The current outbreak of Legionaire’s disease got me to wondering this. A whole-house humidifier mounts on the outlet duct of a house’s furnace. My understanding is that there’s a wicking pad that picks up water from a trough in the bottom, and the passing air absorbs moisture from the pad
So how is it that these things aren’t breeding grounds for massive mold/bacterial colonies? Do the pad and trough dry out completely between furnace heating cycles?
I’m interested in this, too, since I used to work for the company that made the chillers implicated in the original Legionnaires’ breakout (long before my time, didn’t even have to give a deposition). Without bleach in the equation, and the house smelling of it, it seems to be exactly as you described.
Bleach?
That’s what used to chlorinate swimming pools.
Top-shelf bleach is 6% sodium hypochloride (or some such)
The “pool chlorine” you see in big box stores is 10%.
TL;DR:
There is never any standing water in the humidifier.
More detail:
The whole house humidifier that I installed on my furnace is mounted to the outflow side with a flexible pipe connected to the inflow side of the furnace. I had to cut out a large square section (about 1 sq ft) of the of outflow duct and it is entirely covered with a filter. When the furnace is running, most of the air moves right past the humidifier to the rest of the house. But a lot of air does move through the filter to the inflow side.
The humidifier is also connected to a water supply line and a drain. When the humidity drops to a point set on the humidifier, it opens a valve that lets water into the humidifier at the top of the filter and the water trickles down the filter. As the air moves from the outflow side to the inflow side through the filter, it gets humidified.
The water moves from the top of the filter to the bottom where any excess drains out of the filter and into the floor drain that the AC drains into.
Once the thermostat turns off the furnace, the water will stop flowing into the humidifier. Any excess moisture either drains out the bottom of the humidifier or evaporates. No mold, no mildew, no Legionaire’s.
As was said above, fresh water is supplied via tubing/piping, not a trough of standing water.
Additional considerations which are unfavorable to mold growth:
According to Overcooking to Kill Bacteria: Food Myth | Dr Terry Simpson , “Most bacteria do not live above 120°F, and as you increase the temperature you kill more of them. At 102°F most bacteria can no longer reproduce, which is the protective nature of human fevers.” The discharge temperature of most furnaces sit in or above this range. This is most relevant for configurations where the wicking pad is installed directly in the discharge duct of the furnace. Less relevant where the wicking pad is installed in the suction duct.
The water source for the wicking pad cuts off when the humidity set point is reached, giving the pad an opportunity to completely dry.