Heat and Cooling Load Calcs
Here’s the article you’ve been dreaming of. An explanation of the importance of accurate heat and cooling load calculations for retrofits and new home construction. Stay with me here as I know it may sound dry but this is one of the items that is so often done improperly and can have an incredible impact on how a home functions and feels.
People hate being too hot or too cold in their home, and can you blame them? It’s uncomfortable, to be completely literal. For this reason many HVAC contractors use a Rule of Thumb, and then round up for sizing equipment. This means that equipment will always be able to meet the load required, but can have detrimental impacts in other ways, including comfort. Well how can that be? If the space can be provided with enough heating or cooling then how can that make it uncomfortable? Well friends I’m here to explain.
Oversizing
Reduced Comfort: Oversized equipment cycles on and off more frequently, a process known as short-cycling. This leads to uneven temperatures throughout the space, creating hot and cold spots instead of maintaining a consistent, comfortable environment. Temperature stratification in taller buildings can be increased (although new systems that run the fan constantly for ventilation can help reduce this).
Increased Humidity: In cooling systems, oversizing can particularly lead to poor humidity control. Because the system cycles off too quickly, it doesn't run long enough to effectively remove moisture from the air, leading to a damp or clammy feeling that can be uncomfortable for occupants.
Higher Initial and Operating Costs: Oversized equipment is more expensive to purchase and install. Additionally, the inefficiency from frequent cycling increases wear and tear, leading to more frequent repairs and a shorter lifespan for the system. This results in higher maintenance and operational costs over time. Do you hate wasting money? I sure do
Energy Inefficiency: The frequent on and off cycling of oversized systems requires more energy, leading to wasteful spending on higher utility bills. This inefficiency not only affects the occupant's comfort but also contributes to greater environmental impact due to increased energy consumption.
Poor Air Quality: The inability of oversized systems to run long enough cycles can also affect indoor air quality. Short cycling prevents the system from effectively filtering and circulating air, potentially leading to increased dust, allergens, and other pollutants in the indoor environment.
Undersizing
Struggling to Maintain Comfort: Undersized heating and cooling equipment may struggle to reach and maintain the desired temperature, especially during extreme weather conditions. This can leave occupants feeling too hot or too cold, depending on the season. When considering this especially with a heat pump it’s important to remember that auxillary heating can be added to make up for potential shortcomings in the worst times of the year. Take the cold snap that we had in January here in Kelowna. My heat pump is undersized currently for the heat load and -30C was a doozy. Luckily we have auxillary resistive heating to supplement in this very rare instance. The cost savings from buying and maintaining a larger unit can buy a lot of days of back up heat.
Constant Operation: Unlike oversized equipment, undersized systems may run almost continuously in an attempt to reach the set temperature. While this can sometimes lead to better humidity control, it puts constant strain on the system, potentially leading to more wear and tear over time. With heat pumps many people forget that the systems shut down to defrost (which also is why a COP isn’t a true measure of energy used and we need to also look at HSPF but that’s another post) and at that point you lose heat input for the duration.
Energy Efficiency: While constantly running may seem inefficient, undersized equipment may actually use energy more consistently without the sharp spikes in consumption associated with the start-stop cycles of oversized equipment. However, the continuous operation could still lead to high energy costs.
Extended Lifespan with Regular Maintenance: If maintained properly, undersized systems may have a longer lifespan compared to oversized ones because they do not experience the same level of cycling stress. However, the continuous operation can lead to other maintenance issues over time.
While both oversizing and undersizing heating and cooling equipment can negatively impact occupant comfort and energy efficiency, the effects of oversizing are typically more severe in terms of comfort, operational costs, and environmental impact. Proper sizing is crucial for optimizing comfort, energy use, and the longevity of HVAC systems, and you know what the motto is over here at Station Craft: Understand and Optimize. Having a proper room by room F-280-12 done for a home is crucial in solving these problems.
Cheers!