A seldom practiced design procedure, equipment selection, has been part of the International Residential Code for many years, and local MA/RI code at least since 2010.  This critical step is the second part of residential system design, following a Manual J load calculation and prior to the Manual D duct design.  I think confusion around the dated manual has contributed to lack of use and enforcement.  Whether you are installing an air-conditioner, heat pump, furnace, or boiler, there is a definite maximum over sizing that is allowed under the code, based on ACCA Manual S.
     If installing a furnace or boiler, you are allowed to oversize the equipment by 40% above the Manual J heat loss of the home.  This is based on the net or output BTU/hr of the equipment.  Of course, significantly over sizing equipment can lead to many mechanical and comfort issues, but a system could be oversized by 100% and not have it affect the AFUE!
     When installing a heat pump, you can oversize the equipment by a max of 25% above the Manual J Heat Gain of the building.  Under no circumstances are you allowed to size to the heating load, based on International and Local building codes.  This means you will need supplemental heat to make up the difference between the heating output of the heat pump and the heat loss of the home at your design temperatures.
  For A/C systems, some math is required.  I could not hold your attention long enough in a blog to explain the how - we have a tough enough time during class, those who have attended can attest!  Basically, you cannot oversize your A/C by more than 15% above the heat gain of the home.  You will notice that they gave you some room for heat pumps to gain that extra output in heating.  The problems with oversized equipment is only compounding as our ducts and homes get tighter with the new code.  When we (not sure who "we" are) were ok with throwing energy into the attic, that oversized system worked - it blew cold air.  But let me ask you this: Was it efficient cold air?
  Since this Manual S was last updated in 1995, it does not limit how to select variable speed equipment, interpretation treating them and conventional as one and the same.  A two-stage or multi-stage condenser is not what I mean here - there is a big difference with Variable Refrigerant Flow (VRF).  Since the turndown ratio is so large with the updated ductless heat pumps, I would install a ductless system in my own home sized to the heating load knowing that the capacity will ramp down in the summer = I can still feel comfortable.  Carrier, Bryant, and Whirlpool all have conventional split systems with this VRF technology.  So, just because the code (Manual S) is outdated, should the homeowner suffer?  Why does it seem like our industry is already ahead of the code the day it is adopted, and still not enforced?


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Comment by MD on November 2, 2012 at 11:51am

Thanks Christopher. I really enjoyed your post. Cheers. Energy Auditor HQ

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