In the real world, three pascals have become a universal maximum pressure differential (ΔP). Although, there are times when three pascals is even too much of a ∆P.

A pressurized room could cause a reduction of supply airflow to it. This occurs because the door adds resistance to that supply duct. By adding resistance, it effectively creates an increase in the equivalent length of the supply duct.

This is why a closed door can reduce airflow to a room without a return. If a room is pressurized, some of that air is taken from the rest of the home. This situation leaves the main body of the house depressurized with reference to the outside. A house operating under negative pressure will suck in outside air. This action is known as infiltration. The HVAC system causes infiltration in this example.

Did you catch where I said, “A closed door CAN reduce airflow to a room without a return?” If the room has gaps, cracks, or holes connected to the outside, this can be a path for air to flow out of the room.

The problem is the air isn’t flowing back to the equipment. Instead, it is forced outside the building envelope. If this occurs, the house will also become depressurized.

In this situation, there will be little or no measurable pressure difference between the room without a return and the rest of the house. This is another variation of HVAC-induced infiltration.

Building Pressures Impact IAQ

Air leaking into the home from outside won’t always come from ideal areas. It can leak in from dirty areas like a crawl space or an attic, which could hurt the home’s indoor air quality and impact room pressure.

Infiltration can also cause high humidity in a house by sucking in humid outside air. Understanding building pressures can help a technician solve IAQ issues by stopping the problem at its source.

Safety Becomes a Concern

My brother’s laundry room, which shares a wall with the garage, became depressurized if the door was closed while the dryer was running. The dryer depressurized the room in two different ways. The dryer sucks air out of the room through its exhaust.

He has a natural gas dryer, which requires a fuel-air mixture to complete the combustion process.

The air in that mixture must come from the laundry room, which lacked an air path for the dryer. This can lead to a dangerous situation. Air leaking in from the garage could contain fumes from chemicals, gasoline, or even worse, carbon monoxide from auto exhaust.

My brother also has a tankless water heater installed outside next to his condenser. I am from the Chicagoland area, and our winter climate would never allow for this installation approach. Removing a gas-fired appliance from a home makes things a little easier.

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