Hey there, we also have a 37V13 and found the same issue. We pulled the fireplace (very easy) and found they ran the ducting under the back of the fireplace. Additionally, they split the ducting and ran a duct through the pantry into the bathroom SMH.
We removed this duct and carefully flattened the top of the duct base, then sealed the end with HVAC foil tape so the fireplace could fit without crushing the duct. We now have amazing airflow to the garage!
I'll upload pictures when I find them.
Sent from my SM-S906U using Tapatalk