Drain the solenoid body separately from the supply line, pull the battery pack or isolate the 24V transformer from the cold zone, and leave the backflow preventer open per code. A sensor faucet left “winterized” like a manual fixture, meaning the supply valve closed and nothing else touched, will still crack a solenoid body or corrode a battery compartment by the second season. The electro-mechanical parts fail on a different schedule than the pipe.
Why Solenoid Faucets Freeze Differently Than Manual Ones
A manual faucet is a dumb assembly: shut the valve, drain the line, done. A sensor faucet has a solenoid valve body sitting in the water path, usually with a small chamber and o-ring seats that hold residual water even after the supply line empties. That trapped water freezes at the same 32°F (0°C) as anything else, but because the chamber is smaller and the walls are often cast rather than copper, the solenoid body cracks before you’d see a split in a 1/2 inch supply line. You can drain the building’s water lines perfectly and still lose the faucet because nobody opened the solenoid port.
The second difference is that the faucet has power running to it. Battery packs and AC transformers don’t freeze in the sense that ice doesn’t form inside them, but cold cycling causes condensation inside the battery compartment, and that moisture is what kills contacts and corrodes terminals over a winter. So you’re managing two separate freeze risks: water in the solenoid, and moisture in the electronics.
Draining Supply Lines and Solenoid Bodies Correctly
Shut the supply at the stop, then open the solenoid manually (most commercial units have a manual override or bleed screw specifically for this) to let the chamber drain, not just the line above it. If the faucet has no manual override, disconnect the solenoid body at the union and blow it out with low-pressure compressed air, under 50 psi is standard for this, until no water shows at the outlet.
Don’t skip the backflow preventer. Local plumbing code will specify the draining sequence for the backflow assembly feeding the fixture, and in most jurisdictions this means opening the test cocks after the main shutoff is closed, letting the check valves drain by gravity, and leaving the assembly open until spring. An undrained backflow preventer in an unheated mechanical room is a more common freeze failure than the faucet itself.
- Close the supply stop first, not the main building valve, so you’re not draining fixtures you don’t need to touch.
- Open the solenoid’s manual bleed or disconnect the union to clear the chamber.
- Drain the backflow preventer per the local code sequence, typically through the test cocks.
- Leave all valves in the open/drained position until startup, don’t reseal a system you think is empty.
Protecting Battery Packs and 24V Transformers From Cold
Battery-powered units are easier to winterize than hardwired ones because you can just pull the batteries. Remove them, don’t leave them in a faucet that’s sitting unused in a 20°F mechanical room for four months. Cold accelerates self-discharge and increases the odds of a leaking cell corroding the compartment contacts. Check the battery compartment’s IP rating before you decide whether to leave it sealed or open for the winter; a compartment rated IP67 or better can sit closed without trapping condensation, but anything lower should be opened slightly or have a desiccant pack added so moisture doesn’t collect against the contacts.
Transformer-fed units need a different fix. The transformer itself should never be mounted in the freeze zone to begin with, that’s a design decision, not a winterizing step, but if you’ve inherited a job where the 24V AC transformer is in an unheated chase or exterior wall cavity, winterizing means relocating it to a conditioned space if at all possible. If relocation isn’t feasible this season, at minimum verify the transformer’s enclosure rating matches the exposure and that the low-voltage wiring runs are sealed where they penetrate the cold zone, so condensation doesn’t track down the cable into the solenoid connector.
Insulating Exposed Chases, Mechanical Rooms, and Risers
Pipe insulation on the riser feeding the faucet buys you time during a cold snap but it doesn’t replace draining the system for a space that’s unheated all winter. Use insulation to protect the sections you can’t fully drain, like a riser that also feeds other active fixtures, and combine it with heat tape on runs that see sustained sub-freezing air. Don’t wrap the solenoid body itself expecting that to save it if the chamber is still full of water; insulation slows heat loss, it doesn’t generate heat, and a standing column of water in an uninsulated mechanical room will still freeze overnight even under wrap.
If the space has any intermittent heat source, a unit heater on a low set point is more reliable than insulation alone for protecting the electronics and keeps the compartment above the dew point enough to reduce condensation cycling. For spaces with zero heat, like an exterior wall-mounted fixture bank at a loading dock, full drain-down each night during freeze season is often the only workable approach rather than trying to keep the lines live.
Spring Startup Checks Before Reactivating Sensors
Before you close the backflow test cocks and re-pressurize, inspect every solenoid body you opened for hairline cracks around the chamber seam, these don’t always leak immediately under static pressure but will show up as a slow weep once the faucet starts cycling. Reinstall batteries and check the compartment gasket for cracking or compression set from the cold; a gasket that’s taken a permanent set won’t reseal properly even if it looks intact.
Bring the supply up slowly rather than opening the stop fully, this lets you catch a cracked solenoid at low pressure before it sprays at full line pressure. Cycle the sensor manually several times to confirm the solenoid is actuating cleanly, since a solenoid that sat dry all winter can have a sluggish plunger even if the body itself didn’t crack. If you’re specifying replacement units for a space that freezes every year, it’s worth looking at touchless bathroom faucets rated for the exposure you actually have rather than retrofitting a standard interior-rated sensor faucet into an unheated vestibule.
Year-Two Winterization Checklist for Techs
| Component | Winter risk | Action |
|---|---|---|
| Solenoid valve body | Cracks below 32°F (0°C) with trapped water | Drain chamber via manual bleed or union disconnect |
| Battery compartment | Condensation and corrosion, not freezing per se | Remove batteries; check IP rating before sealing closed |
| 24V AC transformer | Enclosure failure if mounted in freeze zone | Relocate to conditioned space or verify enclosure rating |
| Backflow preventer | Trapped water in check valves splits the body | Drain per local code sequence, leave open until startup |
| Supply riser | Standing water freezes overnight in unheated chases | Insulate and/or heat trace; drain fully if unheated all season |
Walk the space once in late fall and confirm every item on that table against the actual conditions, not the conditions when the faucet was installed. A mechanical room that was heated during construction and isn’t heated now is the most common cause of a “surprise” freeze failure in year two. If you find a transformer or solenoid body already mounted in a spot that can’t be reasonably protected, flag it for relocation rather than re-winterizing the same vulnerable point every year.