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.