How to pass a Duct Test

How to pass a duct test the first and every time

Title 24 2016 for installers explained by the Home Inspector San Diego

As we move toward more HERS testing in residential system here in San Diego it is important for installers to recognize a few key points. The target of 6% is not hard to get to if you follow certain methods during install. This adds some time initially but saves time on call backs for system failure. The new code initiated in July of 2016 has caused many more system to be required to be tested here in San Diego.

Use zip tie bands on all connections

Figure 1Banded duct Joint

Common for installers not to do this. it is easy if you are slinging new ducts use zip ties

Why am I required to do this?

It is code and best practice. There have been several test field studies on this and the best way for ducts to have long term survival is bands according the California Energy Commission. The plumbers,  alarm guys and electrician who will crawling in the space years down the road will have a harder damaging and pulling ducts apart  when bands are used.

How is this HERS tested? Visual

Seal with mastic all plenums

Figure 2 sealed plenum with mastic formerly uncondioned platform return

Taping the plenum often does not provide an airtight seal and leaks. Make sure you seal bottom plenum areas before securing air handler to platform.

Why am I doing this?

It is best practice and leakage at the plenum is avoidable during install

Why am I doing this?

The ability to attend to this area during install when it is accessible reduces call back time. Remember we want to get to 6 % and while it is not unobtainable if you start with a leaky plenum chances are even if the rest of the duct system is perfect you might fail. Taking the extra time to do it right is much less time than trying to do it after the system is set.

How is this tested?   Duct leakage test

Seal with mastic any and all star collars

sealed star collar

Figure 3   Sealed star collar

Why should I do this? It is best practice

Star collars are leaky and sealing them with tape is not effective to get a true seal you need mastic. Trying to tape them often causes gaps and the amount of leakage from a star collar can be significant. By sealing with mastic you eliminate this area as being a problem.

How is this HERS tested?   Duct leakage test


Seal duct boots to drywall


Figure 4 duct boot not sealed to drywall

Common failure and systems will not pass without this measure done.

Why am I do this? It is code and best practice

When air from the supply hits the grill it bounces back. If there is no seal around the perimeter of the boot air can and often does push through the gap. This causes less conditioned air to make into the area it is intended to. The result is a duct leakage failure

How is this HERS tested? Visual and duct leakage



Place TMAH in proper location and labeled

Figure 5 TMAH instructions

Easy to do often forgotten

Why am I doing this?

It is code. It is part of the compliance. It will be convenient moving forward for any whom might want to read static pressure or temperature splits.

How is this HERS tested? Visual


Proper Sized Ducting


Figure 6 sufficient sized ducts for system flow

Why am I doing this? It is code and best practice

Contractors are expected to understand and implement proper duct and system sizing and instalation. An airflow test will be performed and without proper sizing this can fail. With current return duct sizing required by title 24 this should never happen and should be caught during city inspection. We need to get proper flow across coils to ensure the system efficiency. Also during the airflow and or fan watt draw Undersized ducts will often raise static pressure cause the handler to work harder and cause a failure of the fan watt draw test

How is this tested?   Airflow test and fan watt draw

Check charge on condenser

Figure 7 system low charged

I know it factory pre-charged but was it done right? Did any leak during shipping or install. Did you install more than 15 feet of line set? The bottom line is it is more usual than not to have the systems low in charge. Charging without checking can lead to over charging the system.

Why am I doing this? It is code and best practice.

A study was done by the CEC and most systems were either over or under charged and few were correct. System efficiency relies on this figure to be right.

How is this HERS tested? System Charge test.

These are the things we HERS Raters test for and must be attended to. It is mainly a matter of process. Once installing contractor understands the rules complying is easy. Should you have any questions feel free to contact us.  We want to help you train your crews so the process is seamless. Reducing costs and improving productivity. At the end of the day this puts all installers on a level playing field. As poor system installs will not pass. Also end customers should have a better systems overall when these rules are followed.

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