Published on January 14, 2026 by Claudio Cabete
Most whole‑home humidifiers installed today follow a design that hasn’t changed in decades. If you have a typical Honeywell or Aprilaire bypass humidifier — the kind that uses an HC22P‑style evaporative pad — it probably works like this:
This “flow‑through” design is simple, but it’s also incredibly wasteful. Only a small fraction of the water evaporates; the rest goes straight down the drain.
And the inefficiency goes deeper than water waste.
Evaporation is a heat‑driven process. Warm water evaporates readily; cold water resists evaporation.
When cold water flows continuously:
You’re essentially trying to evaporate cold water with warm air, and the first thing that warm air does is waste energy heating the pad instead of humidifying the house.
A constantly soaked pad — especially one kept cold — creates the exact environment mold prefers:
Even though flow‑through humidifiers flush minerals, they are not immune to biological growth. A pad that never gets a chance to warm up or partially dry becomes a perfect incubator.
Intermittent flow disrupts that environment. The OFF period lets the pad warm up, and warm, moving air is far less mold‑friendly than cold, stagnant moisture.
A simple pattern works extremely well:
This approach:
The OFF period is doing more than saving water — it’s restoring the pad to the temperature and moisture profile where evaporation is efficient and mold is discouraged.
The legacy design is simple, cheap, and reliable. It avoids mineral buildup by brute‑force flushing. But with modern controllers, sensors, and home automation, we can do far better.
Intermittent flow is the modern, physics‑aligned way to run a flow‑through humidifier.
If your humidifier runs water continuously, you’re almost certainly wasting:
Intermittent flow solves all of these at once. It keeps the pad wet, lets it warm up, maximizes evaporation, and avoids the cold, constantly soaked environment mold prefers.
For anyone running a custom controller — like the Pico‑W‑driven Homatica HVAC optimizer — this is one of the highest‑impact, lowest‑complexity upgrades you can make.