Understanding the role of enthalpy in free cooling

Free cooling, and why it matters at home

"Free cooling" simply means using outside air, instead of your air conditioner, to cool your home. Whenever the air outside is more favorable than the air inside, opening a window (or running a fan to pull outside air in) can lower the temperature and humidity in your home at no energy cost beyond moving the air. Large commercial buildings and condominiums do this automatically, every day, through their HVAC systems — when conditions outside are good enough, the system brings in outside air instead of running the compressor. Energy codes such as ASHRAE 90.1 require many commercial systems to do exactly this ("air-side economizers"), because it can meaningfully cut cooling costs and compressor wear.

Most houses don't have that automation, but the underlying opportunity is the same. The trick is knowing when outside air is actually better than what's already inside — and that's where temperature alone can mislead you.

What enthalpy actually is

In plain terms, enthalpy is the total heat energy held in the air — not just how hot it feels, but how much energy it would take to cool that air down and dry it out. Air holds heat in two ways:

• Sensible heat — the heat you can feel and measure with a thermometer. This is the part everyone thinks about.
• Latent heat — the energy stored invisibly in water vapor. Humid air carries a lot of latent heat even when its temperature reads the same as dry air, because energy was needed to evaporate that water into vapor in the first place, and it will release that energy back out when the vapor condenses (for example, on your AC's cold coils, or on any cool surface in your home).

Enthalpy adds these two together. That's why two samples of air at the exact same temperature can feel completely different, and why a cooler day can sometimes carry more total heat energy than a warmer one, if it's humid enough.

Humidity matters because your comfort (and your air conditioner) doesn't just fight temperature — a big part of a cooling system's job is removing moisture, since humid air feels warmer than its thermometer reading and resists cooling down. If you only compare indoor and outdoor temperature, you're only seeing half of the picture. Comparing enthalpy instead tells you whether outside air will actually reduce the total heat load in your home, temperature and humidity combined.

When temperature alone gets it wrong

Here are a few realistic scenarios where looking at temperature by itself would lead you to the wrong decision, but factoring in humidity via enthalpy reveals that opening the windows is actually the better move:

This is exactly the gap WindowCooling is built to close: it does the enthalpy comparison for you, continuously, so you don't have to mentally juggle both temperature and humidity every time you wonder whether to open a window.

The equation, for the curious

WindowCooling's calculations follow the psychrometric equations published in the ASHRAE Handbook — Fundamentals (Chapter 1, Psychrometrics), the standard reference HVAC engineers use for moist-air properties. The specific enthalpy of moist air is expressed per unit mass of dry air, and is the sum of the sensible heat of the dry air and the latent heat carried by its water vapor:

where:

• t is the dry-bulb (thermometer) temperature, in °C or °F.
• W is the humidity ratio — the mass of water vapor carried per unit mass of dry air (kg/kg or lb/lb).
• 1.006 (or 0.240) is the specific heat of dry air, and1.86 (or 0.444) is the specific heat of water vapor.
• 2501 (or 1061) is the latent heat of vaporization of water at 0°C (32°F) — the energy "hidden" in every unit of water vapor the air is carrying.

The humidity ratio W itself comes from the partial pressure of water vapor in the air, relative to total atmospheric pressure:

where p is total atmospheric pressure and p_w is the partial pressure of water vapor actually present in the air. That partial pressure is what a relative humidity reading is measured against: relative humidity is simply p_w expressed as a percentage of the saturation vapor pressure, p_ws(t) — the maximum amount of vapor the air could hold at that temperature before it starts condensing.

Estimating saturation vapor pressure: the Magnus approximation

Saturation vapor pressure is described exactly by the Clausius–Clapeyron relation, but that equation isn't practical to solve quickly. In practice, WindowCooling (like most weather and HVAC software) uses the August–Roche–Magnus approximation(also known as the Magnus-Tetens formula), a well-established empirical fit that's accurate to within about 0.1–0.2% across the temperature range relevant to weather and building conditions:

This approximation was first proposed by the physicist August in 1828, refined by Magnus in 1844, and later given the widely used coefficients above by Alduchov and Eskridge (1996). It's popular precisely because it's simple enough to compute instantly on a device, yet accurate enough for real-world psychrometric calculations. Once p_ws(t) is known, the actual vapor pressure follows directly from relative humidity:

Chain these together — temperature and relative humidity in, saturation pressure via Magnus, actual vapor pressure, humidity ratio, and finally specific enthalpy — and you get a single number for indoor air and a single number for outdoor air. Whichever one is lower represents the lower total heat load. That comparison is the calculation at the heart of WindowCooling.