AHU frost protection: keeping coils from freezing and heat recovery from icing up

A frozen water coil is about the most expensive way an AHU can fail. The ice splits the tubes, and when it thaws you’ve got water pouring into the plant room or the ceiling below. Replacing a big coil means downtime, sometimes a crane, and a nasty bill. It’s usually discovered on the first properly cold Monday of the winter.

Good frost protection isn’t one device. It’s several layers, so that when one fails the next one catches it.

How coils end up freezing

A coil freezes when very cold air passes over it while the water inside is barely moving, or not moving at all. The usual ways that happens:

fresh air dampers left open, or failed open, while the unit’s off
the heating valve shut (or failed shut) during a cold start
the LTHW pump stopped, or flow lost because of a blocked strainer or an air lock
cold air stratifying across one part of the coil
frost protection bypassed or switched off “just for now”

Chilled water coils are at risk too, because in winter they’re often valved off and full of still water.

The frost thermostat

The last line of defence is the frost thermostat. It’s a capillary thermostat with a long sensing element spread across the face of the coil, so it reacts to the coldest point anywhere on that face, not just one spot.

It usually sits on the leaving-air side of the first water coil, with the capillary looped evenly across the whole face. It’s set a few degrees above freezing, commonly around 4–5 °C, though the spec decides the actual figure.

When it trips, it should:

stop the fans
close the fresh air damper
drive the heating valve fully open
make sure the heating pump is running

It’s normally manual reset, so someone has to look at why it tripped before the unit runs again.

Crucially, those trip actions should be hardwired, so they still happen if the BMS controller has died. The BMS should also monitor the frost stat and raise a high-priority alarm.

BMS low-limit control

You don’t want to rely on the frost stat every cold morning. Well before it trips, the BMS should step in with a low-limit routine. It watches the air temperature after the heating coil (or the mixed air temperature) and, as that falls towards a low threshold, it:

forces the heating valve open
on units that mix return air, cuts back the fresh air damper
raises a warning

That “soft” protection keeps the AHU running through cold weather instead of tripping out first thing every day.

Cold starts

The riskiest moment is starting up on a freezing morning. A sensible routine starts the heating pump and opens the heating valve before the dampers move. It waits until the coil is warm. Only then does it open the dampers and start the fans, with hot water already flowing. Where the design allows, some units also do a short warm-up with the dampers shut and the air recirculating.

Preheat coils

Where it gets properly cold, or where the unit runs on 100% outside air, a preheat coil warms the air before it reaches the main coils. It might be electric, or an LTHW coil on a glycol circuit that can’t freeze. It’s usually controlled to keep the air just above freezing, and an electric one needs its own high-limit protection.

When the unit’s off

Frost protection doesn’t stop when the AHU does. In winter:

the dampers should be shut, ideally with spring-return actuators so they also close on a power cut
heating valves should stay partly open, with pumps running when the outside temperature drops below a threshold
chilled water coils on units that run through winter need flow, draining down, or glycol

Heat recovery icing up

Heat recovery brings its own frost problem. Warm, moist extract air gets chilled by the cold incoming air, and the moisture condenses and freezes on the extract side, gradually blocking the exchanger. How you deal with it depends on the type:

Plate heat exchangers usually have a bypass damper that opens to stop the plate getting too cold. It’s driven by the exhaust air temperature or a rising pressure drop across the plate.
Thermal wheels slow down so they recover less heat, again usually based on exhaust air temperature.
Run-around coils use glycol, plus control of the circuit pump or a mixing valve to stop the extract coil getting too cold.

You lose some heat recovery in the coldest weather. That’s normal, and the heating or preheat coil picks up the slack.

Test it before winter, not after

Frost protection should be proven, not assumed. Before the cold weather arrives:

Trip the frost stat for real, for example with freeze spray on the capillary, and check every action happens: fans stop, damper shuts, valve opens, pump runs, alarm reaches the front end.
Check the capillary covers the whole coil face rather than being bundled up in one corner.
Make sure it needs a manual reset, and that the BMS doesn’t just restart the unit by itself.
Test the BMS low-limit by overriding the sensor value and watching the valve respond.
Kill the power to the damper actuator and check it springs shut.

Things that go wrong

The same mistakes turn up again and again:

frost stats linked out because they “keep tripping”, instead of anyone finding out why
the BMS restarting an AHU over and over after frost trips
capillaries bundled in a corner
damper springs or linkages that have failed, leaving dampers open all night
summer settings still in place in a cold snap, with heating valves closing completely

All of this sits inside the wider AHU control strategy. Get the start-up sequence right and the frost stat should hardly ever need to trip.