We wanted to make sure our home was very energy efficient, which meant strategically choosing which trees we kept in place, and then angling the house right so that the trees provided shade in the summer, but that those trees didn't pose a risk in the occasional winter storms (trees grow and root based on prevailing winds, so once you cut a few out and change the air-flow, they can become more susceptible to blowing over).

Not sure if it has to do with the air flow, type of filter, etc. 

Is the crawlspace treated for moisture (usually pea gravel and plastic) Is there free airflow between the crawl and the basement? 

So not touching the ground and there will be air flow in the base ( I assume).Please advise!  

It seems that hand-holding the client is more important than knowing how the airflow of the plenum affects the temperature of occupied space.

Get a high quality air purifier/scrubber, seal up(caulk) any cracks or gaps that could be allowing air flow between the units. 

The most common ways we have found that brings the CO2 levels down to "acceptable levels" is by either installing a larger HVAC unit or opening up windows and adding fans to increase air flow.

These are also available at Walmart, etc. and act to keep heat from leaving windows.2) Caulking around windows and doors to help seal the air in.3) Putting plastic sheeting over louvered windows that won't crank shut, to at least restrict outside cold air flow.

Meaning low airflow from a dirty coil can cause the high pressure switch to cut the unit off.

We have the tenants open the sink base cabinet doors in kitchen and bath to keep warm air flowing under and into the cabinet.