[wongism1]

I have an electric furnace and it has started to blow cold air. I replaced the batteries in the thermostat and the warm air came back until it heated up to the set temperature then when the house cooled back down and the furnace came back on it would only blow cool air again. I changed the filter, but this did not help. I took the batteries out and put them back in and after a couple times the warm air came back, but the cool air came back just like last time. I replaced the thermostat and it worked OK for the night, but started doing the same thing again.
Any help would be most appreciated
Mike

[labman]

This is an electric furnace depending on resistance elements, and not an outside compressor for heat?

Go to furnace. Likely in the compartment where the filters are is a control board where the wires from the thermostat connect. Connect the R terminal and W terminal. More complicated furnaces may have RH and W1, but look for the red and white wires. If the furnace then heats, the problem is in the thermostat or wiring to it. If not, the problem is likely on the control board. Before changing it, check for a separate relay controlling the heating elements. It should have 6 wires, 2 heavy ones from the house, and 2 more like them to the heating elements. The other 2 will be small wires likely white and blue coming from the control board. If you have power to them, and power from the house, but not power to the heating elements, the relay is bad. You should be able to replace it.

You could have a sensor or limit switch signaling the control board that the furnace is too hot. That keeps the circuit board from sending power to the small wires. See what you can find and post back. If the thermostat, relay, and sensors are OK, likely the circuit board is bad. They are hard to troubleshoot and expensive to replace.