Sizing - Air Conditioner Sizing - Part I


Air conditioner sizing means finding the air conditioner that can balance the heat gains and losses within a building. There is a famous way of sizing an AC. It is the square feet method. However, there the-also-famous Manual J method available, to size ACs accurately.

By TS Mohamed Rasied

We hear a lot about air conditioner sizing, whenever we make the important choice of purchasing, or installing one. What does the word sizing really mean?

Air conditioners are designed for heat removal, or addition (if heat pump is included in the single package). Therefore, the "size" of an AC, will be specified in terms of heat removal, or addition capability.

Units that we use for air conditioner sizing, are BTU/hr (British Thermal Units per hour), Ton, and Watts. Some countries utilise one unit, more than the others, in air conditioner sizing.

It is excellent if you can remember the relationship between BTU/hr, Ton, and Watts. You can quickly gauge the AC size, in case if your friends use units - that are unfamiliar to you;

  • 1 ton is equivalent to 12,000 BTU/hr. And,

  • 12,000 BTU/hr is equivalent to 3,516 Watts - or 3.516 kW (kilo-Watts).

Concept and fundamental of air conditioner sizing

It is based on heat gain, and/or losses in a building. It is obvious that you will need to remove the amount of heat gain into your house - if it is hot outside. Similarly, you'll need to add in the heat loss from your house - if outside temperature is cold.

In short, heat gain and loss, must be equally balanced by heat removal, and addition, to get the desired room comfort that we want.

What determines the amount of heat removal and/or addition that we want?

It is determined by,

  • the temperature difference between outside temperature, and our desired temperature. It is common sense that if you want a cool room, in a very hot climate, you'll need to remove a lot of heat. On the other hand, it does not take much to maintain a cool room, on a mild day

  • the construction material, and arrangement of your building. Let's say, that you have two identical houses. One is build out of glass, and the other out of brick, and well insulated.

    Of course the one built with glass would require much more heat addition, or removal, compared to the other - given a same day.

    Two identical house, with different orientation with respect to the direction of sun rise and fall, will also influence the air conditioner sizing

  • the surface area of the walls. The larger the surface area - the more heat that can loose, or gain through it. Hence, effects the AC size that you would need

  • infiltration plays a part in determining our air conditioner sizing. Door gaps, cracked windows, chimneys - are the "doorways" for air to enter from outside, into your living space. Only you know how frustrating it can get, to feel a draft of chill, or hot air from outside

  • activities and other equipment within a building. Cooking? Hot bath? Is it a gymnasium with a lot of intense exercise?

  • the occupants. It takes a lot to cool a town hall full of people, doesn't it? Oh yes, we generate heat.

Square feet method?

You might have heard about the square feet method in air conditioner sizing. In fact, it is used often by many contractors, that we sometimes use it arbitrarily. This method have been generalised, and simplified by contractors, from sizing many ACs.

The most common relationship is, "1 ton for every 500 square feet of floor area". This method might work for a contractor, working in a small area, with similar type of houses or buildings, and similar wall material.

But, the relationship will not work, if you change the wall construction type, or if you move into a different area, with different climate. Simply said, this method of air conditioner sizing will only work for a confined group of building, in a given area.

However, this is method is reasonably useful for installing window, small split, or portable air conditioners.

Manual J

Now, this is the way we should do AC sizing. Manual J is a database, with methodologies, formulas and guide for sizing and AC. This manual has been developed by ACCA (Air Conditioning Contractors of America) - with regular reviews.

It will provide you with necessary data for heat gain and heat loss calculations. Wall construction, building orientation, infiltration, shading, number of occupants, the temperature you would like to set, outside temperatures, are among the most important values it will look for, for proper air conditioner sizing.

This is indeed, an extensive manual, which is used by certified and experienced air conditioning contractors.

Manual J is just a tool. The important things are, the values that are put in it. If we put in accurate values, then our results will be come accurate. Otherwise, it's not going to help much.

What are amongst the important information, you should have for air conditioner sizing using Manual J? These are,

  • required temperature range within your space,

  • approximate temperature distribution within your area, to find out the number of months you have winter, summer, and mild weathers,

  • wall surface area and material. Window surface area,

  • any tinted window? Any insulated space?

  • how many person is going to live within each space?

  • what are the activities carried out throughout any day? How much cooking? How much of hot water utilisation?

  • orientation of your building, with respect to the sun's rise and fall direction, and,

  • sources of infiltration

TS Mohamed Rasied,
Provides information on air conditioners and air conditioning in her site,
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