Did this with the fireplace direct vent gas too.
Cold air coming in from direct vent gas fireplace.
Plug the holes with rokwool insulation or high temp caulk.
When we bought our house new i tried poking around everything i could.
A direct vent gas insert will stop those pesky drafts because the insert fits snugly inside the fireplace opening and the insert itself is a sealed firebox.
This unit is five years old and installed when home was built.
Install our magnetic fireplace vent coveron the lower vent to stop the inner air circulation from coming inside.
Unfortunately i didn t figure out how to set the glass cover back correctly and we had to endure a couple weeks of sub zero temperatures to 5ft around the fireplace until the builder showed me what had gone wrong.
What is a bit unusual in your case is that these vents usually don t pull cold air into the house without a fire present.
The dog house is vinyl siding and there is an 8 inch vent out the back of it.
I ve noticed it when it s not in use especially at night but it seems inconsistent and not necessarily predictable by the temperature or audible wind.
I have a superior dvt8 cens direct vent gas fireplace.
In the winter alberta we get huge drafts coming in the grills of our gas fireplace insert.
From the outside there is a fresh air intake on the chimney and there is a metal chimney exiting the top of the old stucco chimney.
I am getting a cold draft from the bottom vent top of unit where fireplace meets mantle and even surrounding area where floor meets mantle.
A direct vent gas inserts is flued up the chimney but because it is a sealed firebox the cold air cannot escape back into the room.
Use our direct vent fireplace termination cap coveron the outside cap to stop the cold outside air from coming in through the flue.
I suspect that your vent outside is not lower than the fire place which would allow colder heavier air to find it s way into the house.
Cold air issues from the bottom of direct vent fireplace can usually be eliminated by checking underneath.
I am going to make the assumption that the builder put no.