As HVAC (Heating, Ventilation, and Air Conditioning) engineers strive to perfect the indoor climate, a common debate arises: "Is it okay to leave the window open while running your HVAC system?" This question often divides households and workplaces, as people weigh the desire for "fresh air" at the expense of the system’s energy efficiency. The misconception is that opening a window with the AC running will provide the best of both worlds: a cool environment and fresh air. To illustrate this concept, we will explore both qualitative and quantitative reasons why you should not leave your window open with your system on, and how bad it can truly be for the system.
Negatively Impacts Energy Efficiency:
The primary concern when leaving the window open is energy efficiency. When warm outside air enters your home, the air conditioning system must work harder to cool it. As well as the opposite in the colder months. This consumes more electricity and increases your electric bill.
Negatively Impacts Humidity Control:
One of the main functions of your air conditioning system is to reduce the space's humidity. When a window is open, warm humid air enters the space. This typically happens when the outside temperature is warmer than inside. High humidity creates a damp and uncomfortable environment, promoting the growth of mildew, mold, and other allergens. Excessive humidity can cause your system to work harder to dehumidify the air, causing reduced energy efficiency and increased wear on your system.
Worsens Air Quality:
Leaving a window open allows dust, pollen, and other pollutants into your space. Overall, this will worsen your air quality, can trigger allergies, and can even damage your respiratory health. Depending on your system, allowing pollutants into a single room can cause poor air quality throughout the entire house. Excessive dust and debris can also coat the inside and outside of the thermostat which can cause a faulty reading. Dust in combination with high humidity builds up and sticks to surfaces.
Depending on the system and thermostat location an open window can cause faulty thermostat readings and uneven cooling or heating throughout the zone. One room can be hot, while others will be cold or vice versa. Air infiltration through the open window can also cause the system to prematurely turn off or on, which can lead to added wear and damage to the system, in addition to negatively impacting comfort.
Load calculations were run on a representative small office space with an hourly analysis program to model the added load of an open window on the system. This office space has two exterior walls and two interior walls. The west wall has one 3’0”- 4’0” adjustable window. The space is also “slab on grade” and has a generic flat roof. The space has a floor area of 144ft2 and a ceiling height of 9ft.
This small office space was modeled in the Boston, MA area using the weather parameters depicted below.
Ventilation requirements from ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) Std. 62.1-2013, for an office space, is 5.0 cubic feet per minute/person or CFM/person, and 0.06 CFM/ft2. The loads were assumed to be from overhead lighting (0.7W/ft2), electrical equipment (200 Watts), and the number of people (1 person accounting for 245 BTU/hr sensible and 205 BTU/hr latent). Sensible is the heat that can be sensible is the same as your thermostat reading. Latent load is the energy required to dehumidify a building or space. British Thermal Unit also known as “BTU” is the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit.
The load on this space was modeled with the window closed, half opened, and fully open. When simulating this, some parameters needed to be estimated. Infiltration through an open window is a difficult parameter to calculate due to its dynamic nature. It was estimated that with the window fully closed, the space has an infiltration rate of 0.35 (Air Changes per Hour), 1.25ACH for half open window, and 2.5ACH for a fully open window. This parameter was estimated by taking the square footage of the window, 6ft2, then multiplying it by an estimated velocity across the window of 9ft/min. This results in 54CFM of airflow crossing through the open window. Once the airflow is calculated, we can solve for the infiltration, in air changes (ACH), using the formula [(Volume x ACH)/60) = CFM] is used. Using our estimated numbers, [(144ft2 x 9ft x ACH)/60) = 54CFM] which solves to 2.5 = ACH. Then for a half open window divide “2.5” by 2. It should be noted that these estimates are overly simplified; in reality the infiltration through the open window will be driven by temperature and pressure differences between the outdoor and indoor environment. This estimate was confirmed to be in the right ballpark when checked against other resources [Dimensioning tool of natural ventilation (windowmaster.com)].
Half Open Window (3ft2):
Open Window (6ft2):
The load models indicate that zone load increases as the window is opened. The closed window has an overall peak cooling load of 4,613 BTU/hr and a peak heating load of 4,694 BTU/hr. For the half open 5,154 BTU/hr, and 6,194 BTU/hr. Lastly, the open has 6,083 BTU/hr and 8,209 BTU/hr. From this data as the window is opened more, there is an increased load on the system. These increases are driven by the “window transmission” and the “infiltration.” Also, from this data it can be observed that the latent cooling load increases more than four times compared to the closed window. Any equipment properly sized to cool the space will not have the capacity to remove 4x more latent load (moisture). Latent load is the amount of energy necessary to dehumidify the air.
To get the benefits of opening a window without the consequences, here is a list of alternatives:
· Use a fan — A fan can help circulate air throughout a space and give a nice breeze just like a window!
· Use an air purifier — This can help remove dust, allergens, and other pollutants, improving air quality.
· Install new air filters — Similar to an air purifier, air filters can help reduce dust and other pollutants from the space.
· Use an ERV (Energy Recovery Ventilators) — This is a mechanical system that controls and filters the outside air.
There are also a few specific instances where turning off the air conditioning and opening windows can be beneficial.
· During the winter months when the air is cooler than inside, open the windows wide! Ventilate the space briefly but intensely. This will help the air exchange occur faster and prevent excessive cooling of the space. About five minutes is enough to provide the room with fresh air.
From this data and the negatives listed above, the answer to the age-old question, “Is it okay to leave the window open while running your HVAC system?” is no. Leaving the window open with the air conditioning on negatively impacts the energy efficiency, air quality, humidity, air flow, and overall longevity and performance of the system. When the air conditioning is running, it is best practice to leave the windows closed and use a few of the alternatives listed above.
Mechanical Design Engineer
2021 ASHRAE Handbook. (n.d.). ASHRAE CLIMATIC DESIGN CONDITIONS. ASHRAE Climatic Design Conditions 2009/2013/2017/2021. http://ashrae-meteo.info/v2.0/
Carrier. (n.d.). Hourly Analysis Program: Carrier Commercial Systems North America. https://www.carrier.com/commercial/en/us/software/hvac-system-design/hourly-analysis-program/
Tech Controllers. (2019, December 19). Fresh air in your house all year round - learn how to air your rooms properly - tech controllers. English. https://tech-controllers.com/blog/fresh-air-in-your-house-all-year-round---learn-how-to-air-your-rooms-properly#:~:text=Open%20the%20windows%20wide.,right%20amount%20of%20fresh%20air
Dimensioning tool of natural ventilation. Indoor climate solutions. (n.d.). https://www.windowmaster.com/resources/calculators/dimensioning-tool-of-natural-ventilation/imperial/?Input_area=144&Input_goal=1&Input_load_vent=.375&Input_strat=1&Input_bug=1&Input_height=25&Input_width=50&height_placeholder=24&Input_length=15&Input_temp=76&Input_temp_diff=5&Input_wind_speed=4&Input_discharge=0.61