Following air conditioning sizing recommendations saves homeowners money. Without proper guidance from a home heating and cooling professional, many people will spend extra money buying air conditioners that are too large for the space. By buying the proper size of air conditioner, homeowners save money in the short-term, with a smaller cash outlay for a unit that is not too large for the space. Long-term operating costs go down with smaller, more efficient units as well. Air conditioning sizing does not follow the “bigger is better” adage. In fact, an air conditioner that is too large for a room won’t cool the area uniformly. Additionally, a smaller unit that stays on continuously to create a consistent temperature in the room operates more efficiently than a large unit that turns on and off frequently. A unit that continues operating for longer periods of time at a lower level also dehumidifies the area more efficiently. Removing humidity from the air makes a room feel even cooler.

Experts provide guidelines and recommendations for the size of air conditioning unit that is appropriate for any home. If you’re unsure about the size of air conditioner that your space requires, ask a professional contractor or salesman before you buy. The heating and cooling industry rates air conditioners’ cooling capacities based on British thermal units (BTU) per hour, or by the “ton,” which is 12,000 BTU per hour. Most room air conditioners have cooling capacities ranging from 5,500 BTU to 14,000 BTU. Depending on how hot your local climate gets in the summertime, a 5,000 BTU air conditioner will cool a room of 100 to 150 square feet. Here are some additional points to consider: These numbers apply to room air conditioners. Check with a professional installer to decide the appropriate size of central air conditioner for your home. Keep in mind that passive cooling techniques, including proper insulation, shaded windows, and even heat-reflective coatings on your windows will help reduce your air conditioner sizing requirements

Written The main assumptions include outside and inside design temperatures; 35C outside may be appropriate for where you are geographically, but 19C inside is too cold. air conditioner unit no windowThe common indoor design temperature that most people consider “comfortable” is 22–23C, and many energy-conscious folks set the setpoint at 25C and put shorts and a tank top on, realizing the cooler it is, the more energy the AC unit will use.ac unit not working insideYou could design and install a system to maintain 19C when it’s 35C outside but that would be rare, and so most standard AC systems would be undersized for that, as you’ve noticed.fix my window air conditionerWritten I can say with certainty that reaching 19C is NOT one of them.

My assertion is that your demands are too optimistic about the energy conservation needs of the world at large. In addition, most people would find 19C uncomfortably cold. Having said that, the assumptions are widely available from ASHRAE and others, they include an indoor design temperature, an outdoor design temperature varied by area, humidity, number of air exchanges, the type of walls, the type of windows and number the type and number of doors, the quality of weatherstripping, the amount of wall, ceiling, and floor insulation, and the number of cubic feet of space. Add occupant load, equipment heat, and lighting to that and you begin to have enough information for a “proper” guesstimate of loading on your air conditioning equipment. Frankly, for window units it isn’t bothered with. In a house these units are ineffective. You can and will overcool certain areas and be uncomfortably hot in others. The air flows from room to room.Your assumption that A/C units are sized at 9000 BTU for average rooms implies a few things about an average room.

Is your average room 9 x 12 or is it 30 x 45? Just what is “average” in your view? Do you live in the Yukon or in Panama? These are significant factors.Written The normal assumptions would be amount of insulation, relative humidity, outside air temperature, the number of times the air changes in an hour, room volumes and proportions, number of people in a room, their activity levels, amount of sunlight that can enter the room, the level of lighting in the room and what kind along with other heat sources such as cooking and computers. Another factor is that the 9000 btu/hr air conditioner may not be 9000 btu/hr or may not be 9000/btu per hour for the temperature difference you are trying to get.The first thing I noticed is that I think your low temperature is 5°C lower than it is assumed to be. Since I don’t know where you are other than not likely in the US, I can’t even look up the assumptions.Written In my humble opinion, it would be better to have a slightly undersized forced air cooling system than a slightly oversized system.

The reason being that there are a lot if variables to take into account like insulation factor, actual cubic footage, and window types for example. While it is true that oversized units will be installed in a structure that is known to have an insulation deficiency or, say, high ceilings (more cubic footage of total air); in a modern well insulated home system are sized based on the average high temperatures of the hottest month when cooling is being used.Expect a properly sized cooling unit to run continuously after this average high temperature is surpassed. If the temperature of the particular day soars over that average high temperature, then the cooling unit will lose a little ground on maintaining the number for which the operator has set for the thermostat to call. A little discomfort may be felt by the inhabitants of the subject home, but; The unit will not cut off and will run continuously removing humidity from the air.Now consider an oversized unit in the subject application.

On the very hottest days, an oversized unit may perform better than your neighbors smaller unit, but; it's tell is that far more numerous average hot days that will spoil the victory. An oversized unit will satisfy the thermostat repeatedly on average hot days and the time lag in between anticipated run times is when the people in the dweling will start to feel some humid stuffiness. As the too large unit satisfies the thermostat, the people may feel too cold for a short span of time before the stuffiness sets in and they begin wondering, “When the heck is the AC going to turn back on?”It's true that a force air cooling system does lower the air temperature inside a dweling, but; perhaps a far more important role of an air conditioning system is to remove humidity from the interior air so when inhabitants sweat it will evaporate from their skin and give them a sense of comfort, as opposed to first being cold and then getting a little sticky before the too large unit kicks back in and gets them cold again.