Air conditioner sizing is a 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 on air conditioner sizing. 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: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.Is the heat contained in the water vapour, carried by the air. The higher the water vapour content, the higher the latent heat. ac unit room sizeExamples are, your perspiration, cooking by steam, or hot water vapour from the bathroom.central air conditioner used saleThis is the vertical component of temperature rise, as illustrated in the psychrometric chartAir 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.ac only unitSimply said, heat gain or heat loss, must be balanced by heat removal or heat addition. 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,Square feet method,Using “Manual J”, andIncorporating “Manual J”, psychrometric chart, comfort chart, and duct sizing charts into selecting the best air conditioner size for the applicationAll 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, andThe specific heat of the objectRadiation, 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 emissivityConduction heat gain is proportional to the thermal conductivity of the wall divided by the wall’s thickness, andConvective heat transfer is proportional to the heat transfer coefficient of the fluidContractors, 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, andHeat gain or loss is proportional the surface area of the wallThere 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 roomsThus the square feet method for air conditioner sizing comes to,QuantityPerSensible heat gain1 ton500 ft2 of floor areaHeat gain from person380 BTUper personHeat gain from cooking1,200 BTUper kitchenThe 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 SizingOtherwise, use Method 2: Air conditioner sizing by “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!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.Data for gain in ductsHumidity factors and values, andSensitivity factors for arrangement of a residential or commercial buildingHow to use “Manual J”?The starting point is, you have set the desired temperature and humidity you want within the room/space.Then find the desired temperature and humidity difference between outside, and inside of the room/spaceUse the database and template in “Manual J”, to calculate heating and cooling loadWe 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. 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;Use comfort chart from American Society of Heating & Ventilating Engineers to identify the comfort region within an air conditioned space.Calculate heat gains in all room using “Manual J”, based on outside and inside design conditionsCalculate 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 valueData 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 LondonOutside and inside design air condition, andRoom slope valuesDetails will be explained in psychrometric chart pageCalculate the duct sizes into each room using these data:Maximum air draft at design conditionDuct resistance values, andUse the duct sizing chartsDetails will be explained in duct sizing pageQuite a comprehensive step, but it will make sure that you will make a lot less error compared to simple square feet method.