Recommendations For Sizing Air Conditioning

Sizing matters when it comes to air conditioning because it helps you live a healthy and comfy life without any hassle. If you buy a smaller unit, then it will always struggle to keep your room at your comfortable temperature. And if you buy a larger unit then it will cool your room really quickly and will not be able to remove the humidity. The larger unit will not only add to its price but more energy will also be needed to run it and that means higher bill cost.

Important Cooling Terms

In order to know if your air conditioning is of proper size you need to know few key terms.

Sensible Cooling Load

Due to conduction, infiltration, appliances, solar radiation, pets and people, the heat gain of the house adds the sensible load. This sensible load directly affects the dry bulb temperature.

Dry Bulb Temperature

It is the temperature that is directly measured by the thermometer.

Web Bulb Temperature

The standard thermometer is wrapped by the wet wick and air is then blown across its surface which makes the water to evaporate and lowers the temperature below the wet bulb temperature and it depends on the moisture content of the air.

Latent Cooling Load

The moisture is added to the inside air because of the infiltration, plants, cooking and people etc, that can be calculated using the wet and the dry bulb temperatures.

Design Conditions

Due to variation in the inside and outside conditions of temperature the cooling loads might vary. Therefore based on locality, a set of specific conditions are necessary to properly measure the accurate value of the cooling load. 75oF and 50% relative humidity is takes as guideline while the onsite conditions depend on 2.5% design point.

2.5% Design

Basically from June to September the outside temperatures and the moisture content increases by 2.5% of the hours.


While continuously running the amount of cooling an air conditioner can do is called it capacity which is measure from the sum of latent capacity, which is its ability to remove any moisture from the air, and the sensible capacity that is the ability of the unit to lower the dry bulb temperature. The capacity of an air conditioner is dependent upon the outside conditions if outside gets hot then its capacity decreases.

Tons of Cooling

It’s the standard way of measuring the capacity as the 1-ton air conditioner has the capacity of around 12000 Btu however there a 3-ton air conditioner might not produce 36000 Btu cooling so the capacity has varieties.


EER is the Energy Efficiency Ratio and it defines the efficiency of the air conditioner. When the Btu/hour capacity if air conditioner is divided by the electrical input in watts it gives us ERR.


SEER is the Seasonal Energy Efficiency Ratio and it is used as a standard method nowadays to rate the air conditioner. Three test rates the performance of an air conditioner running at 82pF outside conditions and 80oF inside conditions. The first test includes the humid indoor conditions while the second one includes the dry indoor condition while the last test is a cycling test running the air conditioner for six minutes after every 24 minutes of rest. The result of these tests defines the rating of the air conditioner.

Manual S

To make sure that the selected equipment will meet the cooling load requirement the ACCA has a standard of selecting the air conditioning equipment which is called Manual S.

Manual D

To make the air condition more efficient the ACCA has a standard method of designing the air conditioning duct system and it’s called Manual D. It is time consuming and laborious however this makes the air condition more effective and saves money on your utility bill.

Selecting the Right Air Conditioning Sizing

Before you can design an effective as well efficient air conditioning system you need to calculate the load properly so that you can get the proper air conditioning equipment to meet your cooling demands.
The calculations must be free from any errors including both the input as well as the calculation and formula to avoid any kind of hassle in the process. As the right sizing is so important in keeping the cost down and for air conditioner to work efficient therefore you need to take into account all the factors such as the inside and the outside conditions of your area to make the best possible calculations.

The Proper Air Conditioning Sizing

Theoretically when your air conditioning unit runs for continuous 73 hours during the year with outside temperatures rising about 2.5% of the design point then it means that you have got the proper sizing
A properly sized air conditioner not only provides you the best value for your money by keeping your utility bills down as well but also your contractor by getting him more customer referrals. If you find your air conditioning unit cycling in the afternoon of the hottest summer day, then you definitely have an oversized air conditioning unit. However, if your AC is running for continuous hours on a hot day that doesn’t necessarily means that your unit is undersized sometimes the ducts are leaking or the air flow is too low across the inside coil so you better look for those before assuming that its undersized.

Finding the Right Size

The typical cooling capacity of the air conditioning unit ranges for the 5000 to 12000 Btu. Generally, 20 Btu for each square foot is needed by the air conditioner and you can calculate that by multiplying your room length with its width however you need to take in to account the height of your ceiling as well as the windows you got because those things directly affect t the performance of your unit.
You also need to take into account certain indoor and the outdoor conditions such as if your room is heavily shaded then you 0should reduce your unit’s capacity by 10% and it gets a lot of sunlight then increase the capacity by 10%. If the air conditioner is going to be used in the kitchen, then increase its capacity by 4000 Btu and if more than 2 people occupy that room regularly for a long period of time then add 600 Btu per additional person.

Install It Correctly

If you are installing it yourself then you need to be very careful and make sure that every area of the room in which you are installing it gets the cooling. It needs to be leveled perfectly for its drainage to work properly. Don’t put the lamps or any other electrical appliance near the air conditioner thermostat as the heat signature of that device will make the air conditioner to run for longer time. Similarly, on humid days using a slow speed for fan as the slowly moving fan removes more humidity from the surrounding area.

Select the Right Contractor

If you don’t want to deal with all the hassle of calculations as well as the installations of the air conditioning unit, then you need to find the right contractor for the job. Most of the contractors go for the oversized air conditioning unit. Why? Because oversized air conditioning units usually keep their customers satisfied by not only cooling quickly but also hiding the flaws of the designing of the unit and the duct systems, if any! But these units are not only expensive but also increases the utility bill you if a contractor tells you to get a buy an oversized unit, it’s time that you better find another contractor. Remember always ask for the copy of all the calculations that the contractor has made in finding out the right sizing for you.


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