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Combustion appliance zone (CAZ) pressure too low

user-270695 | Posted in Mechanicals on

On an energy retrofit project during the test out I learned that under worst case conditions, without the air handler on, the unfinished basement combustion appliance zone (CAZ) pressure was at the threshold of the BPI guidelines of -5 pascals. 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.

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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  1. David Meiland | | #1

    Measuring the CAZ pressure is only part of the test. Testing spillage, draft, and CO are also required. Was this done, and what were the results? And, what specifically was done to "accommodate the combustion appliances"?

  2. user-270695 | | #2

    All done. Spillage occurred when the CAZ went to -7pa, but not before when at -4.7. The units run clean and are as efficient as can be, given the technology.

    I need a solution to offer "make up air". In the past, a passive fresh air vent would be installed, but I'm looking for a more controlled approach. Any suggestions?

  3. Tony Olaivar | | #3

    In conditions where CAZ depressurization is severe, we've been installing a ducted intake with barometric damper. This sort of damper is designed for oil burning furnaces and can be obtained at your HVAC supply house. In your case, the depressurization is probably minor enough to get away with a 6" damper or even smaller. Just extend the duct through the hopper window opening, elbow down and feed to another elbow or "T" with closure on the bottom. Install the damper inside and use a grill on the outside to prevent animal entry. Adjust the weight to the greatest degree of sensitivity, (probably .02" WG which is equal to 5pa). This will allow intake air without creating a gaping hole in the house. Tony Olaivar - HERS Rater, BPI Professional - Lanz Heating and Cooling.

  4. user-270695 | | #4


    Great idea. I had thought of this, but was concerned with how to integrate it with the window or even a new glass block window. The glass block comes with a 4" duct tile, for dryers. Also, would I have to install the damper backwards since it will need to draw from outside and not the basement like on a boiler?

  5. Riversong | | #5

    correction: 0.02" WC is equal to 5pa

    You're mixing your metaphors (or your fluids).

  6. Tony Olaivar | | #6

    I'm not sure that this damper can be installed backwards. In any event, the answer is no, the door will swing both ways. Just make sure not to plant any sheet metal screws where they'll interfere. Better yet, use pop rivets. Tony at Lanz Inc

  7. user-270695 | | #7

    Thanks for your help.

  8. David Meiland | | #8

    correction: 0.02" WC is equal to 5pa

    .02" of water gauge is also equal to 5pa. Toe-may-toe, toe-mah-toe, etc.

  9. Riversong | | #9


    You're correct that you'll have to install it in reverse from the typical installation in a tee on the furnace stack because the air must be drawn out of it rather than into it, but those units are not reversible. You'll have to use an inline barometric damper which can be installed in either orientation.

  10. Riversong | | #10

    .02" of water gauge is also equal to 5pa

    Inches of water column ("WC) or inches of mercury ("HG) and Pa (N/M²) are absolute pressure units. Water gauge (or any gauge pressure) is the difference between system pressure and ambient air pressure.

  11. user-270695 | | #11

    Robert- thanks for the clarification and distinction between units.

    Someone else recommended an Eqalizair ( This unit provides combustion air and make up air to the return through 2 ports with dampers. It does not appear to be adjustable in terms of pressure activation.

    Is anyone familiar with this approach?

  12. Riversong | | #12


    I think you'll be better off making up your own plenum with an inline adjustable barometric damper. The Eqalizair system may work, but it is not adjustable, offers no specs on the pressure differential required to open the flaps (which would depend on duct size, type and length). And whoever created that that website doesn't have any comprehension about the subject. It's the most ridiculous and self-contradictory web site I think I've ever seen (using analogies that make no sense and quoting one "expert" as saying "it won't have customers knocking down your door").

  13. David Meiland | | #13

    Water gauge

    I've heard a number of technicians and instructors use water gauge and water column interchangeably, right or not. What Tony was saying was clear to me.

    It's hard to see the merit of a damper that opens at 5 or 7 pascals or anything close to that, at least in my area. The wind would be blowing it open constantly, except in the summer when it's calm and you don't need a damper. Maybe I'm overly risk averse, but I would not be likely to propose an ad hoc solution to an owner in a situation like this. They may be better off leaving the hopper window open a bit.

  14. Riversong | | #14

    How is a zero-Pa damper (an open window) preferable to a calibrated 5 Pa damper, when the purpose is to prevent a negative pressure differential more than 5 Pa?

    The duct could be run out the window, elbowed down and then elbowed outward and terminated with a tee with two (screened) horizontal openings. This would allow wind to pass through yet allow air to be drawn in when the CAZ pressure drops.

    Seems like a pretty clever and simple "ad hoc" solution.

    P.S. All pressure readings are gauge pressures except barometric pressure, which is absolute. But the technical difference is annotated by the units psia and psig. Water gauge is equivalent to Pa differential (which is what a blower door measures).

  15. Todd C | | #15

    Robert - check out Field Controls, they make products for your situation.

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