IndoorComfortKit Water output, not tank size
A humidifier's tank volume tells you how much water it holds. Its rated daily moisture output tells you how much it can add under its test conditions. Compare the result with that output rating, the operating time and the manufacturer's room guidance. This tool estimates outdoor-air replacement load, not a guaranteed product size.
Why the air-change assumption matters most
ACH is the number of room volumes exchanged with outdoor air each hour. It excludes air merely circulated by an indoor fan. A blower-door ACH50 reading uses an imposed pressure and must not be entered as natural ACH. The 0.25, 0.5 and 1 ACH rows are site-defined sensitivity scenarios, not certified tight, average and leaky building categories.
Internal doors, mechanical ventilation and leakage elsewhere can make a single room behave as part of a larger space. Change one assumption at a time, then compare actual humidity over repeated checks. Reducing required ventilation is not an appropriate way to make the estimate smaller.
The moisture balance behind the estimate
At the fixed reference pressure of 1013.25 hPa, Pv = RH × es(T) / 100 and W = 0.622Pv / (1013.25 − Pv), in mass of water per mass of dry air. Saturation pressure uses 6.112 × exp(17.62T / (243.12 + T)) hPa. Cold inputs use liquid-water RH, not ice-relative RH.
Water in lb/day = ACH × area(ft²) × height(ft) × 24 × 0.075 × max(0, Wtarget − Woutdoor). We approximate the reference dry-air mass per room volume as 0.075 lb/ft³. US gal/day = lb/day ÷ 8.34; L/day = gal/day × 3.785411784. The density and 8.34 lb/US gal water conversion are rounded planning assumptions. Pressure, temperature and the product's rating conditions can change actual performance.
A small-room calculation you can reproduce
For 300 sq ft, an 8 ft ceiling, 0.5 ACH, 21 °C / 40% target indoors and 0 °C / 80% outdoors, the estimate is about 0.81 US gal/day or 3.07 L/day. Doubling ACH doubles this load. It does not prove that a room actually exchanges that much air.
Check moisture already in the room
People, cooking, showers and damp materials can supply moisture, reducing the amount a humidifier must add. Adsorption by furnishings, startup from a dry room and moisture transfer through the building are omitted. Zero output means no positive outdoor-air replacement load under these assumptions; it is not a diagnosis of the actual room.
Check windows before increasing humidity. The optional window scenario uses the same model as the condensation limit calculator. Its threshold is not a safe target: edges can be colder, and an unknown window leaves this limit unknown. Stop adding moisture if surfaces become wet and investigate the cause.
References for these winter assumptions
- NFRC: windows and condensation
Supports: Whole-window U-factor and the distinction between glass, edges and frame.
Does not support: Our surface-temperature estimate, a safe RH, or a condensation guarantee.
Source checked:
- EnergyPlus Engineering Reference: air system humidifiers
Supports: Moisture mass balance using air mass flow and humidity-ratio difference.
Does not support: Our assumed ACH scenarios or a specific household humidifier capacity recommendation.
Source checked:
The examples are educational planning scenarios, not building diagnosis, medical advice or whole-house HVAC design. See fixed arithmetic test cases and the source registry.