You set your thermostat temperature to 75 degrees and your furnace cycles on and after sometime it reaches that temperature of 75 degrees. My question is for example, for an hour your house temperature is still reading 75 degrees on your thermostat should your furnace still cycle on (When it cycles on for roughly5 minutes stays off for roughly 15 to 20 minutes)? If so for how long?
Furnace questions
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Related Solutions
I had the exact same symptoms with my natural gas furnace, and it turned out to be the flame sensor.
It's an L-shaped metal bar (but can also be straight, depending on model) that detects if there are flames actually coming out, and turns off the gas if not. It had a white high-temp wire connecting it to the control board. Taking it out, cleaning it with some steel wool fixed the problem for me.
If yours is like the above, you'll only see the back part of it and the bracket, as the L-shaped end is inside the heating unit.
Can you look at the furnace during the time it is blowing cold air and see if the flame is on?
The thermostat has the ability to control the fan, but typically the thermostat sends a "turn heat on" signal to the furnace and the furnace controls are setup to turn the flame on for a X amount of time (to warm up the air) and then turn on the fan once the timer expired (so the air coming out of the vents is not cold). When the heat is turned on is the flame and fan going on at the same time?
Not sure about your furnace, but some have a couple of LEDs that are lit up and can be used to read a status or error codes. If you have the manual you could try to decipher the code while the cold air is running and see if it gives you a clue.
I would start by trying to figure out if the problem is with the thermostat or the furnace by installing a new thermostat and seeing if the problem goes away. If the problem is still there you can just return it. I had a furnace issue where it was going off randomly and would work in heat mode but not cool mode. It turned out that the thermostat was bad and sending intermittent signals.
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Best Answer
Thermostats come in 2 styles mechanical and electronic. Both have a dead band. What is a dead band this is a zone that the thermostat allows the furnace to continue heating above the set point or continue cooling below the set point.
Some have a dead band of +-2 degrees or A 4 degree differential from turning on to turning off. The dead band increases the run time so less starts are required lowering the power Bill. On my electronic thermostats I run +0-3 for cooling and +3-0 for heating the 1 degree costs slightly more but reduces the temp variation in the home.
If there was no dead band the furnace may be cycling on and off within 1 minute and starting the fan and heating the ductwork or starting the compressor and cooling the ductwork costing much more and rooms farthest away from the thermostat may see extremes in their temperature.
So how long would be based on the dead band and outside temperature. Mechanical thermostats may be off by a few degrees especially if mounted on an external wall. Electronic thermostats usually have the ability to shift the calibration point. Mechanical stats can be adjusted but I have seen many that are damaged because folks did not understand the built in dead band.