Product Sizing and Selection

Careful consideration must be made when deciding what size and capacity of system to install in your home.  An undersized system will not perform or keep up with your heating or cooling load, leading to unsatisfied occupants.  An oversized unit will appear to heat and cool the space properly but cycling will increase, and thus efficiency will be decreased.

There are several avenues for you to explore when deciding on the right system size for your application:

1. What is your existing system size and how does it perform? If you are retrofitting an existing building this is usually the first step.  Consider how your system has performed in the past.  Does it keep up? If not, your system may be too small. If you start up the system when the space temperature is well below or above set point how fast does it heat or cool the space (called recovery)?  Is this satisfactory?  Under design days (the coldest or hottest time of the year) Does your system cycle often? If so, the system may be too large.  Have home improvements been made such as an addition, insulation, window change out?  If so, these may make your system over or undersized for its current load.

2. Professional opinion - If you are in the market for a new HVAC system and are comparison shopping, you must have or definitely should get some proposals for complete installation by your local HVAC contractors.  In these proposals, they should have done a load calculation, if not then they are just replacing like with like, and you should move on to another contractor.

3. Rules of Thumb - There are several resources on the web for rules of thumb for sizing of equipment.  These consider your location, building size, type of construction etc.  This should be used as an initial sizing and a quick check of your existing system, as most will have a disclaimer that they are not to be used for system selection.

4. Load Calculation - There are several commercially available load calculation programs available.  These all should use the industry standard Manual J calculation. A great little download that can be used by the homeowner to do their own load calculations can be found at Hvac Computer

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