Yes, it can. A furnace humidifier running on hard well water collects scale faster than one fed by softened or municipal water, and that scale is where the trouble starts. Scale narrows the water path through the pad or the wheel, clogs the drain line, and lets water back up or stand inside the housing instead of draining off after the furnace cycles down. The EPA's guidance on mold and moisture notes that scale buildup can turn into a breeding ground for microorganisms, and that water standing in a central humidifier is a problem worth correcting rather than living with. On Hunterdon County's well water, that is less a question of if and more a question of how soon it shows up.
How an evaporative or steam humidifier works
Most central humidifiers tied into a furnace fall into one of two types. An evaporative unit holds a foam or paper pad in a plastic frame, with a water line from a saddle valve on the supply pipe feeding a tray or trough that keeps the pad wet, and the furnace blower pulls air across that wet pad into the supply plenum. A steam unit heats water inside a sealed canister using electrodes or a heating element, and sends steam straight into the duct through a small port, with no pad involved. Both draw straight from the house's water supply, and both depend on minerals staying out of the way of moving parts that were sized for clean flow, not scale.
Where hard water scale builds first
On an evaporative unit, the pad is the first place scale shows, turning from an open foam or honeycomb surface into a stiff, chalky block that water can no longer pass through evenly. The tray underneath picks up a crusted ring at the waterline, and the saddle valve that feeds the line can narrow from the inside until it is passing a trickle instead of a full flow. The drain line, where the unit has one, is the part most likely to plug outright, since mineral flakes and pad fragments collect at any bend or low point in that tube. A steam canister scales from the inside out, building deposits on the electrodes or the heating element until the canister stops producing steam at the rate it should, and most makers treat a scaled canister as a replacement part rather than something to clean out.
What standing water does to the plenum and the duct near it
Once a drain clogs or a pad stops draining the way it should, water has nowhere to go but back into the housing, and from there it finds the sheet metal underneath. A rust streak or a water stain on the plenum below the humidifier is usually the first visible sign, along with a musty smell near that stretch of duct that was not there before. Left alone, scale and mineral flakes that break loose from a failing pad can travel a short distance into the supply duct itself, especially if the humidifier sits close to a seam or a low spot in the plenum where moisture can collect instead of moving on with the airflow.
The shutdown steps that matter at the end of the heating season
The sheet that came with the humidifier has the exact steps for that model, and skipping them is how a unit sits full of water through a humid Hunterdon County summer instead of dry. In general terms, that means shutting off the saddle valve on the supply line, draining the tray or trough completely, pulling the pad for inspection and usually replacement if it is more than a season or two old, and checking that the drain line runs clear before the unit goes back together for the next heating season. A steam canister gets swapped on its own schedule rather than cleaned, since scale inside the sealed chamber is not something a homeowner can flush out.
What mold treatment and removal covers, and what the heating contractor handles
The pad, the valve, the drain line and the canister are parts of the humidifier itself, and a heating contractor who services the furnace is the one who replaces or adjusts those. Mold Treatment and Removal steps in once growth has reached the plenum, the humidifier's housing, or the stretch of connected duct nearby, which is where treatment stops rather than spreading further into equipment the mold work does not cover. Air Duct Cleaning often follows that work to clear scale flakes and residue from the supply trunk the humidifier feeds into, and a section of duct or plenum that rusted through from standing water is a job for Air Duct Repair rather than a cleaning alone.
Keeping indoor humidity from feeding the problem
The EPA's guidance on mold and moisture points to keeping indoor relative humidity at or below 50 percent as one of the more useful steps a household can take against mold growth generally, not just inside a humidifier. A humidistat set higher than that keeps the pad wetter for longer stretches between cycles, which gives any growth that has already started more time to spread before the unit dries out again. Checking that setting against a simple humidity gauge placed away from the supply registers is a quick way to see if the humidifier is running higher than the house calls for.
Hard well water across Hunterdon County
Private wells supply much of Readington and the townships around it, and well water across Hunterdon County tends to run hard enough to leave a chalky film on a kettle, a showerhead, or a furnace humidifier pad well before a municipal water supply would show the same buildup. A furnace humidifier that has run through several heating seasons on that water without a pad change or a shutdown routine is the kind most likely to turn up a scaled valve, a clogged drain, and a stained plenum underneath it once someone finally looks.