It has been a windy week out here in Phoenix, AZ this past week.
I have a few tricks that I have used that works only sometimes. I use an empty plastic water bottle and put my reference to outside hose inside and try to shield it as much as possible. This has not worked so well the past few days where we have had gusty winds over 20+ mph.
I am open for some input on this, as my calendar is now chaos from all of the rescheduling.
A1 is spot on. I have done hundreds of tests here in Maine and have gotten different numbers from morning to afternoon depending on wind speed. My record is -35 pa with the fan not running. (ended up with a .32 ach using tectite later in the day when the wind died off)
There have been several studies over the years looking at windspeed effects on blower door tests. A study by LBNL used a dataset from the University of Alberta where an automated blower door repeatedly performed blower door tests on small test homes in different leakage configurations. There were many tests in each configuration over a range of windspeeds. This allowed comparison of low to high windspeed tests directly and estimates of bias and RMS errors. There are few simple conclusions:
- Higher windspeeds lead to lower estimates of leakage - i.e., there is a bias in the results - it is not just an increased uncertainty
- As others have pointed out doing both pressurization and depressurization is a good idea
- Up to about 6 m/s (13 mph) multipoint testing is more accurate, above this single point testing is better
- Single point testing has RMS errors of about 15%-20% at all windspeeds. Multipoint testing has RMS errors from about 4 to 40% as windspeeds increase from about 2 mph to 25 mph.
Here is a link to the paper: https://eta.lbl.gov/sites/default/files/publications/iain_walker_-_...
I agree with many of the observations in previous replies:
- longer time averaging helps
- sheltering the outdoor pressure tube is good
- sheltering the blower door fan itself is also an issue (we need to be careful about averaging pressures that so not reflect the true flows because the relationships between pressures and flows for both the envelope and blower door flow meter are not linear), so, if there is an attached garage you might install the blower door in the door between the garage and home and open the main garage door as this can shelter the blower door fan and flowmeter assembly from the wind.
- Putting pressure taps on all four walls helps (as recommended in ASTM E779)
Other researchers have used analytical techniques to estimate errors from mean wind speed (not including pressure and flow fluctuations:
François Rémi Carrié, Valérie Leprince, Uncertainties in building pressurisation tests due to steady wind, Energy and Buildings, Volume 116, 2016, Pages 656-665, ISSN 0378-7788, http://dx.doi.org/10.1016/j.enbuild.2016.01.029.
Sorry that's behind a paywall so here is the abstract:
"This paper deals with the quantification of the uncertainties due to wind in building pressurisation tests.The steady wind model error stems from the heterogeneous pressure distribution around the building. Wehave analysed this error based on a simplified one-zone building model with one leak on the windwardside and one on the leeward side of the building. Our model gives an analytical expression of this errorthat depends on the leakage distribution and pressure coefficients. Using test and reference pressuresat 50 Pa in this model, standard measurement protocol constraints contain the steady wind model errorwithin about 3% and 11% with wind speeds below 6 m s−1and 10 m s−1, respectively. At 10 Pa, the error isin the range of 35% and 60% at 6 m s−1and 10 m s−1, respectively. This paper also includes an estimate ofthe combined uncertainty including other sources of errors which can be used to assess the relevance ofmeasurement protocols. Under idealised conditions, using 50 Pa as reference pressure and a default flowexponent, it can be expected a combined expanded uncertainty of 6%–12% for wind speeds of 6–10 m s−1,respectively."
Other personal observations from more than 30 years of blower door testing:
- When doing pressurization testing move the indoor pressure hose as far away from the blower door as possible
- Use the smallest ring possible on the blower door - larger rings are much more sensitive to wind pressure fluctuations
- empty disposable coffee cups and garbage cans make great wind pressure shelters for the ends of tubes
- time-averaging pressure meters (and "cruise control") are your friend
- be careful with routing of pressure tubing - avoid having it move or vibrate during a test. Sometimes its better to get the tubing away from the blower door fan and not put the tubing through flexible fabric (although less convenient).
- don't let the sun shine on any vertical pressure tubing
- At some point it's just too windy and you need to wait (somewhere around 15/20 mph). Brandon's original question was about testing in this very windy regime and he is right - the results are not good when its this windy.
One more point on standardized testing dealing with wind: the CGSB Standard (149.10) also refers to putting pressure taps on all four walls, but also includes advice on using capillary tubes for time-averaging if you don't have at time-averaging meter (which is increasingly unlikely these days). It also basically says above 20 km/h (15 mph) testing is invalid.
Thanks everyone for all the info! This has been VERY helpful.
I have been reading how multi-point tests are LESS accurate than single point tests in high winds. I would not have expected that. Then it occurred to me that the single point test has no way to measure the flow exponent. So the exponent has to be assumed for the single point test. Is there any reason I can't assume the "normal" exponent and use it for the data set I have from testing on a windy day?
I wish I had measured both pressurization and depressurization. Until I was questioned and started researching it, I didn't think about doing both would help compensate for the wind.
As far as the case goes, I cannot comment on that right now. I'm sure people will understand why. Maybe once the case is over I will be able to give more details. If so, I hope it will help everyone do better tests under less than optimal conditions.
How cool is that -- a database of 175,000 blower door tests!!
Hi, Don. I suppose I should give the link directly : resdb.lbl.gov