Engineering room airflow may present a real challenge when balancing an HVAC system. Most calculations only use the heat loss or gain of a room to decide on required airflow and often don’t take into consideration required room ventilation needs. Let’s take a look at how an air change calculation may simplify this step in your air balancing. What is an Air Change? An air change is how many times the air enters and exits a room from the HVAC system in one hour. Or, how many times a room would fill up with the air from the supply registers in sixty minutes.   You can then compare the number of room air changes to the Required Air Changes Table below. If it’s in the range, you can proceed to design or balance the airflow and have an additional assurance that you’re doing the right thing. If it’s way out of range, you’d better take another look. To calculate room air changes, measure the supply airflow into a room, multiply the CFM times 60 minutes per hour. Then divide by the volume of the room in cubic feet: In plain English, we’re changing CFM into Cubic Feet per Hour (CFH).

Then we calculate the volume of the room by multiplying the room height times the width times the length. Then we simply divide the CFH by the volume of the room. Here’s an example of how a full formula works: Now, compare 7.5 air changes per hour to the required air changes for that type of room on the Air Changes per Hour Table below. If it’s a lunch or break room that requires 7-8 air changes per hour, you’re right on target. If it’s a bar that needs 15-20 air changes per hour, it’s time to reconsider. Let’s look at this engineering formula differently. For example, what if the airflow is unknown and you need to calculate the required CFM for a room? Here is a four-step process on how to calculate the room CFM: Step One – Use the above Air Changes per Hour Table to identify the required air changes needed for the use of the room. Let’s say it’s a conference room requiring 10 air changes per hour. Step Two - Calculate the volume of the room (L’xW’xH’). Step Three - Multiply the volume of the room by the required room air changes.

Step Four Divide the answer by 60 minutes per Hour to find the required room CFM: Here’s an example of how to work the formula: When designing or balancing a system requiring additional airflow for ventilation purposes, remember this room will normally demand constant fan operation when occupied. This may present a problem for other rooms on the same zone, so take that into consideration. Many of these rooms may require a significant amount of outdoor air. The BTU content of this air has to be included in the heat gain or heat loss of the building when determining the size of the heating and cooling equipment. Practice these calculations several times in the shop or office. Then do the calculations in the field several times over the next week to check airflow in rooms with uncommon ventilation requirements. Study the Air Changes per Hour Table to become familiar with the rooms that need more ventilation than the heating or cooling load requires. Rob “Doc” Falke serves the industry as president of National Comfort Institute, an HVAC-based training company and membership organization.

or call him at 800-633-7058. for free information, articles, and downloads. Top Tips for Choosing an Air Conditioner All air conditioners are designed to cool—that is, to remove heat and humidity from interior spaces. If you are in the process of choosing an air conditioner, heed these important considerations before you buy. 4. Central or whole-home units Window units are the most popular air conditioners, and for DIYers, they are the easiest to install and remove. what is the cost of a new ac unitThis type of air conditioner fits in a window opening and vents to the outside.how to open window ac unit Through-the-wall units are larger and heavier than window units. ac unit size for homeRather than sitting in a window, a through-the-wall unit rests within a weight-supporting air conditioner sleeve set into an exterior wall.

Electric circuitry must be reviewed, especially in older homes, because these units often require greater amperage and voltage. Versatile by design, portable air conditioners—ideal for windowless spaces—exhaust through temporary ducting and can be moved from room to room. Central or whole-home units cool the entire house at once. Frequently combined with heating for a complete HVAC system, they are the most expensive option and typically require expert installation. Cooling Capacity of the Air Conditioner Air conditioning capacity is measured in units called BTUs. A small window air conditioner can deliver as few as 3,000 BTUs, while a through-the-wall unit can exceed 20,000 BTUs. Room size is perhaps the most important factor in determining the size of air conditioner you need, but consider also: 1. The number of windows in the room 2. The room’s exposure 3. The quality of room insulation 4. Other appliances that may be in use 5. The number of people in the room

Consult a handy BTU calculator to figure out exactly how many BTUs your air conditioner should have to keep your space cool. Energy efficiency is measured by the Energy Efficiency Ratio rating (EER)—the ratio of the cooling capacity in BTUs per hour to the power input in watts (the higher the EER rating, the more efficient the air conditioner). Air conditioners, especially older units, are among the most expensive appliances to run. Choosing an appropriately sized unit is critical, because a unit that is too small will not meet your cooling demands, and one that is too large will cost more to run and will not properly control humidity. In making your decision, first identify units capable of adequately cooling your space and then purchase the one from that group with the lowest BTU capacity and the highest EER rating. When and How to Buy During the summer, air conditioners are an in-demand commodity, which means they are in short supply and consequently more expensive.