- Home
What Are Heat Loss and Heat Gain Calculations Used For?
A well-maintained furnace will generally last for somewhere between 15 and 20 years. However, air conditioners typically have slightly shorter lifespans at anywhere between 10 and 15 years. Unfortunately, this lifespan means that most homeowners will have to replace their essential HVAC equipment several times during their lifetime.
According to the US Department of Energy, our homes' heating and cooling account for at least 48 percent of total household energy usage. Fortunately, newer models of air conditioners, furnaces, and energy-efficient heat pump drastically lower the amount of energy we need to purchase to keep our homes at a comfortable temperature throughout the changing seasons.
Regrettably, many of the efficiency advantages of newer and improved HVAC equipment are nearly lost when they are not sized correctly for individual homes. Many HVAC companies and private contractors will routinely recommend larger units to boost their profit margins and to "be on the safe side" in the event of extreme weather events.
When an air conditioner or furnace is oversized, this leads to increased energy consumption and less efficient heating or cooling (we will detail this issue in the next section). Therefore, heat loss and heat gain calculations are used by HVAC technicians when consulting on the best unit for a home. However, many companies will revert to generic formulas that only consider the home's generalized square footage.
If you are undergoing a more thorough energy efficiency retrofit of your home, hiring an independent home energy auditor is an excellent place to start. An energy auditor will utilize heat loss and heat gain calculations and dozens of other computations and estimates to design the most effective strategies for improving your home's energy efficiency and thermal performance. Besides identifying areas of low insulation in your walls, ceiling, and foundation, an energy auditor will also be able to recommend the most efficient furnaces, air conditioners, and ventilating equipment to keep your home comfortably and efficiently heated and cooled.
Heat loss or gain (Q) is the result of: the U Value of the building materials (U) x the surface area of the building material (A) x the difference in outdoor and indoor temperatures (ΔT)
U-Values are usually associated with energy-efficient windows. However, all building materials can calculate their U-value, which is a measure of thermal transmittance. Thermal transmittance is defined as "the rate of transfer of heat through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure."
To make matters even a bit more complicated, accurate heat loss or heat gain calculation should also include:
- A combination of the R-values of the various building materials with the exposed surface area
- An estimate of the adequate hole size in the building envelope (this is measured by an energy auditor using a blower door test)
- The total glass area, shading, and direction it faces relative to the sun
- The BTUs per hour provided by lighting, appliances, and other equipment
- The "people load," measured in BTUs per hour, and
- The ventilation load for fresh air entering and exhaust air leaving the building
What Software Exists for Heat Loss and Heat Gain Calculations?
At this point, the $300 to $700 charged by an independent energy auditor might be seeming like quite a deal. However, if you don't want to tackle the calculations independently, there are also online resources available to get you your results.
Is It Worth Discussing Heat Loss and Heat Gain Calculations With Your Contractor?
The National Renewable Energy Laboratory (NREL) finds oversized HVAC equipment is responsible for higher energy use rates due to increased on/off cycling and parasitic off-cycle power. Essentially, the extra power of larger central air conditioners and furnaces will more quickly get your home to its desired interior temperature, thus causing it to cycle on and off more frequently. Unfortunately, this cycling requires extra energy usage and can also negatively affect the lifespan of your HVAC equipment. As a valid comparison, the short bursts of energy that happen with oversized HVAC equipment can be likened to the worsened gas mileage drivers get when driving through stop-and-go traffic during rush hour.
Though your contractor may charge extra money for a detailed heat loss and heat gain calculation, this should ensure that you properly size the HVAC equipment for your home renovation. This step usually results in lower upfront costs as smaller equipment is generally cheaper. It will also save you money in the long run through increased operational efficiency.
Finally, an oversized system will result in reduced comfort. An oversized air-based system will result in uneven temperatures throughout the home unless the fan is set to run independently of the need to heat and cool. In addition, an oversized air conditioner won't be able to remove as much humidity that is responsible for much of our summertime discomfort.
This article has been reviewed by:
David Elfstrom of Elfstrom Energy Solutions.
Tobias Roberts
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.
