[Strawbale] Blower Door test set-up

[Mark Bigland-Pritchard]

Derek -

It's a while since I've used one of the things (I used them routinely in 
London in the late 80s / early 90s), but I doubt that things have 
changed much since then.
Firstly, the seal with the door must be as good as it can possibly be.  
Otherwise you get silly results.  I got through vast quantities of 
masking tape eliminating air gaps between the apparatus and the door 
frame on a mix of Georgian, Edwardian and 1960s doorframes, most of 
which seemed to suffer from some sort of irregularity....

Secondly, the thing works by measuring static and dynamic pressure 
differences between inside and outside at a range of pressures.  That 
ensures that wind pressure is already taken account of in the readings.  
(But it also means that gusting can result in problems in equilibration 
and so in unreliable results.)  From the data collected, it is possible 
to calculate the air flow rate, and hence the air exchange rate, for a 
range of pressures.  The value for 50Pa pressure difference is found by 
interpolation of these results.

If you are not sure of the results, you can always operate the fan in 
the other orientation - i.e. do both pressurisation and depressurisation 
tests.  They should give very similar results to each other.

Now, 50Pa is actually quite a lot more than you would get under normal 
circumstances.  You don't get to find out what the average "actual" air 
exchange rate is by this means - for that you have to apply a fudge 
factor which depends on the local windspeeds, possibilities for stack 
effect in the building, pressure differences owing to mechanical 
movement of air, etc.  [Some simple energy assessment packages (like 
HOT2000, for example) will decide for you what fudge factor to use and 
factor it in when calculating consumption.]

Mark



Derek Roff wrote:

>Shawna said:
>  
>
>>If you could find a set of magnahelic gauges, you could possibly make
>> a frame system, but sheesh, to make it airtight is a challenge.
>>    
>>
>
>Mark said:
>  
>
>>>The blower door method gives you - by extrapolation of results -
>>>a standard figure, usually at 50Pa pressure difference, but relating
>>>that standard figure to what the actual air exchange rate would be
>>>under real conditions is problematic.
>>>      
>>>
>
>At a blower-door demonstration, I asked the energy auditor about the 
>seal around the door (which didn't look perfect to me) and about the 
>wind conditions.  He said that the blower door apparatus and 
>instrumentation compensated for all that, and other variables, by 
>taking direct pressure measurements inside and outside.
>
>At the moment, I can't imagine exactly how that would work.  Getting a 
>perfect door seal and a windless day would be difficult, but 
>compensating for hard to measure variables is also problematic.  Were 
>these claims a bit of smoke and mirrors?  Please tell us more about the 
>theoretical and real world issues that affect the accuracy of blower 
>door testing.
>
>Derelict
>
>Derek Roff
>Language Learning Center
>Ortega Hall 129, MSC03-2100
>University of New Mexico
>Albuquerque, NM 87131-0001
>505/277-7368, fax 505/277-3885
>Internet: derek at unm.edu
>
>
>_______________________________________________
>Strawbale mailing list
>Strawbale at listserv.repp.org
>http://listserv.repp.org/mailman/listinfo/strawbale_listserv.repp.org
>
>  
>