Heat load calculation

Need help figuring out what a heat load calculation is, and what they are used for?

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If you are considering upgrading or replacing your home’s heating system, there are many things to take into consideration and one of those is heat load.  A heat load is the amount of energy needed to condition the air inside a home or building.

The amount of energy needed depends on several factors and a complete survey of the home or building is the first step in performing a heat load calculation. We must also be mindful to consider if there are any planned upgrades to the home or building which could change the heat load in the immediate future.

Here’s what you should know:

When sizing and specifying a replacement system an HVAC professional will perform a heat load calculation for your installation, considering the existing square footage and volume, insulation values of the walls, ceilings, floors and windows, as well as how tightly sealed the house is (infiltration) and finally the outdoor temperatures for your region throughout the year.

The full heat load calculation adds all these factors together and then adjusts the total for the elevation of your home. The heat load is expressed in terms of BTUs per hour of output needed to properly condition the home or building. When recommending specific systems for your home we must also factor in the efficiency level of the equipment you are considering.

A heat load calculation for an average 1,500 square foot home takes between an hour and up to four hours after all the survey data has been gathered. If the home design is very simple, the windows are all the same and the construction data is typical they can be done very quickly.

A system that can keep your house at 70 degrees F when it is 100+ degrees F outside is going to be over-sized in capacity when the outdoor temperature is 75. What you end up with here is a system that only needs to run for a few minutes when it is mild outside, let’s say 75 degrees, and that can spell trouble. Why? An air conditioning system is intended to remove humidity as well as reducing the air temperature. If your system only runs for a few minutes, it will not be on long enough to remove much of the humidity in your home. So, what do people do then? They turn the temperature setting lower because the humidity in the air makes it feel warm. This equates to higher electric bills because the system is now going to cycle on and off several times per hour. Additionally, the excessive moisture in the home can encourage the growth of biological matter (we can’t legally call it mold) in the evaporator coil and potentially the home.

A well-calculated heat load will help ensure that the equipment isn’t too big or too small for its intended use. A properly sized system will cycle on and off anywhere between 2 and 4 times per hour depending upon the outdoor and indoor temperatures.

A few final considerations: If the original installation of your HVAC system was done with a proper heat load calculation then you may not need to have another one performed when replacing your equipment. If it was not done properly, your duct work might also need to be replaced or modified. The duct work should be sized to deliver the total volume of air that your system can produce.

Today’s high end variable capacity systems can help with improperly sized duct work as well as helping people who live in areas where the outdoor temperature has a wide swing, such as Texas. The average temperature in San Antonio, TX. is 68 degrees but in the summer, it is often over 100 degrees. In this case we would perform a heat load calculation considering the average high temperature of San Antonio which is 80 degrees. If designed properly, when it is 100 degrees outside we would expect a single stage system to be able to maintain 80 degrees inside with proper system cycle rates and adequate humidity removal. If a variable capacity system is to be installed, we could use one that has a higher cooling capacity for these hot temperature days and it would run at a lower capacity when the outdoor temperature is closer to the average. Be sure to check out our articles on system types for more details on variable capacity systems. These will soon be the industry standard and heat loads calculations a thing of the past.

If you have questions about this please feel free to reach out to us, we would love to help.