Freeze Stat Tripping and Frozen Coils: What GTA Property Managers Need to Know Before the First Cold Snap
It's the first night of the season that drops well below freezing. At 6 a.m. the property manager gets a call: the corridors are cold, the make-up air unit is off, and the building automation system shows a low-limit alarm. Someone has already reset it twice, and it tripped again both times.
That is freeze stat tripping, one of the most common winter HVAC problems in commercial buildings. The good news is that the freeze stat did its job. The bad news is that it tripped for a reason, and if that reason isn't found, the next trip might come too late to save the coil.
What a Freeze Stat Actually Does
Air handlers and make-up air units that heat with hot water or steam have a coil full of water sitting in the airstream. In winter, that airstream can include a lot of outdoor air. If the water in the coil freezes, it expands, and the copper tubes split.
A freeze stat is a low-limit safety control that protects that coil. Most use a long sensing element strung across the face of the coil. If any short section of it senses air close to freezing (they are usually set a few degrees above 0°C), the control shuts the fan down and closes the outdoor-air damper to stop cold air from reaching the coil. Many are manual reset, so someone has to physically reset them before the unit will run again.
Why Coils Freeze and Freeze Stats Trip in a GTA Cold Snap
A freeze stat trip is a symptom. The cause is almost always one of these:
Outdoor-air dampers that don't close. A failed actuator, a slipped linkage or blades that no longer seal let outdoor air pour across the coil, even when the unit is supposed to be off.
Cold air that doesn't mix. When outdoor air and return air don't blend properly, a stream of very cold air can run along one part of the coil while the rest stays warm. That's often why only one section of the element trips.
Heat that isn't reaching the coil. A stuck heating valve, a failed pump, air in the piping or a boiler that has shut down all leave the coil with cold air and no heat to fight it.
Low or no glycol. Systems that rely on glycol for freeze protection lose it over time through leaks and top-ups with plain water. A diluted mix can freeze at a much higher temperature than the system was designed for.
Cooling coils left full of water. A chilled-water coil in a unit that brings in outdoor air can freeze in winter if it isn't drained or protected for the season.
Units that shut off overnight. When a unit cycles off and the heating loop stops circulating but the damper leaks, the coil can sit in cold air with no flow for hours.
Freeze Stat Tripping: What to Do When It Happens
Don't keep resetting it. If it trips again after a reset, the cause is still there. Each run puts more cold air across the coil.
Never jumper or bypass it. Bypassing a freeze stat to get heat back on is one of the most common ways coils burst.
Look for water. Check under the unit, in the drain pan and on ceilings below the mechanical room or rooftop curb.
Keep the building warm another way while the cause is found, so pipes and occupants are protected.
Call for service. A technician needs to find out why the air got that cold: dampers, valves, pumps, glycol, controls. Our emergency service team handles these calls through the winter.
Signs a Coil Has Already Frozen
A frozen coil doesn't always leak right away. The split often shows up only when the ice thaws, sometimes days later. Watch for:
Water in the drain pan, under the unit or on ceilings below.
A heating loop that keeps losing pressure, or a boiler make-up water line that keeps running.
A coil that stays cold in one section even when hot water is flowing.
Hissing or dripping inside the unit once it warms up.
If a coil has split, the repair usually means replacing it, and the water damage can cost more than the coil. That's why prevention matters.
How to Prevent Frozen Coils This Winter
Many freeze-ups can be caught during a fall start-up. Before the cold arrives:
Stroke every damper. Make sure outdoor-air dampers close fully and actuators, linkages and seals are in good shape.
Test the freeze stat. Confirm it trips, shuts the unit down and closes the damper the way it should.
Test the glycol. Check the concentration where systems rely on it, and top up with the right mix, not plain water.
Check valves and pumps. Heating valves should open fully on a call for heat, and pumps should run in cold weather.
Drain or protect unused cooling coils in units that bring in outdoor air.
Review the controls. Outdoor-air dampers should close when a unit is off, and low-limit alarms should go to someone who will act on them at 3 a.m.
A maintenance agreement puts these checks on a schedule every fall, instead of after the first freeze.
Getting Ahead of Freeze Stat Tripping
Burban Air Systems services make-up air units, air handlers, boilers and the rest of the equipment found in GTA commercial buildings. If your freeze stat is already tripping, or you want your units checked before the first cold snap, call (416) 757-3271 or send us a message.
