Discover Ways for Air Conditioner Sizing...

Air conditioner sizing is a very important step before you buy, or rent a unit. Get the sizing wrong, and you’ll be facing problems such as insufficient cooling, insufficient moisture removal, and unnecessarily high electricity bills.

It is a mistake, you can’t afford to make.

I will explain to you on this page, about variations on air conditioner sizing. Let us begin.

Purpose of air conditioners:

Heat transfer occurs whenever there is a temperature difference between two solids, fluid (liquid or gas) sections, and through vacuum.

It may happen through conduction, convection or radiation, or any combination between these three.

Hence, heat transfer between outside and inside of your dwelling space is inevitable. It either transfers out of your house (during cold season), or transfers in (during hot season).

Air conditioning units are designed to control heat gain or loss within conditioned air. These are:

  • Sensible heat, and

  • Latent heat

Sensible heat

Is defined as the heat of dry air. It is the “dry heat” from anything that has the temperature above 0 Kelvin, be it from electrical appliances, lighting and sun’s radiation.

This is the horizontal component of temperature rise, as illustrated in the psychrometric chart.

Latent heat

Is the heat contained in the water vapour, carried by the air. The higher the water vapour content, the higher the latent heat. Examples are, your perspiration, cooking by steam, or hot water vapour from the bathroom.

This is the vertical component of temperature rise, as illustrated in the psychrometric chart

Air conditioner sizing has to be carried out such that, sensible heat gain or loss in your space, is balanced by the sensible and latent heat removal or addition to your space.

Mouthful isn’t it? Simply said, heat gain or heat loss, must be balanced by heat removal or heat addition.

Conversion for heat and power units:

  • 1 kW is 1.34 horsepower,

  • 1 kWhr is 3,413 BTU/hr (British Thermal Units),

  • 1 ton is 12,000 BTU/hr

Three common ways of air conditioner sizing:

Sizing for air conditioners are carried out using,

All three methods have been used by air conditioner contractors throughout the world, in near-equal distribution. Let us look at these in detail.

Method 1: Air conditioner sizing by square feet method:

Our trip to heat transfer page has shown that heat content in a space is proportional to,

  • The mass of the object,

  • The temperature of the objects, and

  • The specific heat of the object

Radiation, conduction and convection page has also stated that, for a given temperature difference and contact surface area between two objects,

  • Radiation heat gain is proportional to the surface emissivity, ε. White paints have very low emissivity, whilst black paints have very high emissivity

  • Conduction heat gain is proportional to the thermal conductivity of the wall divided by the wall’s thickness, and

  • Convective heat transfer is proportional to the heat transfer coefficient of the fluid

Contractors, or D.I.Y. installers use square feet method over and over again, from “experience” and using the facts of,

  • Heat gain or loss is proportional to the volume of air contained within a space, and

  • Heat gain or loss is proportional the surface area of the wall

There are however, assumptions made before coming to the generalisation of the method (which will be given in a short while). Those assumptions are,

  • The surface emissivity is CONSTANT for all rooms,

  • Wall thickness is CONSTANT for all rooms,

  • Temperature difference is SAME throughout the area,

  • Thermal conductivity of wall is CONSTANT for all rooms

Thus the square feet method for air conditioner sizing comes to,

Sensible heat gain1 ton500 ft2 of floor area
Heat gain from person380 BTUper person
Heat gain from cooking1,200 BTUper kitchen

The major problem with this way of air conditioner sizing is that, all the assumptions made are NEVER constant.

There are variations in wall construction, presence of plants and trees and facing direction of the house (East, West, South or North), will have impact on heat gain and loss of the house.

Furthermore, the numbers I have stated, vary from a contractor to another. Which is why, you have to use the numbers with caution.

You can, however use the numbers for purchasing or renting smaller units, and in small quantities, say, portable, window, or small ductless air conditioners, where the stake is generally less than USD 2,000.00.

Using more detailed way of air conditioner sizing, might not provide you with much of an added value of your time – if that is the case.

Ways of AC Sizing

Otherwise, use

Method 2: Air conditioner sizing by “Manual J”:

“Manual J”? It is the accepted method for calculating heating or cooling load for residential and industrial application. This method is used widely by contractors, and the method is approved by ANSI.

This manual, is produced by ACCA (Air Conditioning Contractors of America) is close to 150 pages thick!

Why so thick? Because it contains,

  • Various factors and values for conductive heat transfer coefficient, from glass windows to variations of brick walls,

  • Various factors and values for radiant heat transfer,

  • Factors and values for air infiltration. Infiltration is air leakage through designed opening, cracks, unsealed edges and crevices.

    Value for infiltration is easily calculated if there are designed opening like chimney, or flue stack. It is not very straight forward for crevices or cracks. Which is why, “Manual J” has included safety factor to cater for difficult calculations.

    Nevertheless, Miller & Miller in "Air Conditioning, Home & Commercial" has stated that, volume flow rate of infiltration is proportional to the surface area of either the inlet, or outlet, square root of the temperature difference between outside, and inside air, and square root of the height between inlet and outlet.

    Relationship of infiltration in air conditioner sizing

  • Data for gain in ducts

  • Humidity factors and values, and

  • Sensitivity factors for arrangement of a residential or commercial building

How to use “Manual J”?

  1. The starting point is, you have set the desired temperature and humidity you want within the room/space.

  2. Then find the desired temperature and humidity difference between outside, and inside of the room/space

  3. Use the database and template in “Manual J”, to calculate heating and cooling load

Know your design conditions for air conditioner sizing

We have to bear in mind that, using “Manual J” for air conditioner sizing could be overwhelming for initial use, but it will be easy once we get used to it. Which is why, hiring a contractor is a good idea, as they have experience in this.

ACCA has also simplified the manual in electronic (CD-ROM) format, for easy data entering and calculation.

The major disadvantage of this way is the time consumed, and the risk of over-estimation, or under-estimation from contractors.

Best bet is, tell them not to add more safety factors, as the manual has already included it. You have to ask for the calculation sheet, to verify the data. Not that you don’t trust them, but to double check. People make mistakes.

This method is useful for installing larger sized split (ductless) air conditioner, and central air conditioning systems, with budgets less than USD 7,000.00. The time spent will be much more useful.

Ways of AC Sizing

Method 3: Air conditioner sizing incorporating “Manual J”, psychrometric chart, comfort chart, and duct sizing charts:

This method is more comprehensive than “Method 2”, but it will only take a little longer time, than the previous one.

You would want to use this method if, you’re installing a central air conditioning to cater for an approximate area of 800 m2 (~8,600 ft2) or more.

This method of air conditioner sizing involves these steps;

  1. Use comfort chart from American Society of Heating & Ventilating Engineers to identify the comfort region within an air conditioned space.

    This chart has been designed based on conditions within The United States. However, most people around the world find the comfort zone, as comfortable

  2. Calculate heat gains in all room using “Manual J”, based on outside and inside design conditions

  3. Calculate the mass flow rate of air into each room, ratio of fresh air to re-circulated air, and determine the size of air conditioning unit from the cooling load value

    Data that has to be used are:

    • Value of 5 litres/s, per person, per compartment (room), for air supply, as suggested by Dr Nightingale of University College London

    • Outside and inside design air condition, and

    • Room slope values

    Details will be explained in psychrometric chart page

  4. Calculate the duct sizes into each room using these data:

    • Maximum air draft at design condition

    • Duct resistance values, and

    • Use the duct sizing charts

    Details will be explained in duct sizing page

Quite a comprehensive step, but it will make sure that you will make a lot less error compared to simple square feet method.

Ways of AC Sizing

Summary or air conditioner sizing:

You can use any of those methods that you want for air conditioner sizing. However, please bear in mind that rough and simplified methods may not always work, especially with different building material.

Use simplified method for cheap, small, and relatively localised air conditioning.

Use more comprehensive methods whenever making the wrong decision will throw thousands of dollars of your hard earn money.

After all correct air conditioner sizing will give you the required comfort, at an expected utility bill.

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