The short answer is “the right size”. This topic is near and dear to me because the house we bought two years ago here in Brandermill has an oversized heat pump. I’ve tried several “tricks” to improve performance without any success. We can’t control humidity from spring to fall without the aid of a dehumidifier. It doesn’t have to be that way…
Every house is different, one size doesn’t fit all, rule of thumb sizing doesn’t work (Well, sometimes it works, kind of like how a broken clock is right twice a day). First step of properly sizing HVAC equipment is doing a heat load calculation, sometimes referred to as a Manual J. Typically this is done by inputting information about your house like square footage, insulation type, duct location, widow area, etc. into a computer problem and the amount of heat loss and gained per hour is calculated. The more accurate and detailed the information that is inputted, the more accurate the results are. Once you know how much heat loss and gains the house has equipment that fits those parameters can be selected. Unfortunately sometimes heat load calculations are performed “quick and dirty” just to be used as a sales tool for the appearance to be providing value. This often leads to oversized equipment being installed.
Why is this important? Bigger is better right?
When it comes to heating and cooling your home bigger is definitely not better. Equipment runtime plays a huge roll in overall comfort.
In cooling long runtimes are required for humidity removal. It can take 10-15 minutes for the indoor coil to become cold enough to start removing moisture from the air. An oversized air conditioner runs for less time per hour when compared to a right sized one. Less runtime = less moisture removal. Larger air conditioners also consume more power to operate, so less runtime does not necessarily = lower power bills.
Oversized heating equipment (most common when using fossil fuels) satisfy the thermostat too quickly. This can cause several comfort issues. The air in the living space is heated so quickly (then the thermostat turns off the equipment) that large temperature swings occur. Short cycling the thermostat also contributes to surfaces not being heated. Surfaces can be significantly cooler than the air, then radiant cooling occurs. An extreme example of this is the grocery store’s refrigerated section. The air might be 72° but it feels very cold. Our bodies reject heat to the cold surfaces making it seem much cooler than the air temperature. If a whole house humidifier is present longer runtimes allow for more humidity to be added. (Most humidifiers are the evaporative type and require heat to evaporate the water.)
Replacing your equipment can be a 15 year decision. If you are experiencing comfort issues this is a great opportunity to address them. Make sure your contractor does their due diligence by performing a heat load calculation, and selecting the right sized equipment. Bigger is not always better.