Zonal pressure diagnostics (ZPD) can create a misconception between customers and practitioners. The ZPD readings we get on interior rooms show a ratio between two openings: between the room and the rest of the house or between a zone and the outside. It doesn’t tell you the size of the hole or the CFM50 of the leakage; a 25-Pascal reading would tell you that the holes are the same size, but it wouldn’t tell you anything about the size of the hole or the extent of the leakage.

Without knowing the hole size or the CFM50, a ZPD reading doesn’t hold a lot of value from a technical standpoint. However, ZPD readings can be a decent piece of evidence for establishing that a client has leakage; these readings can help you close a sale and ultimately end up helping the customer.

When using a blower door, you bring the space under -50 Pascals of pressure with respect to the outside. In Genry’s video, he looks at an indoor laundry room with a supply and a return path through a vent that has been taken off. Under the first set of conditions, it would appear as though the laundry room is 84-86% connected to the outside, indicating a leaky room. However, when we open up the return hole a little bit, the pressure drops a lot, and it only appears to be 54% connected to the outside. However, we didn’t affect the hole connected to the outside. So, the ratio is flawed.