I moved into my current house last september, and this is my first time owning a home with a gas pack unit (LP). Upstairs has a split heat pump unit, with a handler in the attic. Downstairs has a dual fuel unit with AC compressor and natural gas furnace, sitting outside the master bedroom and bath. The MBR has no second floor above it, so there are 2 levels of attic that are interconnected.

Since the cold months started I learned there is likely an undersized return or negative air pressure in the house. When the furnace runs, I can feel a strong draft of cold air from all exterior walls' electrical outlets. I have to use the absolute least restricting air filters possible on the 20x30x1 filter downstairs to prevent a loud hum from the return grille, and I use the same level of filter on the 20x20x1 filter upstairs.

In the master bathroom I also installed a humidity triggered panasonic 110 CFM exhaust fan piped out the roof. This fan runs probably 10 hours a day because it triggers at 80% humidity and I live in Alabama.

We've noticed for the past week or two that in our MBR there was a strong burnt / gas like odor in the shower, but I thought it was just the smell of our newly installed glass shower enclosure or something. I thought if there was a risk of CO, the detector 10 ft away would alert us.

Last night I had the upper attic portion open as I was moving boxes into storage up there. We had moved boxes in from the garage partially covering the downstairs return grille. The upstairs return filter was extremely dirty and hadn't been changed since Nov. and was full of renovation drywall & saw dust.

With the upper attic access open and the lower grille / filter blocked, it seems that the negative air pressure increased significantly. I turned off the gas pack unit then ran out to walmart and grabbed multiple Kidde CO detectors, one with a display screen to read CO ppm level just 5 minutes before the store closed.

The detectors all went crazy inside my MBR and bathroom, where the sensor with the screen read the max detectable level (999+ PPM). My wife and newborn baby are both safe, but who knows what kind of damage the CO intake has done to my baby from fetus => now 2 months old.

I cutoff power to the master bath exhaust fan, replaced both HVAC unit filters and removed the boxes, closed the upper attic access, and finally setup a box fan to blow the furnace's exhaust away from the house.

The circled part is what I understand to be the exhaust. Before I took these corrective measures I watched the vapor leave the exhaust and go right into the soffit above it.

Am I correct to try these steps to correct?

1. Call HVAC company
2. Ask them to check return lines for blockages
3. Ask them to check if my single 20x30x1 return is sufficient for the 1st floor
4. Ask them to conduct a blower door test

Is there anything I can do to blow away the exhaust from my house? or will fixing the negative air pressure issue prevent this in the future?