Load Calculations Matter

Load Calculations Matter

Guest Blogger: Chris Morin, Owner & Founder, HVAC Pro Blog

‘Hi, my name is______, and I don’t do Load Calculations.’

Just for the record, I have completed more than my fair share of Manual J Load calculations.  Don’t get me wrong; I have tried all of the short cuts in my 20+ years in the industry!  What I have found is that not a single short cut helps you, and will come back to haunt you, to include this one!  Never mind the fact that Manuals J, S, and D are Residential Code Requirements across the U.S. as part of the International Residential Code (IRC), International Energy Conservation Code (IECC), and International Mechanical Code(s) (IMC).

When I speak with HVAC Contractors who use a shortcut, for various reasons, most are convinced that they don’t need to calculate the heat loss or heat gain on a building.  That they have quoted so many systems in their career they know the A/C system should be X tons for that home.  Their “process” looks a bit like this:

  1. X,XXX sq ft. / XXX sq ft. per ton = X Tons
  2. Select the next size up, ‘just in case’
  3. Install the always-oversized system, and never have a call back because they don’t answer their phone anyway!

I can assure you, your experience does not equate to being above the laws of physics or thermodynamics.  I can also honestly say that if you are doing a calculation for heating only, you are probably close to correct in sizing your system – since you always work with the same temperature differences.  Here are the Factors that affect the heating & cooling loads:

  1. Temperature Difference 

Heating: about 75% of the load calc

Cooling: about 10% of the load calc

  1. Air Exchange

Heating: about 25% of the load calc

Cooling: about 15% of the load calc

  1. Solar Heat Gain

Does not impact Heat Loss

Cooling: up to 60% of the load calc

  1. Internal Gains

Does not impact Heat Loss

Cooling: about 15% of the load calc

Since Solar Heat Gain and Internal Gains cannot be easily calculated without software, and can make up as much as 75% of your heat gain, I hope you can see the need to complete the first step in Residential System Design: ACCA Manual J (v8).  The problem with not completing this step, or not doing it completely and correctly, is that it will impact the rest of the design process with Equipment Selection (ACCA Manual S) and Duct Design (ACCA Manual D).  This usually equates to increased equipment size/cost, increased duct size/cost, and an increase in customer energy use.  Worse, these items decrease customer comfort and satisfaction.

Of course, this is an easy process for new construction homes, as all of the information of the building materials is literally on a piece of paper for you.  If you are retrofitting any residential system in existing construction, a strong understanding of building materials and construction is required for a proper site survey.

So, the first step to recovery is admitting you have a problem, right?  If so, don’t worry as many have come before you and laid out a time-tested, peer reviewed system design process!  Trust me, when you start using the process correctly, at the very least, it will help you (and your customer) sleep at night!


Chris Morin is the Author, Owner & Founder of the popular HVAC Pro Blog.  For Advice with Residential HVAC System Design, Quality Installation, and System Diagnosis, you can find Chris’ Articles on HVACProBlog.com, or popular social media outlets Facebook, LinkedIn, and Twitter.



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