Duct Testing in the San Francisco Bay Area/Silicon Valley

Reasons why HVAC Contractors (and the Developers they work for) Fail HERS Tests.

This week I had two notable tests where the developers of a brand new houses failed his HERS test. These are new houses built under the 2013 California Energy Code.  Both houses had HVAC systems that failed to move enough air to satisfy the Total Airflow requirement when there is air conditioning. Both systems had the largest residential air conditioner that one can buy, a 5 ton condenser and matching evaporator coil.

With a 5 ton condenser, to provide 350 CFM of airflow per ton, you’ll have to provide 1750 CFM. This week’s problem units had 1230 and 1410 CFM. Given those numbers what does an HVAC contractor do to solve the problem so that he can satisfy his client and pass the code requirements so that the house can pass final inspection? Pick one strategy.

A.      Rewire the control board so that the system is actually in the highest speed.

B.      Replace ducts, especially the return ducts, with bigger diameter replacements so that the airflow will increase.

C.      Change out the motor in the furnace or air handler to an electronically commutated motor to get a higher airflow.

D.      Change out the condenser for a smaller size so that the airflow/ton rule is met.

E.       Tear out the system and hire an HVAC contractor who will actually perform a manual J, Manual  D, manual S and Manual T so that the system is guaranteed to move the proper amount of air to each duct and each room.

I’m still waiting to hear back from the owner/developer to hear how these went. However, in the last 6 months I’ve seen contractors and owners do A, B, D and E.  I’ll comment on each strategy.

A.      Rewire control board connections. - This is a common problem I see weekly where a tech will change the wiring and/or the dip switches and the unit immediately moves more air to pass the AF test. It shows why the HVAC contractor really wants to have a tech on the job when the HERS testing happens.

B.      Replace ducts. I see this fairly often. The HVAC contractor adds another return to the master bedroom or enlarges the return duct from 16 to 19 inches, more airflow happens, and the problem is solved.

C.      I have never seen this one done but know from PG&E sponsored commissioning classes that one can use Motormaster software and dial-in a properly sized motor for any application. That same class will tell you how extremely common it is to find wrong sized motors, pumps and fans throughout the built environment.

D.      Change out the condenser for a smaller unit. If you have a 5 ton system that is delivering 1230 CFM, changing to a 3.5 ton condenser works.  Just changing the condenser may be easier than re-doing the ducts.  Make sure it is 14 SEER, 13 no longer passes.

E.       Owner tells HVAC contractor to pull out non-code compliant system and hires contractor B who follows code and performs load calculations, duct design and then tests the system himself to make sure that every duct and every room is getting the airflow that the right-soft software said was necessary for proper performance. 

This one happened to a contractor a few weeks before Christmas. The contractor was expecting to get the final $14,000 payment mid December and pay his supplier and employees. He probably would have used that money to buy some Christmas presents for his family. Instead, that guy got a tough lessen in why a contractor needs to conform to the code.

Filter Grill Requirements Update from the CEC

While testing at BECT.us I have come across a number of air conditioning systems that do not meet the total airflow and fan/watt draw levels of performance that the code calls for. When this happen the HVAC contractor has to redo the return duct design and then install it at significant cost of time and reputation. It shows that the system was never actually designed but pieced together using a best guess. There has got to be a better way.

The CEC has just sent out an update of their filter grill requirements which are a critical part of the return duct system. Here is there update.

"Filter Grille Area Requirements

When complying with the Alternative to Section 150.0(m)13B, the nominal size of the air filter media should be used to calculate the minimum total return filter grille gross area. If the air filter is not located at the filter grille, use the nominal size of the filter grille to determine the area. The calculated area must be equal to or greater than the values in TABLES 150.0-C or D.

Additionally, TABLES 150.0-C and D also require that:

 1. Each return duct must be no longer than 30 feet.

2. Each return duct must not have more than 180 degrees of total bend.

3. If the total bending of a return duct is more than 90 degrees, one bend must be a metal elbow.

4. Return grille devices, which include the air filter and return grille locations, must be labeled in accordance with the requirements of Section 150.0(m)12A.

 5. The label must state the grille’s design airflow rate and a maximum allowable clean-filter pressure drop of 12.5 Pa (0.05 inches water) for the air filter, as rated in accordance with AHRI Standard 680 for the design airflow rate for the return grille. Please note that additional air filtration requirements of Section 150.0(m)12 may apply."

I've seen some contractors who were shocked at how much filter area the code was calling for. Of course, the best way to make sure that the filter grill requirements are met is to use a proper duct design system such as Right Soft. The vast majority of installations that I see are not designed at all.

I hope that bringing this to light could alleviate some of the pain I've seen in the eyes of someone who has to redo their hard work often in a sweltering attic.

BECT's Duct Test and Sealing Video

BECT's video at http://bect.us/contractors/duct-test-and-sealing-video/ demonstrates how to get your HVAC system to pass a code duct test. It'll show you how to seal the connections on any HVAC system. When you understand that, you can seal all the connections from the furnace to the plenum to the start collars to the ducts to the boots to the interior of the house. When you do that there is not much room for the system to leak air.

You'll need the right supplies first. California Code requires that all supplies in the duct system must be marked UL 181. At any big-box home supply store you can buy air duct mastic, tape (Separate types for flex duct and metal to metal connections),  big zip ties, a disposable paint brush and disposable gloves. You'll also need a zip tie tensioning tool.  

Tape alone won't get you as far as you need go to get ducts sealed properly. The best technique is to apply a thick coat of air duct mastic over every single connection. This will seal up any imperfections.

The day before installation, paint all the metal "wyes", start collars and plenums with mastic. This will create solid connections where air is less likely to leak. When you connect the ducts to the start collars and boots make sure to use a hand tensioner tool and get the connections tight. There is an inner and an outer lining to flex ducts, make sure that you zip tie and tape both layers. Then, on the outside layer, seal around the whole connection with air duct mastic.

Be comprehensive about the this process. It is much easier to get it done right the first time rather than staring at a seemingly sealed duct system and wonder where it is leaking. I've spent too much unpleasant time with with contractors standing there telling me that "It can't be leaking!" when the manometer tells us otherwise.

Your work will pay off. All the technical reports I've read about well-sealed duct systems say that they just work better, are more energy efficient, stay cleaner and need less maintenance than leaky duct systems. While new duct systems can get away with 6% leakage in California, crews who compete with others in their own company routinely get down to 3%. There are some elite HVAC companies who call themselves Home Performance Pros who make it a matter of pride to get their ducts so tight and well constructed that the test equipment we use can't even register the duct leakage, it just says, "low". "Low", that is something to shoot for when constructing a new duct system. Go for it.