fine. The white rodgers thermostat does not display any warning of low battery at all. The batteries are about 1year and 6 months old and the thermostat is 2 years old. I have 2 years left on the furnace warranty.
Any idea what could have caused the initial problem?

What is the model number on the thermostat? Most thermostats don't run off the batteries, they are just there as a back up if the power goes out. Also what is the make and model # o the furnace?

My White-Rodgers 1F78 t-stat DOES operate off the batteries but also has a "LOW BATTERY" display. When batteries are low for a period of 4 weeks, the stat will revert to a 10 degree higher setpoint for cooling and a 10 degree lower setpoint for heating. I like this stat, it maintains a consistent temperature and features a +/- 3 degree adjustable room temperature display (I adjusted mine 1 degree according to my digital temperature meter).

That being said, I don't know how your stat might malfunction once the batteries are low or dead. Seems odd that the heat would come on by setting the stat to the "fan on" position, but I've heard of stranger things happening. Can't hurt to change the batteries but suspect you may need to reprogram the stat.

The White Rodgers thermostat is a 750 which runs off the battery. I measured the voltage in the circuit as 1.7 volts. I put in two new alkaline batteries and these measure under load 3.14 volts. The furnace company is going to change the limit switch in the fall since the furnace is still under warranty. The thermostat has a relay on it that gets enerized to turn on the furnace.
This can be veried when you hear the click sound for the White Rodgers. I am going to replace this two year thermostat incase it is an intermittent problem. I checked the programming and that looks ok. The furnace is a two stage high efficiency 80,000 btu with an electrically commumated motor (Brushless DC Motor). The furnace runs the first stage for about 20 minutes and if the temperature is not safisfed then it switches all of the burners on and the motor starts running at the faster speed.

Those furnaces are sweet. I installed a Trane 90% efficient furnace in my "real" home with a variable speed blower motor that I could set by DIP switches to get the temperature rise and cfm exactly where I wanted it. It was also a 2-stage gas valve and control module. I miss it. What make/model is yours?

Gordo48111, It is a York which is the same as a Coleman which is manufactured by Johnson Controls.
It is about 8 years old. It also has a ventor motor. The startup capacitor went last year on the motor and then 4 weeks later the motor seized up. At least it was fixed under warranty.
You can also use a regular single stage thermostat on it.

So, I would take your make and model information and call a few HVAC dealers and ask about the probability that you could get a new AC system that uses the old evaporator coil if it is still working when the compressor fails. Also ask about the expected life of the make and model you have now. If they tell you the coil won't work with any modern system and the expected life is 15 years, its an easier decision than if they say you can still find a system that works with that coil and they usually last 20 years. I just don't know the answer for your make and model, but an HVAC shop should be able to give you some idea.



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