In 2002 I purchased a 90K BTU mid efficient carrier gas furnace and it was installed in a home that was build 30years ago. We have our ducts cleaned every 3years and this May was the year for the cleaning. It has been cold of late and we have started our furnace. We noticed that in the 2 top front rooms, we have 2 registers and they are blowing cool air; especially in my 6month old son's room where we find really cold at nights. In the remaining parts of the home in the back and on the ground floor the registers are blowing warm air. Can you please advise of what the problem can be before I have to make a service call? I received 1year warranty on my gas furnace and we have a 20year manufacturer warranty on the heat exchange. I would really like to resolve this as it is getting colder here in Ontario, Canada. Also, this greatly affects my 6month old son.
Any information or suggestions will be greatly appreciated. Thx!

I don't understand what is happening at all. Except for your PM, I would have let this question go. Actually, I would be just as happy if TTop or Acetc would give a good answer to it and other questions.

It has worked fine the last 4 years, but now part of the system is blowing cold air? Could anything have changed over the summer to expose the ducts supplying cold air to the outside air?

I advise you to contact a reputable HVAC company and find out exactly what is going on with your system. You can check your ducts to make sure they are all connected and also check your air filter to be sure it is clean but other than that what you have is a very strange situation and anything we post here would be just guess work. The air from the supply duct should not be cold.

Thanks for the advice. I did check the register in my son's room and found that it was somewhat disconnected.