Important considerations for HVAC Load Calculations
In the HVAC industry calculating the Heating/Cooling loads of a building are critical to the overall design of the building. There are many variables that need to be examined to appropriately size the HVAC system. In the past, all variables either could not be measured or were not taken into consideration, and safety factors were applied to compensate for the unknow heating/cooling load. This can cause the system to be over sized, and can cost the building owner additional money in installation and operating costs. Properly sizing HVAC systems can maintain comfort, humidity levels, and increase energy savings. A better understanding of the principles behind how buildings lose and gain heat can help with discussions between building contractors, and HVAC engineers.
Location of the building is a critical design consideration for Heating/Cooling Loads. For example, the cooling load will be much less for a building in New York City vs. a building in Miami, and vice-versa for the Heating Load. Usually there are two temperatures to design around, the Warm Season Temperature, and Cold Season Temperature. These temperatures represent the coldest winter day and the warmest summer day respectively. Temperature data is collected and shared by ASHRAE for cities around the world, and can be found in ASHRAE Fundamentals Handbook. The Warm and Cold Season Temperatures are a starting point in understanding how much heat is gained or lost by the building due to the outdoor ambient temperature.
Another aspect to look at is how well the building is insulated. A well-insulated building will be able to maintain comfortable temperatures without increasing energy usage. The measurement of how well heat travels through a given material is known at the “U-factor”. The U-factor is the amount of energy that travels through the material per area of material within an hour for every degree difference across that material. The metric system uses the units kWh/m2-C-hr, and the imperial units are Btu/ft2-F-hr, to define the U-factor. Building materials with relatively low U-factors are good insulators, and keep heat from transferring quickly.
The temperature difference used in the U-factor equation is the difference between the indoor design temperature and the outdoor temperature. The outdoor temperature used is one of the seasonal temperatures that was discussed above. The indoor design temperature is set depending on the activities in the building, typically this temperature is 70oF. To find the buildings Cooling Load the Warm Season Temperature is used for the outdoor temperature, while the Heating Load uses the Cold Season Temperature. To find the heat loss through a building wall, ceiling, or floor, the equation is used. Looking at that equation Heat Loss (Q) is equal to the walls area (A) times the materials (U-Factor) times the change in temperature across that wall.
You can see that heat loss calculations can become complicated quickly because building walls, ceilings, and floors are not usually the same material throughout. The percentages of materials used can simplify the calculations. For example, if a wall is 10 feet high by 50 feet long with five, 4’ x 2’ windows, we can assume that 8% of the area for heat loss is the windows and the rest is the normal wall. The same assumption can be made with the make-up of the wall, like if it is 80% insulation and 20% lumber.
Infiltration and exfiltration need to be taken in consideration when calculation heating/cooling loads. This is when air leaves the building through cracks, or forced out via ventilation for air quality purposes. There are tables found in the ASHRAE Fundamentals Handbook – Chapter 17 that tabulate the building material, whether it’s a widow, door, or even an elevator shaft, and give it an air leakage coefficient. These calculations are usually very complicated and use system software in conjunction with the blower-door test to calculate heat loss rates.
Occupation, activity, and equipment within the building is another variable that need to be taken into consideration. Internal loads can range from computer equipment that needs to be cooled, to people moving in a factory. These values of heat gain can be found in Chapter 18 of ASHRAE Fundamentals Handbook.
When all the variables for a building are taken into account the Heating/Cooling load for a properly sized HVAC system can be installed. This will keep the building occupants comfortable, while keeping cost and energy usage to a minimum.