Deep Dive into Blower Door Testing

Deep Dive into Blower Door Testing

Over the last several years I have accumulated a range ofquestions about how blower doors and blower door testing work. And I want toshare them with you to know more about Blower Door Testing.

How does a blower door work?

The fan motor and blades push air out of the buildingwith the fan sensor located where the air is being pulled into the fan [more onthis below]. Channel A reads the hose pressure [with the reference outside],while Channel B measures the pressure at the fan sensor and just upstream ofthe sensor, converting this pressure difference to flow.

NOTE: The relationship between the pressure difference inside tooutside the building and the air flow resulting from the pressure differencecan be expressed by a power law equation:

Q = CΔpⁿ

Q is the flow (cubic feet per minute), C is a constant, P ispressure, and n is the exponent of the power law equation.

The exponent can vary between 0.5 and 1.0, withessentially big holes in the building making an exponent of 0.5 the best fit(curved line) and very tiny holes making the exponent of 1 the best fit (astraight line). The cool thing about this power law equation is that it is justabout the relationship between pressure and flow. Yes, other factors such astemperature and air density have impact, but they are easy to correct for anddon’t fundamentally affect the pressure/flow relationship.

Why doesn’t “necking” down thefan with different rings create turbulence and mess with the key relationshipbetween pressure and flow?

Think of the air just upstream of the fansensor—where the air is being pulled in—as the water just above two rocksrestricting flow in a stream. That water is smooth while the water downstreamis turbulent. A similar situation takes place in the blower door and thisis why the fan sensor is on the “calm” upstream side of the set up and whyrestricting the blower door with rings does not really disturb what happens atthe sensor.

Link : https://www.greenbuildingadvisor.com/article/blower-door-qa-an-interview-with-collin-olson-from-the-energy-conservatory

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Calm water isupstream of chute formed by two rocks in this river, while the water below isturbulent. This is analogous to the air before and after entering the blowerdoor fan.

The other thing going on here is that the air ismoving so fast, regardless of which ring is being used, that there is notreally time for turbulence right where the air is being pulled past the sensor.

When performing a blowerdoor test, is depressurizing the house inherently more accurate thanpressurizing the house?

This is an interesting question with a couple ofanswers. First, since to pressurize you just spin the fan around, the sensor isstill upstream in the “calm” spot. And the fan does not really care or knowwhich way it faces.

However, during a depressurization test, the airupstream of the sensor is on the inside of the building and that air is alwayscalm. During a pressurization test, the air upstream of the sensor is outsidethe building, where wind—either steady or gusting—can create “noise.”

Wind creates problems that can be overcome with anaveraging baseline for depressurization, but the wind can create an addedproblem during pressurization testing.

How do we know whatexponent to use for anyone building?

It turns out that for most buildings, the exponentsare fairly clustered around .65, and if you are just doing a single-point testand using the cfm50 result, the impact is not really worth worrying over.However, if you extrapolate the ELA at 4 Pa from that single-point test with anassumed exponent—to better assess infiltration—then you could be introducingsignificant error.

To really compare how a building performs at 50 Pa ofpressure difference to how it performs at 4 Pa pressure difference, amulti-point test can generate a curve and a better value for n, the exponent.But it is harder to get accurate pressure readings at low pressures and thisalso does not tell us anything about where the leaks are, an important part ofunderstanding the air leakage a building experiences.

How different is using a ductblaster instead of a blower door fan for building air tightness testing?

The only thing that really changes is the shroud. We should be using a duct blasterfor building air tightness more often. It works for homes in the 3 to 5 ACH50range. And for multi-family unit testing you can literally leave the fan in thepanel as you move it from floor to floor and unit to unit, assuming that theunit entry doors are the same size. In Europe, they call TEC’s duct blaster amini-blower door.

What’snew with the TEC DG1000?

When we start to take our efficiency industry asimportant as the tile in the lobby, we will have in our schedules and in ourplans accommodations for testing and people in our industry won’t have to worknights and weekends. It’s a matter of people recognizing the importance of airtightness testing and giving the issue the attention, it is due.

The TEC DG1000 is a huge breakthrough,essentially combining a pressure gauge and a computer.

One of the latest updates adds Bluetooth capability.This was a really difficult technical hurdle, even though we had the antennaalready built into the gauge. Before, if you pulled up the TEC app on yourphone to run the blower door remotely, it pretty much shut down the othercapabilities of your phone. With Bluetooth, a point-to-point connection is madebetween the gauge and your phone, so you can still use say, google, on yourphone

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