It came to my attention recently that there may be a widespread misinterpretation of a requirement of the BPI standards.  The bottom of page 13 of the BPI Technical Standards for the Building Analyst Professional states the following:

When CAZ depressurization limits are exceeded under worst-case conditions according to the CAZ Depressurization Limit table, make up air must be provided or other modifications to the building shell or exhaust appliances must be included in the work scope to bring the depressurization within acceptable limits.

I would assume that ‘building shell’ refers to the exterior building envelope or pressure boundary of the building.  And if ‘makeup air’ refers to adding a hole in the building shell, then ‘other modifications to the building shell’ could be interpreted as sealing holes in the building shell.  I would like feedback from BPI instructors and BPI students to find out if it is being taught that sealing up a building and lowering the Blower Door cfm50 is a remedial action recommended in the work scope to solve worst-case depressurization issues.

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if it is being taught that sealing up a building and lowering the Blower Door cfm50 is a remedial action recommended in the work scope to solve worst-case depressurization issues

 

This is not at all what was taught in the class I took.


This has never been taught in my class. We teach that the ideal remedy is to upgrade to a sealed combustion appliance to eliminate the interaction of house pressure on the appliances. If this is not a reasonable upgrade then there are other test which would give us an ideal of the cause of depressurization, sometimes it maybe return duct leaks in the CAZ zone(seal the ducts), sometimes it maybe large fans in the area (can be reduced in size ), the area around the appliance could possibly be isolated from the house and add air from outside, and other times I have had to add makeup air for dryers in the CAZ. Every situation is different but I have never or have never taught my students that just sealing the house tighter would be a proper corrective action for bringing depressurization within acceptable limts.
What to you think the standard means by 'other modifications to the building shell to bring the depressurization within acceptable limits.'  It mentions makeup air earlier, so I do not think it is suggesting making the building shell leakier.  Are there other modifications to the building shell they might be referring to that could bring the depressurization within acceptable limits?

Paul,

 

In fact I do think the standard is suggesting making the building shell leakier.  Obviously this would be a last resort, but it would be preferable to make a house leakier than to have a combustion appliance that is backdrafting. 

 

But usually excessive depressurization problems can be solved with pass through grills between various zones of the house, or jump ducts, or undercutting doors so they maintain at least a 1" return air pathway on the bottom.  Next steps would be to mechancially disable the highest speed fan setting on the kitchen fan or other large fan that is causing worst case depressurization.  

 

Another option is to put natural drafting combustion appliances inside their own sealed off and insulated utility room with make up combustion air coming in directly from outside.   Either that or move them to the garage outside the conditioned space.  If all else fails, then you have to start replacing natural drafting combustion appliances with either electric or sealed combustion units. 

 

One big misinterpretation I have seen people make, is that they confuse or conflate make up air needed for combustion appliances and worst case CAZ depressurization, with mechancial ventilation needed if you get a house below 70% of the Building Airflow Standard. 

 

If you have a tight house where mechanical ventilation is required, and it also is failing the CAZ depressurization test, you cannot solve both issues with one fix - unless you literally just add a big hole to the house which makes the CAZ and the whole house a lot leakier.   Instead, the CAZ issue needs to be addressed with one of the options I outlined above, and the mechanical ventilation needs to be addressed with an exhaust or supply fan of some sort, or an HRV.  But adding mechanical ventilation can't and won't solve a CAZ depressurization problem. 

 

 

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