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Six places an air handler freezes in an Albany winter.

Most winter coil failures in commercial buildings start in one of six spots. Here they are on a cross-section — and what we check at each one before the cold arrives.

The cross-section

Outside air in on the left, warm air out on the right.

Cross-section of a commercial air handler with six numbered freeze points Outside air and return air enter a mixing box on the left, pass a filter, a heating coil and a cooling coil with a drain pan and trap, then a fan, and leave as supply air on the right. Red numbers mark the outside-air damper, the mixing box, the heating coil, the cooling coil, the freezestat across the coil face, and the condensate trap. Outside air Return air Mixing box Filter Heating coil Cooling coil Fan Supply air 1 2 3 4 5 6
Simplified drawing of a typical commercial air handler with a heating coil and a cooling coil. Your equipment may be laid out differently.

  1. Outside-air damper

    A damper that sticks open, or opens too far at start-up on a cold morning, pulls sub-freezing air straight onto the coils. Linkages and actuators are checked every fall.

  2. Mixing box

    Outside and return air don't always mix. A cold stream can hug one side and hit part of a coil. Damper settings or air blenders fix it.

  3. Heating coil

    If hot water stops flowing, or a steam coil doesn't drain, the coil can freeze and split. Pumps, valves, steam traps and glycol strength are checked.

  4. Cooling coil

    Water left in a chilled-water coil over winter can freeze. It's drained or protected with glycol before the cold.

  5. Freezestat

    A low-limit sensor across the coil face that stops the fan and closes the damper before a coil freezes. It only helps if it works, so we test it every fall.

  6. Condensate trap

    Where the drain runs through a cold space or outdoors, it can freeze and back water into the unit. Insulation or heat tape helps.

General guidance. Rooftop units, fan coils and heat pumps have their own versions of these risks; we check whichever you have.

A technician testing the freezestat sensor strung across the face of a coil inside an open air handler
The fall visit

What gets done before the first hard freeze.

  • Outside-air dampers stroked and set to close fully.
  • Freezestats tested — trip, reset, and the right response from fan and damper.
  • Heating coil flow, valves and steam traps checked; glycol strength tested.
  • Cooling coils drained or protected; condensate traps insulated.
  • Rooftop gas heat sections fired and checked.
When one freezes anyway

What to do in the first hour.

Stop the air

If the freezestat hasn't already, shut the unit off so no more cold air crosses the coil.

Don't force it

Don't reset and restart repeatedly — a frozen coil can split when water is pushed through it.

Call

Tell us which unit, what the building needs to stay open, and whether any water is leaking.

A technician opening the heat section of a rooftop unit on a snow-covered commercial roof on a clear winter morning
A technician checking a steam trap on the piping beside a heating coil in an older mechanical room
Homes

Houses freeze in different places.

A house doesn't have a mixing box, but it has its own cold-weather weak points — and they cause a lot of January no-heat calls.

Furnace drain lines
High-efficiency furnaces make water. Where their drain runs through a cold garage or outdoors, it can freeze and shut the furnace off.
Exhaust and intake pipes
The plastic vent pipes on the side of the house can ice or drift over with snow. Keep them clear.
Heat pumps
Some frost is normal and melts on defrost. A solid block of ice isn't — call.
Before the first hard freeze

Book the fall visit.

For a building or a house — October is the month to do it.

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