The Furnace Humidifier
A furnace humidifier is a small water appliance sitting inside the air you breathe, and it needs attention every heating season.
Short answer
Each fall: replace the pad or clean the drum, clear the drain, check the water supply and the solenoid, clean the distribution tray, and set the humidistat sensibly. Each spring: turn the water off and the damper to summer. Neglected, it either stops working or becomes a wet surface in the supply air.
The fall service
Replace the evaporator pad on a flow-through unit, or clean and descale the drum and pad on a drum unit.
Clear the drain line and the drain pan, because a blocked drain on a flow-through unit backs water into the plenum.
Check the water supply, the saddle valve or tee, and the solenoid, since a stuck solenoid either floods or does nothing.
Clean the distribution tray and the orifice, which scale up in Toronto water.
Check and set the bypass damper to the winter position, which on a bypass unit is the thing everybody forgets.
And check the humidistat and set it to a sensible winter number rather than leaving it where the installer put it.
The spring step
Turn the water supply off and set the bypass damper to summer. A humidifier left connected and the damper left open over a Toronto summer means a wet component in the cooling supply air, which is a mould source, and it also short-circuits the cooling airflow through the bypass. Two minutes, every spring.
The humidistat setting
Lower than most people set it, and lower as it gets colder outside.
Too high produces window condensation, and in a Toronto winter that water runs into the sill and the wall.
Persistent window condensation is the practical signal to turn it down.
Thirty to forty percent is a reasonable range, reducing toward the bottom of that in a cold snap.
An outdoor-reset control does this automatically and is worth having if the unit supports it.
And measure with a separate hygrometer in a living space rather than trusting the humidistat on the duct, which reads a different place.
What goes wrong
A pad never changed, which scales up, stops evaporating and does nothing.
A blocked drain, which backs water into the plenum and the furnace cabinet.
A stuck solenoid, which runs water continuously — frequently straight down the drain, invisibly, on the water bill.
A bypass damper left open in summer, which is both a mould risk and an airflow problem.
A humidistat set too high, which is the cause of most window condensation complaints in a Toronto house.
And a steam or atomizing unit with no maintenance, which in hard water scales up quickly and can discharge mineral dust into the air.
Common questions
What humidity should I set in winter?
Thirty to forty percent, lower in a cold snap. Window condensation means turn it down.
Do I need to do anything in spring?
Turn the water off and set the bypass damper to summer. Two minutes.
How often does the pad need changing?
Every heating season, and sooner in hard water.