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. best ac wall unitThis may present a problem for other rooms on the same zone, so take that into consideration. how much does it cost to repair car acMany of these rooms may require a significant amount of outdoor air. ac unit turns on then offThe 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.So how do you know which size air conditioner you need given the many different sizes available? It depends on the size of the room or rooms you are trying to cool. To start you need to measure the area that needs to be cooled and this will give you a good idea of the correct size air conditioner you will need. If you are looking for an air conditioner that will cool your entire house then please call us as there are many other factors needed to calculate this. To calculate the size you need to multiply the length times the width of the room or rooms you are trying to cool. 

When trying to cool two adjacent areas determine each space separately and add the totals together. Then refer to our chart to the right to see how many BTU’s your air conditioner needs in order to cool the room(s) properly. Example 1: You want to cool just one room that is 10′ long and 12′ wide. 10 X 12 = 120 By looking at our chart you would need an air conditioner with 5,000 BTU’s to cool this room of 120 sq. ft. Example 2: You want to cool two adjacent rooms. Room #1 is 10′ long and 11′ feet and Room #2 is 8′ wide and 10′ long. Room #1: 10 X 11 = 110 Room #2: 8 X 10 = 80 Total of both rooms is 190. You would need an air conditioner with 6,000 BTU’s to cool these rooms. Once you have determined the number of BTUs that will most efficiently cool your rooms, you need to make adjustments based on other factors as well: How many BTUs to cool a room? Here are some common room sizes and the needed BTUs: 8 X 10 = 80 sq. ft. |

9 X 10 = 90 sq. ft. | 10 X 12 = 120 sq. ft. | 11 X 13 = 143 sq. ft. | 12 X 12 = 144 sq. ft. | 12 X 14 = 168 sq. ft. | 13 X 19 = 247 sq. ft. | 14 X 18 = 252 sq. ft. | 16 X 20 = 320 sq. ft. | While you can calculate the square footage on your own, one of our trained air conditioning professionals may be the better choice to perform a BTU calculation on your home. Give us a call and we will be more than happy to help you get the correct air conditioner you need.Select the right air conditioner to cool off and save on energy costs with this buying guide.Air Conditioner TerminologyChoosing the right air conditioner is easy with the right information. The first step, before you go shopping, is to learn to speak the language.BTU: The British Thermal Unit (BTU) is the international measurement of energy. A BTU is the amount of heat required to raise 1 pound of water 1ºF Fahrenheit. In air conditioner (AC) jargon, a BTU measures the amount of heat an AC unit can remove from the room.

As the BTU rating increases, so does the size, weight and cost of the AC. Room ACs are generally 5,000 to 24,000 BTU. Matching BTU requirements to room size is important. The room(s) won't cool efficiently if the BTU rating is too low or too high for the size of the room.EER: The Energy Efficiency Ratio (EER) is calculated by dividing the BTU by the number of watts. The higher the EER number, the more efficient the unit should be. A higher EER-rated AC will also be more expensive, so decide if the higher price tag is worth the energy savings you may be getting. The ratings range from 8 to 11.5. If it's really hot where you are, go for a rating of 10 or higher.Thermostat: The heat-sensing thermostat adjusts the temperature of the air coming from the AC unit. They can be programmable or manual. An adjustable thermostat combined with a variable-speed fan is an effective cooling machine. AC units are available with a remote control, 24-hour delay timers and digital temperature readouts.Filter: Clean air is essential for efficient operation.

Look for units with filters that can be easily cleaned or replaced.Fan: The fan moves the air. A variable-speed fan combined with an adjustable thermostat is an effective cooling machine. Look for louvers that adjust up / down and right / left.Chassis: The chassis is the frame that supports the main parts of an AC.Packaged Terminal Heat Pump / Air Conditioner: This type of large heating or cooling unit, installed through the wall, is used in hotel and motel rooms.Shop Air ConditionersMeasurements to ConsiderMeasure the RoomDetermine the square footage of the room (multiply length x width).There are lots of variables to consider when choosing a room AC: number of windows, number of people in the room, which direction the room is facing, ceiling height, etc.This chart can help you find the right size room air conditioner to meet your needs. Area to Be Cooled(square feet)Capacity Needed(BTU per hour) You may need to adjust the BTU rating needed based on whether the room to be cooled is sunny or shaded.  

Measure the Window DimensionsMost window air conditioners are designed to fit double hung windows. Measure the inside dimensions. Take an exact measurement, not just an estimation.Take your room dimensions and window measurements with you when you go shopping.Check the Power SupplyWhat kind of plug does the unit have? What kind of plug is your wall outlet? Make sure the circuit can handle the operation of the unit. Smaller ones work with a standard 115-volt outlet. Larger 115-volt units may need a dedicated circuit. The largest AC units require a 230 / 208-volt circuit.Safety Considerations for Your Air ConditionerWeightHow heavy is it? Get some help when carrying and installing the unit. An AC unit can easily weigh over 100 pounds.NoiseAlthough newer units are much quieter than older models, be prepared for some additional noise in the room.InstallationFollow the manufacturer's instructions carefully when installing a window unit. It's essential that the unit be stable for safety reasons.

It's also essential that the unit be tightly sealed to keep the cool air inside where it belongs. Apply weather stripping as needed to prevent airflow around the unit. Put the AC in a shaded window if possible for added cooling efficiency.MaintenanceAC problems can be easily avoided with simple maintenance. Keep the filter and coils clean or replace them as recommended by the manufacturer. Don't need a new AC just yet? Here's some preseason maintenance advice for your old unit:Remove the inside grill. Carefully take out the foam filter. Wash in warm soapy water. If the filter's extremely dirty, wash it a few times. Rinse and allow the filter to dry before replacing. Filters that are beyond the reusable state can be inexpensively replaced.Filter maintenance alone will increase your AC's efficiency. Get the most from your unit by cleaning it thoroughly.  Take the unit from the window and remove the outside housing. Vacuum the interior to remove dirt, bugs and leaves. Cover electrical parts with plastic, and then wipe the interior with a damp cloth.