Your New Addition: Sizing Your HVAC Equipment

Climate & HVAC Sizing

House Orientation Must be Considered

As an owner-builder, you want to make sure your new home addition will be of the highest quality with the fewest possible problems.  So make sure you require your HVAC subcontractor to run a manual J calculation (or equivalent) to check the equipment sizing for your existing home with new space added and give you a copy prior installation.  The last thing you want is for your new space to be too hot, too cold or inefficient.

No Rule of Thumb: Could you imagine Ford or GM sizing a car engine based on what the designer “feels” would work best?  They always do the engineering calculations for all of the items affected by the engine to make certain it performs optimally.  So, why do we let our HVAC subcontractors look at your home and use a rule of thumb method to size your furnace or air conditioner?  I can’t tell you how many heating guys I have come across that tell me, without looking at a new architectural plan, exactly what type of system is required.  When asked how they came up with the design they typically answer “from years of experience.”  Ok, great.  You’ve been around a long time and you’re good at what you do.  So prove to me that you know what you’re talking about and do a manual J calculation (or equivalent) to prove it.  For tips on finding the right HVAC contractor for a great price go to our video…Finding Quality Subcontractors.

What is a Manual J Calculation?: This calculation takes into consideration a number of different factors that affect the performance of the heating and cooling system for your particular home.  Below is a list of the most important factors with a description of each.

• Window types and locations (including glass size and type):  Careful consideration should be given to locations of windows in your new design.  These can have a dramatic affect on your HVAC system.  Also important are glass type including Low-E coatings, gases between panes, and number of panes as well as the frame type (i.e. vinyl, wood, aluminum, fiberglass…etc.)
• How tight your home is or will be.  Newer homes are being built much tighter than homes built even ten to fifteen years ago.  With foams, caulks and building science methods, we are keeping more of the cold air out.
• Insulation levels:  All insulation must be considered into the design including ceiling walls, cantilevers, basement, crawl space…etc.
• Size, spacial configuration and orientation of your home:  Having more space typically means bigger equipment will be required (with all else being equal).  The configuration of your new space can also have an impact.  An example of this would be volume ceilings…in a cold climate, high ceilings can make a room difficult to keep warm but in the hot weather, a vaulted ceiling may actually save on cooling costs.  The orientation of your home as a huge impact on the design of your system as direct sunlight can put additional loading on your system.  Roof overhangs, careful window placement, and certain building materials will help control solar heat gain and can reduce equipment size requirements.
• Occupants (age and quantity):  Occupants give off heat themselves…the more people you have in the home, the bigger affect it will have on heating and cooling capacity.
• Climate:  Obviously the outside temperature will play a major role in the sizing of your HVAC equipment.

The most common misconception for heating and cooling systems is the notion that bigger is better.  If your system is over-sized, it becomes highly inefficient and creates wider temperature swings in your home.  It may also require more maintenance and need to be replaced sooner.  So, check the calculation your subcontractor has performed on your specific home and make sure the equipment being installed corresponds with the results of the calculations.   To see an overview of preplanning activities for your new addition go to our previous post titled Tips for Adding On To Your Home.

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