# Zonal Pressure Puzzle

I have a wonderful little (16 x 14 x 8) test cabin at Bristol Community College in Fall River, MA.  There are 3 'rooms', one of which stands in as the bathroom.  It has a bath fan in the ceiling and a small, hopper window.  When I depressurize the house to 50 Pascals, the pressure between the bathroom and the main body of the cabin is just 4.3 Pascals.  When I stand in the bathroom with the door closed, air is pouring in through the bathroom fan, there is enough flow to make the fan rotate backwards!  Doesn't that mean that the room is well connected to outside?  Shouldn't the pressure in the room WRT the main body be closer to 50?

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Comment by Rich Manning on August 12, 2016 at 8:32am

Thanks Paul,

I look forward to your conclusions. Will you post them?

Comment by Paul Raymer on August 12, 2016 at 6:13am

This is a test cabin inside a building.  The cabin has four outside walls, a floor, and a ceiling.  The room that the fan is in has a ceiling between the room and the laboratory which is serving as 'outside'.  The cabin was depressurized to 50 Pascals WRT the lab space.  The pressure in the 'bathroom' WRT the main body of the cabin was just 4.3 Pascals.  The pressure in the 'bathroom' WRT 'outside' was 45.7 Pascals (as it should have been).  But there was a significant breeze through the fan.

The problem was that the electricians never installed any ducting on the fan so that all the air that did leak through the fan's backdraft damper was focused on the blower wheel causing it to spin at a rapid rate generating a small vortex under the fan which felt like a major breeze.  Small hole.  High pressure. High velocity.  Low volume.  Like putting your thumb over the end of a dripping hose.

I held my first ANSI/RESNET 380 class in the cabin this week where we were able to go through all the ventilation test protocols in the Standard.  Powered flow hood, static flow hood, inflatable device, large vane and small vane anemometers, averaging flow stations, pitot tubes and compare results.  I will put together a report on the results of the 10 participants and how they compare.  The standard is looking for 5% tolerance.  I don't think that's going to happen with the EXH having a tolerance of 10% out of the gate.

Comment by Judy Rachel on August 11, 2016 at 6:14pm

I think you are mixing up what test you are performing.  With zonal pressure testing you would measure across 2 boundaries. One being the area you are depressurizing ( aka the conditioned space ) to an unconditioned or intermediate zone such as an attic, garage, or basement. The other boundary being form that intermediate space to the outside - across the roof, or garage to outside or basement to outside.  These measurements can be used to quantify the ratio of the interior side leakage rate to the exterior side leakage rate.  While measuring the pressure differences across the interior and exterior surfaces of the zone can provide useful information about the leakage path (for example, the ceiling is leakier than the roof or the basement is leakier to the inside than to the outside), it doesn't tell anything about how much leakage there is.

People are taught to measure across doors as a way to determine where leakage paths are within the conditioned space.  In a sense this is akin to using a pressure pan to determine which duct has the most leakage to help direct duct sealing efforts.  Close a door measure across it's boundary WRT the main body, open that door and then close another door and measure across that boundary WRT the main body, etc.  The room with the largest pressure difference WRT the main body of the house has the most air leakage path.  Whether those paths are a big one or a bunch of small ones cannot be determined by this test.

Comment by Rich Manning on August 11, 2016 at 10:27am

It sounds like the damper may have been installed backwards?

Comment by Dale@EnergyWright on August 7, 2016 at 6:47am
I'd look for a duct disconnect in the attic. What connections are there between the bathroom and the main body of the house? Door undercut, common ductwork, and plumbing chases are a few possible ways to connect the bathroom to the house and move the bathroom pressure towards zero.

Good exercise in zonal diagnostics, Paul!
Comment by Ryan Puckett on August 5, 2016 at 12:58pm

If I'm reading this right, a 4.3 pascal pressure difference indicates 8.6% connected to outside and 91.4% connected to the house. Ideally you would see a 0 pascal pressure difference right? Have you done an add a hole test to estimate the cumulative hole size leakage to outside? A 1 sq inch hole though the back draft damper and a 11 sq inch undercut door would give you the 4.3pa difference.

Comment by Paul Raymer on August 5, 2016 at 5:59am

Ah, but there is a backdraft damper on the fan!  I've tested these before and found that they are generally quite tight, as you say, but something unusual is happening in this case.

Comment by Jim Tenhundfeld on August 4, 2016 at 8:56am

It appears that there is no damper on the exterior of the bathroom fan which allows outdoor air to enter the bathroom putting the bathroom outside of the pressure boundary.  When there is a exterior damper, and you depressurize the house, then this forces the damper to close tighter and then there should not be any pressure difference between the bathroom and the main body of your cabin.

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