This article was originally published in the March/April 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online March/April 1994

Blower Door Cruise Control

The use of pressure differential diagnostics to measure and interpret air leakage is changing the way blower-door users approach buildings. While measuring series leakage paths can give practitioners a better understanding of building dynamics, the techniques involved can sometimes be cumbersome.

Typically, the technician depressurizes a building to 50 Pascals (Pa), takes a pressure reading across the surface in question, and adds a hole to the surface. The resulting pressure drop in relation to the size of the hole is used to assess the extent of leakage. But to achieve accurate readings, a consistent whole-house pressure must be maintained with relation to the outside, before and after adding the hole.

It can be difficult to follow test procedures and keep the pressure constant in large or complicated houses, especially when working alone. Adding a hole or opening an attic door will, of course, change the whole-house pressure difference. To complete the tests, the technician must open the attic door, go downstairs to adjust the fan speed at the front door, then return to the gauges at the attic opening to see the results. In a large home, these trips back and forth, adjusting and readjusting, may be a good aerobic workout, but they can be time-consuming and frustrating.

Technicians working for Mass-Save Incorporated have been using a new flow control system that simplifies the diagnostic process. This system maintains a consistent house pressure relative to the outside by automatically adjusting fan speed. As the technician moves throughout the building opening various size holes into zones, the pressure difference will automatically return to 50 Pa, or any other preset pressure.

This flow control system, sometimes referred to as cruise control for the blower door, was developed at Mass Save and is based on a closed-loop feedback system. The system has two operating modes, manual and automatic. The operator initially depressurizes the building by increasing fan speed with the manual control, and then switches to automatic mode. The system automatically responds to any change in house pressure, increasing or decreasing fan speed as needed to maintain an even pressure difference.

System components include a pressure sensor with two taps set up like the Magnehelic gauges on the blower door. The low-pressure tap is inside the building and the high-pressure tap tees into the tube on the blower-door gauge that extends to the outside. When a pressure change is detected, a process controller sends a signal to the motor control to change fan speed, maintaining a constant pressure. In this automatic mode, the loop is closed, freeing the operator to experiment with the pressure dynamics of the building.

This technology can be applied to any of the many pressure diagnostic techniques, but it is especially well suited to the technique of increasing the hole size to double the zone depressurization with reference to the outside (see In Search of the Missing Leak, HE Nov/Dec '92, p.27). This simple technique can give a fast, visual estimation of leakage area. Opening a hatch to an attic, or a door to a garage, will automatically increase the fan speed until the initial zone-to-exterior depressurization is doubled (see Figure 1). The size of the opening roughly estimates the size of the existing leakage area. The cruise control keeps the sum of house-to-zone and zone-to-exterior pressure at 50, regardless of the opening size.

The applications of this prototype are still growing, changing the methods used to measure interzonal and duct leakage. One refinement that could be addressed is the sensitivity and responsiveness of the fan motor control. Sudden pressure changes due to an exterior door abruptly opening or a strong wind gust can cause the motor to overcompensate before readjusting to 50 Pa.

We hope that continuing development of this technology can be integrated with built-in computer programs to further enhance diagnostic procedures and make our efforts just a bit easier.

-- Bruce Torrey

Bruce Torrey is the building applications specialist for Mass-Save Incorporated, an energy services company based in Massachusetts.


Figure 1a. Initial view. With the whole house depressurized to -50 Pa, take a pressure reading from the house to the zone in question. In this example the attic-ceiling bypasses are causing the attic zone to be depressurized 10 Pa with reference to the outside.


Figure 1b. Second view. Gradually open a hole from house to attic until the initial attic-to-outside depressurization is doubled (in this case from 10 Pa to 20 Pa). To make this procedure work, the automatic flow device keeps the whole house depressurization at a consistent -50 Pa.



Figure 2. Schematic of a blower-door cruise control.



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