Poor Man's AC

Home Assistant integration that decides, on thermodynamic grounds, whether direct adiabatic (evaporative) cooling improves the indoor climate of a room. Evaporating water lowers the temperature but raises humidity, so it is not obvious whether it makes the room more or less comfortable. The integration uses the heat index as the comfort measure and its total differential along the adiabatic cooling path as the decision criterion.

Inputs

• a temperature sensor (°C)
• a relative humidity sensor (%)
• optionally an ambient pressure sensor; without one (or while it is unavailable) the fallback pressure from the options is used

What it computes

From temperature, relative humidity and ambient pressure the integration derives the specific humidity x (water mass fraction, kg_water / kg_moist_air, using the Magnus formula for the saturation vapour pressure) and evaluates the total differential of the heat index along the isenthalpic evaporative-cooling path:

dHI = (∂HI/∂T)·dT + (∂HI/∂x)·dx

dT and dx follow from the energy balance of the adiabatic process (sensible heat released = latent heat absorbed), which fixes the x-dependent slope dx/dT = −cp(x)/L. dHI is reported as the heat-index change per 1 K of evaporative cooling. dHI < 0 ⇒ cooling lowers the heat index ⇒ recommended.

The water vapour density ρ_w (absolute humidity, g/m³) is also computed and exposed as a sensor, but it is not part of the decision criterion.

Entities

• binary_sensor – Adiabatic cooling recommended
• sensor – Heat index, Absolute humidity, Specific humidity, Wet-bulb temperature (cooling limit along the isenthalpic path), Wet-bulb depression (maximum achievable evaporative cooling, K), Optimal water uptake (water that evaporative cooling should add at most: up to the heat-index minimum along the isenthalpic path, clamped at saturation, g/kg), Heat index differential (dHI)
• diagnostic sensors dHI/dT and dHI/dx (disabled by default)

Installation

Copy custom_components/poormansac into your Home Assistant config folder (or add this repository to HACS as a custom repository), restart, then add the integration via Settings → Devices & Services → Add Integration → Poor Man's AC and pick the temperature and humidity sensors, plus an optional ambient pressure sensor.

Options

Settings → … → Configure: the dHI threshold below which cooling is recommended.

Lovelace card

The integration bundles an experimental psychrometric (Carrier) chart card and registers it automatically as a Lovelace JavaScript-module resource — for the default storage-mode dashboards no manual setup under Settings → Dashboard → Resources is needed. Once the integration is loaded the card appears in the dashboard's Add card picker as "Poor Man's AC Psychrometric Chart" (a browser refresh may be required the first time).

If you run your dashboards in YAML mode, Home Assistant manages resources from your own YAML and the integration cannot edit it; the module is still loaded globally so the card works, but if you maintain a resources: list yourself add /poormansac_frontend/poormansac-psychrometric-card.js as a module there.

It plots the current air state as a point on a dry-bulb-temperature (x-axis) vs. water-loading x (right y-axis, g/kg) chart, draws constant relative-humidity curves (including the 100 % saturation curve), and shows the isenthalpic cooling line — the path along which evaporative cooling would move the air (up and to the left, toward saturation).

type: custom:poormansac-psychrometric-card
entity: binary_sensor.poor_man_s_ac_adiabatic_cooling_recommended
# optional (defaults shown):
# title: Psychrometric chart
# t_min: 0
# t_max: 40
# x_min: 0
# x_max: 50
# rh_lines: 5   # number of equally spaced rel.-humidity curves;
#              # the top one is the 100 % saturation curve, 0 % is never drawn,
#              # and 0 hides them entirely
# point_label: [t, x, hi]   # which values to show next to the state point;
#                           # any of t (temperature), x (specific humidity),
#                           # hi (heat index), rh (rel. humidity); [] hides it

The card reads everything it needs from the binary sensor's attributes (temperature, specific_humidity, pressure, dx_dt) and recomputes only the relative-humidity curves, using the same effective pressure the integration used for x.