Calculating your AC size for any given space is a complicated process that requires both technical knowledge and experience. A few factors that can complicate the process are the quality of your insulation, the projected occupancy and the number of windows and doors in your home. It’s much more than choosing a size based on the square footage of your building alone.
Most situations will require a trained professional to truly determine what size unit is needed. Grandmark Service Company wants to provide as much knowledge as possible so you can best understand every aspect of your HVAC situation.
How Are Air Conditioners Rated?
Before asking the question, “What size air conditioner do I need?”, it’s important to understand how air conditioners are sized and the terminology used in the industry to describe various sizes.
Air conditioners on the market are generally rated in “tonnage” or “BTU”. A BTU — or British thermal unit — is a measurement of energy that signifies how much energy is required to change the temperature of one pound of water by one degree Fahrenheit. BTUh stands for British thermal unit per hour and is a unit of measurement that signifies how much heat an air conditioner can remove from a room in an hour.
Tonnage is just another way to describe BTUh. BTUs directly translate to tons: 1 ton is the equivalent to 12,000 BTU. Most residential AC units are between 1.5 and 5 tons, or 18,000 – 60,000 BTU.
The Manual J Calculation
A qualified professional is the only person who can accurately determine what size of AC unit is needed for a given situation. In order to professionally calculate the required BTUh for any situation, a formula must be used called “The Manual J Calculation”.
The Manual J Calculation is to the HVAC industry what E=mc2 is to physics. It’s one of the foundational pieces of information that determines a vital aspect of any AC situation: the HVAC load, or how big the unit will need to be.
The formula involves carefully taking various measurements around the building and inputting them into an equation. Because every HVAC situation is different, the formula requires different measurements in each circumstance. A one-room apartment will have different requirements than a multi-room school.
Some variables that could be measured for the Manual J calculation are:
- Square footage
- Number of windows
- Number of doors
- Quality and type of insulation
- Building Materials
- Environmental Factors
There are many other potential factors that could go into determining the size of your AC. This makes it important that every situation is evaluated by a trained professional before any major HVAC decisions are made.
DIY Air Conditioning Sizing
While we don’t recommend making any decisions based on any unprofessional opinion, there is a quick calculation that anyone can do to get a rough estimate.
To approximate the HVAC load for a room, take the square footage of the space by multiplying the length by the room’s width. That number is then multiplied by 25 BTU. This means that to calculate a room that’s 5 feet long by 5 feet wide, the equation needed would be (5×5)x25. This equals 625 BTUh needed to cool that space.
Again, we don’t suggest making major purchases based on this kind of impromptu calculation, but this method can be used to get a rough estimate of what size AC will be needed for a situation.
Never Be Unsure About Size With Grandmark Service Company!
With the help of our qualified professionals, you can rest assured that the unit you purchase will be the correct size for whatever space you’re trying to keep cool. If you did the work yourself and feel that your unit isn’t adequately sized for your circumstance, don’t hesitate to call us and have one of our trained employees perform a fully comprehensive Manual J Calculation so we can set you on the right path!