Install a Bypass Humidifier

Install a whole-house bypass (or powered) humidifier: mount it on the supply or return plenum and run the bypass duct to the opposite plenum so warm air flows through it, tap the water supply (saddle valve / line) and route the drain, wire the humidistat into the blower / 24V so it runs only on heat-with-airflow, set the water panel, and verify water flow + evaporation.

Approx. 120 min

Before you start

•Type chosen: bypass (uses the pressure difference between supply and return to push air through, needs the bypass duct) vs powered/fan (its own fan, mounts on the supply, no bypass duct). Bypass is simpler/cheaper; powered handles tighter spaces / higher output.
•A potable water supply within reach and a drain for the run-off (flow-through humidifiers waste water to drain), plus a clear plenum face on supply AND return for the bypass run.
•Humidistat location decided (on the return/wall) and a 24V source + the blower signal available to interlock it.

Tools & materials

Drill + hole saw (plenum cutout), template, sheet-metal screws, foil tape · Saddle valve or tee + 1/4 in tubing, drain tubing, adjustable wrenches · Multimeter, low-voltage wire, humidistat · Humidifier unit + water panel (evaporator pad) + bypass collar/duct

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Mount & bypass duct

Cut in and mount the humidifier
Using the template, cut the plenum and mount the humidifier on the supply (or return) plenum, sealed and level so the water panel sits and drains correctly. Kill and verify power before working on/near the air handler.
Tools: Drill + hole saw, Template, Foil tape
Target: Humidifier mounted level + sealed on the chosen plenum
⚠ Verify power off before opening the air handler / drilling the plenum.
Run the bypass duct to the opposite plenum
On a bypass unit, cut a collar in the OPPOSITE plenum (return if the unit is on the supply) and run the bypass duct between them. This routes warm supply air through the water panel and back into the return. A powered unit skips this — it has its own fan.
Tools: Sheet-metal screws, Foil tape
Target: Bypass duct connects the humidifier to the opposite plenum (supply↔return); damper set open in heating season

Water & drain

Tap the water supply
Tap a potable cold (or warm, per the unit) water line with a saddle valve or a proper tee, run 1/4 in tubing to the humidifier's solenoid/inlet, and confirm no leaks. Many manufacturers prefer a proper tee over a self-piercing saddle for reliability.
Tools: Saddle valve or tee + 1/4 in tubing, Adjustable wrenches
Target: Water feed tapped + run to the inlet; no leaks at the tap or fittings
Route the drain
Run the drain line from the humidifier to a floor drain / condensate drain with continuous downhill slope (flow-through units run water to drain by design). No traps or uphill runs that hold water.
Tools: Drain tubing
Target: Drain sloped continuously to a legal drain; no uphill/trapped runs

Wire & verify

Wire the humidistat into the blower / 24V
Wire the humidistat and the humidifier's solenoid so the humidifier only runs on a humidity call WITH the blower running in heating — typically interlocked through the blower (sail switch / fan signal) and the 24V control. Running it without airflow puts water into a dead plenum.
Tools: Multimeter, Low-voltage wire
Target: Humidifier energizes only on a humidity call with the blower running in heat (interlocked via 24V/blower)
Set the water panel and verify flow
Install the water panel (evaporator pad) with its flow direction correct, then call for humidity with the blower running and confirm water flows over the panel, wets it evenly, and the excess goes to drain — and that it shuts off when the call ends. Set the humidistat to a sensible RH so windows don't sweat.
Target: Water flows + wets the panel on a call, drains the excess, stops when satisfied; RH set to avoid condensation