Thursday, February 2, 2012

Arizona duct sealing for IAQ

Just took a random trip out to Arizona to visit the ‘rents.  Leaving North Jersey in late January and getting 4 straight days of 70 degrees and sunny in the desert.  Not bad.  I can certainly understand the appeal of this place…really is gorgeous, especially when it’s not oppressively hot.
Anywho, I always like to poke around in a different type of house when I get the chance.  Adobe style, 2500 sq ft, 2 heat pump HVAC systems, built in the 1970’s with an addition in the 1990’s, slab on grade, flat roof.   North Jersey is not exactly known for slab-on-grade, fully stuccoed, adobe style houses that exist in dominant cooling climates.  So with my pops leading the way, I got a little tour of the house and came across an interesting little building science related tid bit.
My folks had an energy audit done on the house about 6 months ago.  A gentleman from the utility company came out and performed what, by my father’s account, seems to have been a solid and thorough energy audit adhering to BPI standards.  Blower door, some pressure diagnostics, and analysis of the ducted distribution system and AC/heat pump systems….pretty good deal for $100.
The guy ran the blower door with and without all of the duct work taped off to determine the estimated air leakage attributed to exterior duct leakage.  The auditor also determined probable (not measured) HVAC system airflow, and determined that estimated duct leakage to the outside was approximately 25% of total system airflow.  Pretty typical.
With this knowledge in hand, I took a closer look at the HVAC systems.  System #1 is a 3-ton unit located in the garage and services the front (original) 2/3 of the home.  System #2 is a 2-ton rooftop packaged unit and services the master suite and rear 1/3 of the home.  The rooftop unit looked pretty tight, so I focused my attention on the garage unit, which looked something like this:
The field built plenum was banged together with a few standard framing nails, and a very weak attempt was made at sealing the inside of the plenum with Great Stuff.  A non-treated 2x4 sill plate was shot into the garage slab as a nailer for the plywood.  Obviously, the 2x4 is starting to rot away.  A filter slot and an opening for the since-removed evaporative cooler in the supply trunk were basically wide open to the garage.

Re-calling all they did for me growing up, I decided to tackle the job of sealing up this air handler and plenum.  We picked up a few cans of Great Stuff and a tub of water based duct mastic, and I went to work.  I popped off the front piece of plywood and literally crawled into the plenum.  Vacuumed out the nastiness in this area, and then spent about 30 mins and an entire can of great stuff sealing up all accessible penetrations between the garage and the plywood box.  I then pulled off all of the failing metal tape at the seams in the duct work and painted it up nice with the duct mastic.  I sealed off the penetration for the evaporative cooler, and my pops will be weatherstripping the door to the filter slot.  I sealed up the “access door” to the plenum with duct mastic.

This was all a pretty typical case of duct leakage outside of the envelope.  Seal up the ductwork and improvements will be had.  Three specific results are expected:

-          1.  IAQ improvements.   My parents have an IAQ issue where the house sometimes smells like garbage.  The plenum is technically on the return side of the system and as such is under a continuous negative operating pressure.  This negative pressure draws air partially from the garage and distributes it throughout the home.  Garage air is not good….chemicals are often stored there, and CO is exhausted from cars when they are started.  More importantly to my parents, though, is the IAQ issue resulting from the garbage cans that they store in the garage.  During the heat of the summer, they have to leave the garbage cans outside because the house starts to stink like garbage.  Same thing going on here: the duct leakage is drawing the odors from the garbage cans into the system and dispersing it throughout the house.  I’m very hopeful that sealing up the return side of this system will fix that problem.
-          2.  Return air temperature.  My father guesses that the garage gets to around 110-120F on hot days in the summer.  Similar to an attic AHU with leaky return ducts, the hot garage air raises the return air temperature and strains the system.  This leads to increased consumption, and energy savings will result from the sealing of these returns.
-          3.  Supply leakage.  The slot for the evaporative cooler component was just blowing conditioned air into the garage.  Sealing this will direct the conditioned air into the home, where it belongs, and will in the process reduce consumption.

On top of this, the duct sealing should help balance out the distribution throughout the home.  Imbalances owing to duct leakage can drive air infiltration across the envelope.  As such, the duct sealing process should also contribute to reduced air changes.

Essential to any duct sealing package is the performing of a post-work airflow test.  Tighter ducts will operate under increased static pressures and, and with this potentially reduced system airflow rates.  Airflow rates too low, and you can freeze up the cooling coil.  I wasn’t real worried about it at my parents house because, although the air handler and plenum are now considerably tighter, the rest of the duct work in the house is still pretty leaky, and I don’t think we came anywhere close to creating static pressures that will reduce airflow to problematic levels.

The lesson, as always: seal your ducts if your house smells like garbage.


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