Although there is general awareness of oversizing equipment within the residential construction industry, the full extent of the problem and consequences is not often appreciated, including:
- Efficiency – Longer run times reduce cycle losses and thus improve efficiency. HVAC equipment that is too large for the home (and required heat load) will short-cycle resulting in the published efficiency rating of the unit not being achieved.
- Installation & Operating Costs – Oversized equipment costs more to install and operate due to significant increases in equipment and material costs plus increased cycling losses operationally. It is well understood that HVAC equipment operates more efficiently when it can run at steady state instead of “stop and go” when oversized.
- Shorter Equipment Life Cycle - Not only is excessive cycling bad for efficiency, it is also harder on the equipment components. Using a furnace as an example, the rapid, repetitive expansion and contraction of components as it heats up and cools down over and over again puts stress on the heat exchanger and other components.
- Reduced Comfort - Oversized HVAC equipment causes wider temperature fluctuations, especially in perimeter and remote areas of the home. In addition, an oversized system produces more noise than one that is sized under proper design procedures.
To address right sizing HVAC systems, there is a Canadian Standards Association (CSA) standard on how to properly size HVAC equipment that is nationally recognized and referenced by all Canadian Building Codes. It is called CAN/CSA F280-12 “Determining the Required Capacity of Residential Space Heating and Cooling Appliances”. The new F280-12 standard was designed to address the shortcomings of the original standard published in 1990 that resulted in the oversizing of residential HVAC equipment/ductwork and account for the evolution of building codes and growth of high performance homes in Canada.
In British Columbia (BC), the F280-12 standard is referenced in the BC Building Code and other building industry publications including:
2012 BC Building Code states in Section 9.33.5 Capacity of Heating Appliances:
“The required capacity of heating appliances located in a dwelling unit and serving only that dwelling unit, shall be determined in accordance with CSA F280, “Determining the Required Capacity of Residential Space Heating and Cooling Appliances,” except that the design temperatures shall conform to Subsection 9.33.3.”
Homeowner Protection Office states in the Energy Efficiency Requirements for Houses in British Columbia:
“HVAC systems and ducts are required to be sized in accordance with good practice”, such as described in the Thermal Environmental Comfort Association (TECA) reference material, CSA 280, and Section 9.32 and 9.33.”
Within the building industry, there are a number of modelling practices, technology and software approaches with varying degrees of sophistication available to HVAC contractors and builders to guide the selection and design of HVAC equipment in Part 9 construction. Current industry practices range from the guestimate method (X BTUs per square foot) to spreadsheet models (e.g. TECA Quality First™) to NRCan’s HOT2000 to HVAC Design software (e.g. Wrightsoft) and others.
Be sure to avoid the "rule of thumb" and work with a professional who understands and uses load calculation tools that meet the F280-12 standard to avoid the disappointments and issues that arise when HVAC systems are oversized.