Make sure your blower door test meets the standard Technical instruments that measure airtightness need periodic calibration

17 August 2021 by Jason Quinn

If you’re conducting blower door tests—or hiring someone to do it—make sure you understand the Standard that applies to the test that is being performed. That Standard includes a requirement that the blower door micromanometer is accurately calibrated. Read on for more information. If the micromanometer isn’t up to scratch, your blower door test results are invalid.

Designing a building so the wind doesn’t blow through it is fundamental to performance and comfort. Even a certified Passive House is not completely airtight and this is not a goal worth pursuing. Passive House levels of performance aim for a sweet spot that balances affordability and practicality of construction with (a) building durability, (b) occupant comfort and wellbeing and (c) energy efficiency.

From the point of view of energy-efficiency, the less air leakage the better. Air leakage also has implications for building durability as air passing through a building’s walls, floor and roof is likely carrying moisture, which is absorbed into the wall assembly.

Air tightness is precisely measured on-site using a portable blower door. The data provides peace of mind to designers and clients and gives feedback to the construction team. Passive House builders soon realised that air leakage is a reasonable proxy for build quality and became quick to compare their blower door test results. (The current New Zealand record is 0.2 ACHn50, a third of the Passive House upper limit.)

A blower door is made up of a fan with calibrated holes in it. You measure the pressure across the holes and that provides the air flow rate (and thus the amount of air leakage). Then you also measure the pressure in the building versus the outside. Provided you do not physically damage  the structure of the fan and calibrated holes (for example, by cracking or gouging), the blower door should not need calibration. So take care and treat it like the scientific instrument that it is.

Figure 1: Blower door test equipment setup. Photo by Jason Quinn.

However, the device that measures the pressure, the micromanometer, does need calibration. It can measure pressures down to 10Pa to within +/-0.15Pa or +/-1% of the reading (whichever is higher). Manufacturers require calibration every two years. Test standards EN13829:2001 and ISO 9972:2015 require the use of calibrated gear. (If you are conducting air leakage tests and providing reports, you are implying to the client you are competent and have read and understood the standards you are working under. So read the standard.) ATTMA requires annual calibration in their standard TSL1. As the air leakage testing market in New Zealand is just getting started, as certifiers, we accept the two-year interval recommended by manufacturers for Passive House Certification.

There are more than 20 blower door testers in New Zealand at time of writing and they are using several different types of micromanometers to measure building air leakage. There are three models of micromanometer in common use (see below). They all have two channels of sensors and are used over a test pressure range of +/-2000 Pa. Typical use range is +/- 400 Pa.

Figure 2: Micromanometer by Minneapolis Blower Doors DG700 (top). This is currently the most common but no longer being sold. Bottom, newer model DG1000, the second most common unit. Photo from Minneapolis Blower Doors website.

Figure 3: Micromanometer by Retrotec DM32 (only two of these in the country, that I’m aware of). Photo from Retrotec website.

I’m aware that some blower door testers have been ignoring the requirement to calibrate the micromanometers. It creates a risk that the airtightness result is wrong. Unfortunately, up until now the only option has been to ship the devices back to the USA for calibration: expensive and time-consuming.

However it’s now much easier because Auckland-based firm CPS NZ Ltd recently invested in the calibrator that the manufacturers use to make micromanometers. CPS sorted out how to do this for my DG-700, which required using special manufacturer’s software. Do note that CPS can’t reset the device if the sensor has drifted too far from its original calibration. That will still require the device to take a vacation stateside to get fixed. But CPS can provide official verification if your gauge is operating within the manufacturer’s tolerances.

CPS will carry out a 10-point calibration and issue an IANZ endorsed calibration certificate on each channel A/B (see Fig 4).  You need to specify what pressure settings you want calibrated. I suggest testing +/- 1000, 500, 250, 125, 50 P .


Figure 4: Calibration sheets for channel A, channel B is calibrated similarly. Gauge is within the specs and thus I can use it for another two years before it needs recalibration.

In order to use blower door test results for Passive House certification (including PHI certifications such as EnerPHit or Low Energy Building) it is necessary to show that the micromanometer used for blower door testing has been calibrated. This can be done in several ways.

  1.  If the Minneapolis report shows the internal calibration date is within two years of the test, you are done.
  2. If the internal date is older than two years and you have had the gauge independently calibrated by external experts, attach the calibration sheet (to every report). Do check that the report states the gauge is still ‘within manufacturer tolerance’’ (ie CPS testing or others as outlined below). This calibration by must have been less than two years from the date you carried out the blower door test.
  3. If you have conducted a field calibration per the Minneapolis Blower Door company’s protocol, you can attach a letter and extend your gauge out for one additional year beyond the certification established in #1 and #2 above. This documentation needs to be submitted with every report. A statement on company letterhead signed by the person doing the field check is acceptable.

If the gauge doesn’t meet these parameters, then the blower door test was not conducted as per the standard and we cannot accept it. This could in theory cause a project to fail certification if it is not possible to reconduct the test with calibrated instruments.

Here are the contact details for CPS so I don’t have to reply to half a dozen emails requesting them. Call Chris instead.

Chris Woudenberg | Technical Director | CPS NZ Limited
38J Angle Street, Onehunga, Auckland 1061
AUCKLAND +64 9 636 4999

The cost at the time of testing my equipment was $375+GST and shipping.