A few years ago, my brother bought a new construction home near New Orleans, LA. He was nervous about the quality of his new home, but more specifically, the HVAC equipment and installed duct system.

There were news stories about homeowners suing local home builders because of mold issues. After his family moved into the house, he asked me if I could come down and analyze his HVAC system.

It just so happened that the Air Conditioning Contractors of America (ACCA) was having a conference in New Orleans that year, so I flew down and killed two birds with one stone.

When the conference ended, I inspected my brother’s HVAC system. What I found surprised me. There was one central return in the hallway and no jumper ducts or transfer grilles in the bedrooms. Apparently, the installation contractor did not remember that one CFM in equals one CFM out.

What You Supply, You Must Return

Room pressure issues can arise if a room has air supplied to it without a path for the air to travel back to the return. When I brought this to my brother’s attention, he recalled something the builder said during his final walk-through of the home.

He was told, “Make sure you don’t shut the bedroom doors at night.” I couldn’t believe the builder would suggest not using the bedroom doors. This statement blew my mind.

However, if you think about it, this suggestion makes sense. The builder must have learned from previous comfort complaints that by keeping the doors open, the complaints would go away. He stumbled across the fact that the building is part of the duct system, and the interior doors are the biggest dampers in the system.

This was good advice because the multiple supply registers and a lack of return air pressurized the primary suite.

A Room is an Extension of the Ducts

When you close a door to a room that has a supply duct and no return path, you take air from the main body of the house and dump it into an isolated portion of the home. That isolated room becomes pressurized.

Think of the room as a duct and the door as an endcap. In this scenario, the room could have measurable static pressure. The only difference is the scale used to measure room pressures.

Instead of using inches of water column, we use a smaller scale called pascals (Pa). To give you an idea of how small a pascal is, 0.1 inches of water column (in-w.c.) is equal to 25 pascals.

The goal should be to have no measurable pressure difference between a room and the rest of the house.

Click Below for the Next Page: