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.