Published on January 14, 2026 by Claudio Cabete
Most HVAC humidifiers are “flow‑through” designs: a solenoid opens, cold water trickles across a pad, and warm furnace air evaporates a fraction of it. The rest goes down the drain.
That’s the simple story.
The deeper story — the one that explains why intermittent flow is so much more efficient — lives in the physics of evaporation, heat transfer, and microbial ecology.
Evaporation is not magic — it’s a heat‑driven phase change. To turn liquid water into vapor, you must supply the latent heat of vaporization, roughly ~2256 kJ/kg at 20°C.
When cold water continuously flows across the pad:
This creates a two‑stage energy burden: sensible heat load and latent heat load.
Intermittent flow breaks this cycle by allowing the pad and water film to warm during the OFF period, eliminating the repeated sensible heat penalty.
With continuous flow, the pad is constantly cooled by incoming cold water and never reaches a steady‑state temperature.
With intermittent flow, the pad absorbs heat from the furnace air, warms up, and accelerates evaporation.
Continuous flow turns the pad into a heat sink.
Intermittent flow turns it into a heat battery.
The vapor pressure of water increases exponentially with temperature:
Warm water evaporates dramatically faster than cold water.
Intermittent flow allows the water film and pad to warm, increasing the vapor pressure gradient and boosting evaporation efficiency.
Mold thrives in dark, moist, cool environments — exactly what a continuously soaked pad provides.
Intermittent flow disrupts this environment:
Continuous flow = cold, wet, dark → mold‑friendly
Intermittent flow = warm, drying, ventilated → mold‑resistant
Modern controllers make intermittent flow trivial and far more efficient.
Intermittent flow is the modern, physics‑aligned way to run a flow‑through humidifier.