Gas Furnace Blows Cold Air during Programmable Thermostat Cycle
Our furnace works fine when set to a constant temperature, (22 degrees celsius), the gas ignites every time and the blower functions properly.
The problem occurs when I program the Honeywell thermostat to switch temperatures to 21 overnight, 19 during the day, 22 in the evening.
When it's time for the thermostat to bring the temperature up from 19 to 22 in the evening I'll hear the ignition switch click and the gas will not ignite. The blower will then turn on and circulate cold air indefinitely, (we were away for the weekend and came back to a 10 degree temperature inside, the blower had been circulating cold air constantly for 2 days). This problem also frequently occurs, (not every time but more often than not), when the temperature switches from 22 to 21 in the evening. We'll wake up to a cold house, the blower constantly running.
My ignorant solution to this is to flick the furnace ON/OFF switch numerous times, while listening for the gas to ignite. It usually does after 5 or 10 tries. I don't know whether it's a thermostat, (7 years old, it's digital, there's no auto option to switch from cold to heat during a programmable cycle so I don't think that's the problem), or furnace problem, (I thought maybe it was the electric ignition switch but it works every time when set at a constant temperature of 22 celsius). Any thoughts would be most appreciated.
