Understanding Heat Load and Heating Capacity

February 29, 2016

If you’re looking to buy or upgrade your furnace, it’s important to know some of the basic facts about heating and cooling systems. In the case of furnaces, two important terms are ‘heat load’ and ‘heat capacity.’ A beginner in the world of HVAC (short for Heating, Ventilating, and Air Conditioning) might not know the difference between the two, so let’s take a look at what each term means and why it’s important for a homeowner to know about them.

Heat load refers to the amount of heating that a building needs in order to maintain the indoor temperature at established levels. Heat capacity means the amount of heat that a piece of HVAC equipment can provide. When you see the term ‘heat load,’ it is referring to the building’s needs. When you see the term ‘heating capacity,’ it refers to the actual heating equipment. In this case, both terms are quantified in numbers of “btu’s,” the unit of measurement which stands for “British thermal units.”

How do You Determine Your Home’s Heat Load?

When you obtain estimates for replacing your furnace (or boiler), the HVAC contractors that you speak with should all perform a heat load calculation for your home, necessary in order to determine the proper size furnace needed. Many years ago, HVAC companies used very rudimentary sizing parameters, based mainly on the square footage of the home, and the result was frequently far from accurate. To play it safe, equipment was often way oversized, which will result in higher energy costs, more operating noise, and most importantly, an uncomfortable home!

Later, the Air Conditioning Contractors of America developed standards and detailed mathematical formulas to determine a home’s heat gain or loss. How regularly those formulas were used–and how accurately–typically varied widely by the contractor, as well as by the job. Now, computer software makes it much easier to determine the amount of heat gain or loss, while taking into account greater detail of a home’s construction. It’s now easier than ever to determine an accurate heat load for a home–the important first step in choosing a properly sized replacement unit. And even with these advancements, many HVAC contractors pay little mind to proper equipment sizing.

Why is Heat Capacity Important?

Matching the heating capacity of available replacement furnaces with the results of the heat load calculation is the final step in determining the proper sized unit for your home. An experienced HVAC contractor will be adept at choosing a furnace from his product line whose heating capacity either matches or is a little above the amount of heat required. As part of the process of choosing the appropriate size equipment for your home, the contractor will also factor in elevation (above sea level) and efficiency rating of the proposed furnace, both of which are crucial variables to consider while making sure that the recommended unit that is will provide enough heating capacity.

In the HVAC world, balance is key. In heating your home, you need to balance your home’s heating requirements with your heating system’s capabilities. If the heat load and your system’s heat capacity are out of sync, you can end up uncomfortable and paying more for your energy bills. Understanding the difference between heat load and heat capacity is the first step in balancing the two and creating heating harmony in your house.