RTFM! But Wait This House Has No Manual w/ Sam Myers and Genry Garcia
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The house is a system, and occupants may have complaints related to IAQ and comfort. If those complaints can’t be addressed with the HVAC system, we may have to look at the building envelope, which serves as the container for conditioned air. Leakage and distribution of leakage can affect comfort, especially if discomfort is more severe in some rooms than others. The HVAC system or exhaust fans may also change the building or individual room pressures while running. Genry and Sam addressed those factors in an HVAC School article, which you can find at https://hvacrschool.com/what-if-house….
Buildings have an outer skin that separates the inside from the outside, often called a building envelope or air barrier but sometimes known as a pressure boundary. Ventilated attics can be very leaky, especially where the drywall meets the top plate, and locations with penetrations or uncapped chases also often present problems. To test for envelope leakage, you must use a blower door; depressurization is done to exaggerate the leaks in a home. If we know the fan pressure and surface area of the hole, then we can calculate the flow.
You can locate individual envelope leaks by PRESSURIZING the house and using a smoke machine to see if it gets swept up under the doorway. The smoke exposes leaks under exaggerated pressure conditions, so it effectively leads you to the leak.
Equipment sizing is also important for comfort; an oversized system can lead to poor humidity control inside the home due to short runtimes. If the HVAC system doesn’t match the house, then the occupants may feel uncomfortable.
Blower door tests can expose some issues with leakiness, but they don’t address mechanical-driven infiltration from the HVAC. Running HVAC units can drive air into or out of the building via leaky ducts and unbalanced rooms. Balance checks with a manometer can allow you to determine how rooms are pressurized in reference to the main body of the house. In general, individual rooms should not be more than 3 Pascals above or below the pressure of the main body of the house. However, rooms could still be balanced with inadequate supply air reaching the room, which can also cause comfort problems.
Leaky supply ducts can cause too little conditioned air to reach the supply registers. Leaky return ducts can allow too much unconditioned air from the attic to enter the return, which can decrease the indoor air quality.
Some rooms come under negative pressure compared to the rest of the house, indicating that some poorly sealed areas of the room may be responsible for leakage. A relatively inexpensive thermal camera can be used to locate these leaky areas. You can also check for gaps and cracks on the outside of a building to locate potential leaks in the building envelope.
When checking pressure balances, you must still refer to the blower door test results to interpret the individual room pressures. The initial blower door test is a piece of the puzzle when we look at the envelope leakage for the whole building.
Genry and Sam also use a simulator to check the pressure and find solutions for hypothetical cases. The hypothetical room is too positively pressured when the door is shut. In cases like that one where we need to get air back to the return, we may consider a dedicated return, transfer grille, or jumper duct as a solution. When the fan speed is the same as it was in the first case but in a leaky room (with a big, leaky chase), we could consider locating and sealing the leak.
Leakage also depends on the size of the hole and the velocity of air flowing through the hole. Oversizing a system or changing a motor (such as going from a PSC motor to an ECM) can cause leakage to worsen.