AC vent letting cold air in when off.

[NC1]

Hi, I have an air conditioning system in my house. in the lower floors there is almost feels like outside cold air coming out of vent even the the system and hrv is off. Upstairs nothing out of the vents. My tech says there is no leakage and each vent is pulled off the main line. He says air needs something to help it move so he can't figure out why this is happening. He tried to say maybe i had negative pressure in the house but as air is pulled into the ac carrier unit in the attic through a 12 by 12 filtered opening in the top floor of my house. He covered that with plastic and air is still coming in downstairs. He cannot figure it out. Any ideas would be helpful. Thanks.

 

[Dana]

Ducts in a cold attic above the insulation layer will have convection issues related to the denser cold air falling down the vents. The amount of convective drive is higher when there is more vertical involved, and if any of the vertical runs from the attic to the first floor are outside the insulation envelope of the house it'll be even worse. There is probably still convection in the upper floor vents, but the velocity & volume would be a tiny fraction of what happens with a 10-15' vertical run. In fact, the draw from the convecting down the taller runs may be drawing air into the upstairs supply ducts.

Ideally ducts would never be installed in unconditioned attics, and stay completely within the pressure & thermal boundary of the house, but that can be pretty difficult & expensive to do as a retrofit.