Building Code: ACCA Manual S

 For information regarding changes to ACCA Manual S, Click Here.
 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?


  1. Christopher, nice article. You (and your readers) should know that Manual S is being updated. The current language in Section 7.4 Sizing Adjustable-speed Cooling Equipment will also be revised. The current language is pretty solid guidance, but it will be revisited by the review committee and further refined by the public review process. Keep up the good work. Wes

    1. Hi Davis,

      I knew about it and was looking for the right person comment here and got you. I think proper knowledge of anything can have the proper result.

  2. Living in the Pacific Northwest we have weather that allows heat pumps to work well. There are areas which have a substancial heating loads with no cooling loads like Bandon Oregon which has 4577 HDD and 3 CDD 65 Base. Because of this the utilities have adopted performance base specifications that require heat pumps systems to be designed with a maximum balance point at 35 degrees for the home.
    I am a technical advisor to one of the ACCA manual S subcommittee members. I have pointed this out in hopes that changes might be made to make more since for all who enjoy the diverse climate here in the Pacific Northwest. There has been an exception for econimic considerations for those who live in heating dominated cilmates with arid summers, but we need this to be expanded upon so code officials dont keep us form making good decision that compromise comfort and energy savings of the heat pumps in our homes.

  3. The key point in this entire article is the last sentence. The rest of it is rudimentary. I only speak of the existing home market. Can anyone answer the last question?

  4. Thanks for the great info Wes! The contractors in MA/RI will be excited to see the updated version. Thanks for taking the time to check out my blog too - I am sure you guys are busy down there!
    Thanks to Mark for raising the awareness too! Looks like you share the same beliefs regarding Variable Capacity Heat-Pump Equipment Selection as I do. Good to know we will be well represented on the subcommittee.
    I wish I had the answer to my last question in the blog, regarding code enforcement. It may just be a total lack of understanding by local code officials. More importantly, I think those that are educated on system design may be too reserved to question the HVAC contractor's Manual J interpretation. Everyone is so worried about liability these days that no one wants to speak up!

  5. Good info Christopher.

    I find that there are many contractors that let their equipment distributors do the "selecting" for them. Most don't ever reference a "detailed cooling capacity" chart. And, even if this chart was referenced, most capacities are listed at an 80 degree entering-air drybulb temp, which is generally too high for efficient new construction projects that have tight ductwork.

    As a result of this over-estimation of capacity, right-sizing may happen more often than we think!

  6. Can anyone post here?

  7. Yes, we have an open forum, so please comment! If I find the comments inappropriate or clearly an advertisement, they will be deleted...thanks for reading my blog!

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