When considering air conditioning, there are several factors that come into play. The task at hand is to match options with the room that needs to be cooled. While that may seem like an easy task, it becomes harder when we examine some of the considerations professionals think about.
Room Size – This is more than just square footage, although that is the base calculation. The rule of thumb is 20 BTU of cooling capacity for every square foot.1 Using this rule, a 12ft by 12ft room would only need about a 3,000 BTU air conditioner. However, this is likely to be too small and you will be hard pressed to find an air conditioner less than 7,000 BTU in capacity.
This measure doesn’t take into account the ceiling height or the airflow that naturally occurs between rooms. The smaller the amount of air in a room, the quicker that air can be exchanged unless the room is well insulated and closed. The only situation where 3,000 BTU might be appropriate is a small, insulated attic space.
Room Volume – HVAC professionals consider not just the square footage of a room, but the vertical height as well. They will look at the cubic feet per minute an air handling system is capable of producing and compare that with how much air is actually in the space. This will tell them how long it will take to appreciably change the temperature in the room.2 The ability of any particular room air conditioner to move air will also affect whether some parts of the room remain hotter than others.
Room Use – The number of people who occupy a room affects the amount of air conditioning needed. Human beings generate heat. If more than two people will occupy a room regularly, the cooling needed is increased by 600 BTU per person. Other uses may also change estimates – for instance, a kitchen will increase demand considerably because of the heat from cooking.3
Electrical Supply – While many single room air conditioners will run off of normal household voltage, they may require a dedicated circuit if they draw 15 amps or more. A dedicated circuit just means that no other appliances or loads use the same supply from the fuse box. Other installations might require a higher than normal household voltage which would need replacement wiring.
Other Factors – If the room is on a south wall or there are multiple windows, this will increase the cooling load. Poor insulation will also need a higher cooling capacity. Climate also comes into play because heat transfer is faster when there is a larger difference between outside and inside temperatures – this also increases load during high demand.
A sizing estimator can be found here.4
Which type of air conditioner?
The standard, for single room air conditioning is the window unit – except where these cannot be installed or will not efficiently cool the room. In some cases, there is no appropriate window available. A portable air conditioner can be used if there is any venting possible and a split system can be installed with only a small hole through the wall. However, because of the price and convenience advantages, it is better to have a window unit or a similar unit that penetrates the wall.
If cost is not the primary concern, a split system offers installation and customization advantages. The small interior unit (fed by an outside condenser) can be mounted on any interior wall without blocking a window.
Another option is to use a fan as an accessory to move air around when the built-in air conditioner fan won’t reach the entire room.
The most common concerns with air conditioners involve noise and improper functioning. Whenever a unit that is too small for the room is installed, users quickly find they cannot achieve a comfortable temperature. This is a result of underestimating the BTUs required for the installation. This often happens when extreme temperatures aren’t considered – as summer temperatures rise, along with humidity, a previously acceptable air conditioner is found insufficient. Adding a separate fan or dehumidifier will often overcome this problem, but proper sizing is the root cause.
Noise is a common issue that appears when a room air conditioner is either improperly installed, poorly manufactured or the room needs to be particularly quiet. Window units will mention how loud they are in the detailed description (measured in dB) and if the room is used for sleeping, watching television or other purpose where noise may be a problem, this should be checked. Paying more for a quieter unit is worthwhile in these cases.
When an air conditioner is improperly installed in a window, it can rattle against the casement. Poor construction of the fan, condenser and less than adequate insulation in the unit itself can generate higher levels of noise. Sometimes, an increase in noise is a sign of an imminent failure or a lack of lubrication.
Inefficient cooling can also result when the air filter is too dirty to allow free air movement. This causes the fan motor to work harder and creates internal heat. Poor cooling can also be the result of a unit that is too large. Called “short cycling” the unit will kick on and then almost immediately off as the temperature drops quickly. This leads to undue stress on the unit as it continuously cycles back and forth.5
Although this article mentions some of the difficulties and problems associated with correctly installing a single room air conditioner, this is well within the skill set of most homeowners. Forewarned is forearmed. Just paying attention to the pitfalls mentioned above will get you past any difficulties and make your purchase an informed and satisfactory one.
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