Should you open the windows or run the HRV? Compare indoor and outdoor air properly — by water content, not just relative humidity.
An HRV transfers sensible heat but essentially no moisture — its supply air's humidity ratio tracks outdoor air even though its RH shifts with temperature. An ERV also recovers some moisture. Default here models the Fantech SHR150R-style HRV: sensible-only recovery.
This is why comparing 45% RH with 85% RH can be misleading: relative humidity depends on temperature, but the bars above show actual water mass per kg of dry air.
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RH is a ratio between the water vapor present and the maximum the air could hold at that temperature (saturation). Heating a sample of air lowers its RH without removing any water; cooling it raises RH without adding any water. This is exactly what happens inside an HRV.
The temperature to which air must be cooled, at roughly constant pressure and water content, for condensation to begin. It's a direct, temperature-independent readout of how much moisture is actually present.
Mass of water vapor per mass of dry air (g/kg). Because it doesn't change when you simply heat or cool the air, it's the right quantity to compare when deciding whether ventilation will dry or humidify a building — not RH.
A combined measure of the sensible (temperature) and latent (moisture) energy in moist air, in kJ per kg of dry air. Two air masses can have the same temperature but very different enthalpy if their moisture content differs.
Sensible heat is energy tied to a measurable temperature change. Latent heat is energy tied to a phase change (evaporation/condensation) without a temperature change. Air conditioning usually does both at once: cooling the air (sensible) and condensing moisture out of it on a cold coil (latent).
Dalton's work on partial pressures of gases was foundational to understanding gas mixtures — including how water vapor behaves as one component of air.
Their Clapeyron and Clausius-Clapeyron relations connect temperature to equilibrium vapor pressure and latent heat. This relationship is why saturation vapor pressure rises so steeply with temperature — the physical basis for the Magnus-type formulas used in this tool.
The Magnus family of formulas gives practical, accurate approximations for saturation vapor pressure from temperature alone. This tool uses the widely used Alduchov-Eskridge coefficients, an improved variant of Magnus's original form.
Not the discoverer of humidity or vapor-pressure physics, but the engineer who turned this thermodynamics into a systematic discipline. ASHRAE records his 1907 patent for precise dew-point control, and Carrier dates his "Rational Psychrometric Formulae" to 1911 — work that underlies modern air conditioning and psychrometric charts.
Psychrometric charts bring several properties of moist air — temperature, humidity ratio, RH, dew point, enthalpy — into one graphical representation, letting HVAC engineers read the full state of air from just two known properties. The chart above is a simplified version of that idea.
This started as a learning project — a way to actually understand the psychrometrics behind a ventilation decision, instead of trusting a gut feeling about temperature and humidity. It turned out to be genuinely useful for real household decisions, not just an exercise.
The natural next step is wiring this to live data: outdoor conditions from a
weather API, and indoor conditions from a Sensibo-controlled HRV/AC, so the advisor reasons from
real-time readings instead of manually dialed-in sliders. That version will not be public. A Sensibo
API key grants full account access — every sensor, but also full control: it can turn the AC on/off
and change its setpoint remotely. Read and control are not separable at the API-key level, so the key
will never touch client-side code. It lives only in a server-side proxy restricted to read-only calls,
under the same secret-isolation rule this site already enforces for every other credential — JWT
keys, API tokens — in .env, never in the browser. The credential behind "show me the
temperature" can also flip a switch in my house, and the design treats it that way from day one.
Wikipedia (secondary reading): Psychrometrics · Dew point · Relative humidity · Clausius–Clapeyron relation · Willis Carrier.
This is an educational household decision-support tool, not HVAC design software. Saturation vapor pressure uses the Magnus/Alduchov-Eskridge approximation, valid over the normal household temperature range. Enthalpy uses the standard simplified HVAC relationship h ≈ 1.006T + W(2501 + 1.86T). HRV/ERV modeling uses a simple effectiveness model, not manufacturer performance curves. Heat-pump behavior is described qualitatively only. Do not use this tool for engineering design decisions.