Air Conditioning - Should One Window Be Left Open in the Home?

Running your air condition unit can cost up to $1000 per year, which is a lot of money to keep your house cool.

Therefore to get the best value from your air conditioning unit it should be installed and checked yearly by a registered HVACR contractor. This will ensure the system is balanced, the refrigerant charge is correct, and the unit is running at optimum temperature and efficiency.

This is an article in Mechanical Engineering, and in particular, about the efficient running of your home air conditioning unit. The article will include a discussion on the effects on the A/C unit and air quality through leaving a window open whilst the unit is running.

We begin then with a recap on a typical home air conditioning unit’s operation.

The refrigerant gas is fed into the compressor, from where it exits as a high pressure and temperature gas. The gas now passes through the condensing coils over which air at ambient temperature is blown. This condensing unit is normally located outdoors.

This unit condenses the gas to a high pressure liquid at ambient temperature that is now fed through an expansion valve. This valve lowers the temperature and pressure of the liquid refrigerant, before passing it through the evaporator coils.

The air to the living quarters is passed over these cold coils and is cooled; at the same time the refrigerant is converted back to a gas through evaporation, before being fed back into the compressor to complete the cycle.

Meanwhile air is being continuously drawn through ducting from the living quarters by a fan that blows it over the evaporator coils and being cooled before recirculating back to the rooms.

The discharge ducting from the evaporator unit normally has an air filter/air purifier, especially if one of the family has asthma or an allergy to dust. A de-humidifier can also be fitted which removes the moisture from the air preventing dampness and increasing comfort.

An air conditioning layout/flow diagram is shown below; please click on it to enlarge the image.

click to enlarge

In short, the answer to this question is No! All windows and outside doors should be shut whilst the air conditioning is operating for the following reasons:

1. Air conditioning systems are specifically designed, sized and installed to distribute the correct quantity and quality of air to the home. Leaving a window open will affect the loading of the A/C unit.

2. The efficiency of the home air conditioning unit depends on the system of fans and ducting being carefully balanced, normally by a professional HVACR engineer. Leaving a window open will alter these settings, lowering the system’s efficiency.

3. As can be seen from my diagram, on modern A/C systems an air filter and/or an air purifier are incorporated in the discharge ductwork. The filter is imperative to remove dust particles or pollen, whilst the purifier can have a UV/catalyst that removes any chemicals or odors. These components are essential if anyone in the family suffers from allergies such as hay fever or asthma. Leaving a window open will introduce dust/odors/pollen from the garden into the house and into the air conditioning system.

4. The de-humidifier removes the moisture from the airstream. This prevents dampness, condensation and damage to electrical appliances and electronics. It also makes the living quarters more comfortable for the family. Leaving a window open will reintroduce moisture to the system and home.

Reference Webs.

1. Energystar: HVACR recommended practices
2. Progressenergy: HVACR energy saving tips
3. Associatedcontent: operating air conditioning efficiently
4. articlebase: sequence of operation of an AC unit

Related: What is the Best Indoor Humidity Range for Books, People, and Electronics?