Window Cooling Forecast

The Window Cooling Forecast app compares the temperature and humidity of outdoor air to indoor air to show when outdoor air can be used for cooling. Energy codes such as ASHRAE 90.1 require many large buildings to use equipment like air-side economizers to cool the building using outdoor air when its temperature and humidity are more favorable, meaningfully cutting cooling costs and compressor wear. Most residential HVAC systems do not have an air-side economizer but the underlying opportunity is the same. The trick is knowing when outdoor air is actually better than what's already indoors — and that's where temperature alone can mislead you. To determine when outdoor air is more favorable, we need to use a metric called 'enthalpy'. Enthalpy combines air temperature (as sensible heat) and humidity (as latent heat) into a single number representing the true energy of the air. Outdoor air is more favorable for cooling a building when its enthalpy is lower than that of the indoor air. Instead of guessing whether outdoor air will help cool your house, this app calculates the difference in the enthalpy (ΔH) of indoor and outdoor air and advises you when it makes sense to open your windows. It will even give you a 12-hour forecast for effective window cooling availability. The app needs to know the temperature and humidity of the inside and outside air. If you have a thermostat (e.g. Nest or Ecobee) that gives you that information you can enter it manually. The app also needs the temperature and humidity of the outdoor air. The app can query Apple Weather using your location to obtain this information but it is best if you use sensors that measure the temperature and humidity of the air outside your window. For this first version of the app, you can also obtain and use a low-cost Govee hygrometers which can provide temperature and humidity to the app over Bluetooth LE. If you have two of them you can use one for indoor data and another for outdoor data.