Commercial Load Programs are very helpful tools when it comes to calculating cooling and heating loads for a building but some people use their other functions to help them, for example, size equipment or determine airflows. I’ve seen a few common errors when using these load programs for more than just the basic cooling and heating load calculations. Therefore the best thing to do is to step back and verify what goes in, and equally importantly, what comes out.
The first common error I typically see is in the selection of the return air path. Typical return air path options include ducted or plenum and each of these affect the system in a different way. When using the program to size airflows it is important to ensure this is selected correctly. For example, in a plenum return system, some of the building load shifts to the return air and increases its temperature. This will allow the reduction of supply air because of the reduced space loads as there is less heat to transfer from the plenum to the space. Each load program has different default settings for the return air path so it is important to ensure the proper input.
The second usually overlooked item is fan static pressure. When using load programs for equipment sizing, neglecting this may take away from your safety factor increasing the possibility of issues. We can see from the ASHRAE Systems and Equipment Handbook that the temperature rise is a function of differential pressure. Therefore the total heat gain will also depend on your airflow rate.
Load programs are tools which, when used correctly, can make a lot of tasks easier for us to complete. It is important however to look at the beginning and the end results using rules of thumb to make sure the results makes sense.
For more information, refer to the ASHRAE Handbook – HVAC Systems and Equipment.
