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Choosing an Air Conditioning Unit: Why an Oversized Air Conditioner is a Bad Idea

Updated: May 12, 2023


Choosing an Air Conditioning

The reason you get an air conditioning unit in the first place is to control how comfortable you are in your home. And that is the first reason not to get an oversized air conditioner.

When an air conditioner runs, it does two jobs. It lowers the temperature of the air and removes moisture from the air. To do the second one, the AC must run for a while. As the air passes over the evaporator coil, it encounters a very cold surface. Ideally, the air temperature drops about 5° C when the air passes over that cold surface. In places with higher relative humidity in the summer, the other necessary process that happens when the air hits the coil is that the temperature of the coil is below the dew point of the air. As a result, water vapour condenses on the coil. However, in oversized air conditioners, water vapour condenses on coils, too, but you have to get enough condensation on the coil for the water to start dripping down into the pan below the coil. Even then, you still have to get enough water in the pan for it to drain away. Until the water condenses and makes it outside, you haven't dehumidified the air because that water on the coil can evaporate and get back into the air in the home. Oversized air conditioners don't run for a long time because they satisfy the cooling load quickly and then shut off. Properly sized air conditioners run longer, so if you want your air conditioner to dehumidify your home and cool it, don't go oversize with your unit. The thing that wears equipment out is starting up and shutting down. The more it happens, the shorter the life of the equipment - another reason not to get an oversized unit. Oversized units start up and shut down a lot more because it runs for only a short time to meet the thermostat setpoint. Then a few minutes later, it comes on again and runs for a short time. Over a day, an oversized air conditioner can have many more start-ups and shut-downs than a properly sized air conditioner. That creates more wear and tear and reduces the lifespan of your unit. Conventional wisdom was to save money on air conditioning bills with an appropriately sized air conditioner, but in reality, that created only a small saving for the homeowner. Instead, you should save money on the upfront cost because you're putting in a smaller AC. The BTU, or British thermal unit, is the key number you need to know when searching for an air conditioner. The higher the BTU rating, the better and faster the unit should be at cooling a room. If you need help choosing the right air conditioner in London or Hertfordshire for your home, call us, particularly if you're unsure about the right size unit. Atmosfair provides air conditioning services in London to suit all budgets and applications. Don't hesitate to contact our specialist team for expert advice on installing, maintaining, or emergency repair of an air conditioning system in your home.

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