CAZ pressure too low

On an energy retrofit project during the test out I learned that under worst case conditions, without the air handler on, the unfinished basement CAZ was at the threshold of the BPI guidelines of -5 pa. There is an atmospherically vented furnace and HWH down there. When the air handler was turned on AND the basement door closed, the pressure went to -7 pa. I read this as leaky air returns in the basement, many of which are panned joist bays. The homeowner is going to seal these, but will not be able to seal them perfectly. I suspect, the worst case CAZ pressure will not pass the BPI standards, even after the sealing of the exposed returns. This client will be switching to geothermal in a few years and getting a sealed combustion or electric HWH.


In the past, a passive fresh air intake would be installed. There must be something more controlled out there.

I have made temporary provisions to accommodate the combustion appliances, but what would be the best way to remedy this?

He has a hopper window near the equipment that needs to be replaced: Is there a glass block tile vent that would accomplish this: something that would open only when the pressure went below a set point?

Thanks in advance!

Rob Post


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Clients tend to plug holes to the outside when cold air spills in through them.  Relying on a mechanical device to operate and ensure makeup air when demanded is generally not a good solution as it may fail at some point. Plans for switching heating system may be delayed, so install a permanent fix. I think you're on the right track with sealing the return up well. Upstairs ventilation equipment may be pulling air through the duct-work from the basement as well. You don't mention what the whole house pressure is with respect to outside when the CAZ is negative; you may have a supply duct leaking to the outside through a crawlspace or attic which would depressurize the whole house and yield the same negative pressures in the CAZ. I recommend air-sealing the basement ceiling if you haven't already. This helps isolate the CAZ from upstairs ventilation equipment when combined with duct sealing.  If all else fails, I recommend power assist venting.


Dale Sherman

Thanks, Dale.


There is no duct work in unconditioned spaces. The leaking returns in the basement are the culprits. The entire basement  was air-sealed with ccsf with the exception of the duct work.

I got some solid advice on another forum and will be installing a combustion and make up air system. I've ordered an Equaliz-Air and a Skuttle Model 216. I will test both and decide which is best for my situation. These are passive systems

Sounds like you're on top of it, both make-up air vents are good units.  I prefer piping them to the appliances.  And as Ed suggests, you want to be sure the return is well sealed.  If the homeowner does it, just follow up with a QC inspection.  Thanks for sharing.





I think your assessment of the situation is correct.  If you are sure that he will install a heat pump in the near future, I would have no qualms about installing a small supply into the area to pressurize it - something that would be easy to close back up.  Of course, do your best on the returns - don't trust a homeowner with this critical job as there are problems that are not obvious.


Ed Minch



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