Hi
I have a heater/AC unit in my attic. It is gas. I have a two large ducts coming from the main unit that lead to boxes that then feed several smaller ducts. When the heat is not on, I can feel a cold draft coming from most of these vents. The temperature in the duct, when the heat is not running, is 50 degrees. I have wrapped all the junction boxes with additional insulation. I found one disconnected vent pipe that I re-attached. When this occurs, the outside temperature is usually below 40 degress.
Is this normal? Is this caused by an air leak? If so, how do I find it?
Thanks

Are your ducts insulated? I'm asking because if they're just tin then the temperature change would transfer more quickly to the normal attic temperature. Any difference like that would find a way to come back inside.

Further, it's difficult to fully seal a whole house let alone the heating system.

Recognize that when the bathroom fan is functioning it causes a negative air pressure which will be compensated from the outside.

Anyway, how to find it? Do you have any incense sticks? Have your house settle completely say overnight without any blowers. Then on the morning of a windy day light an incense stick (candles are an open flame and not recommended) then walk around the house slowly at any place you'd want to check for air flow. There's a tight seal if the inscense smoke isn't turbulant.

When you see the smoke roll and wave you've found where air is moving and you'll eventually find what you're looking for.

Good luck.