Hi. My gas furnace is going on every 4-5 minutes. The furnace was inspected recently and the tech siad that it was functioning OK. The temperature outside is approx -7 celsius and the wind chill makes it approx - 16 celsius. What could be the problem?

Well, many options here, but the best would be to have the tech back and look over the system now that it is acting up. Most likely furnace was working fine when he was there, but now there it s problem that need to be looked into. Hopefully its as simple as a faulty thermostat. Good luck and please let us kinow how things go.

It must not be running very long when it runs?

First thing I would suspect is the limit switch - did the tech mess with it?
Maybe the low limit (where it shuts the blower off) somehow got adjusted
Higher? Or maybe the limit switch is having a problem.

If not the blower shutting off too soon, maybe your thermostat is
Having problems and shutting the burner down too soon.

Maybe listen & see if the burner is shutting off or if it's the blower,
Then you'd have an idea which check, the thermostat or the limit switch.

You can see the temp setting on the dial in the limit switch -
The upper one is the temp inside the heat exchanger where the blower
Should turn on, the lower one is where it should shut the blower off.

You have to be sure to hold the wheel if you move the pointers.
I was doing some checking on mine lately and turned the wheel
Just a little bit unintentionally and the burner would shut off before
The blower would start. Rotated the dial just a little now it's OK.