Coil Freeze Risk Checker HVAC

Worked example

A 6 AM freezestat trip in January: OAT 5 °F, return air at 70 °F — and the trend shows the return-damper linkage stuck at half, so the box is running 50 % outdoor air.

1. Place the mix: MAT = 50 % × 5 + 50 % × 70 = 37.5 °F.
2. Set it against the fluid — plain water freezes at 32 °F: margin = 37.5 − 32 = 5.5 °F.
3. The stat is set at 38 °F and the mix is under it: the stat was right. A trip here is the protection working — the finding is the damper linkage, not the safety.
4. Now switch the fluid to 30 % propylene glycol: the freeze point drops to 8 °F and the margin opens to 37.5 − 8 = 29.5 °F. The tubes are covered — and the stat still trips, because it reads air, not fluid. Glycol protects tubes, not trips; chronic trips are a mixing or setpoint conversation.

The calculated MAT is the straight weighted average from the Air Handlers lesson’s mixing-box arithmetic. When you need the full psychrometric blend — humidity, enthalpy, mass-weighted streams — the Air-Mixing Calculator owns that math.

Freeze point vs burst point

Freeze point is where ice crystals first form — the mix turns to slush. Burst point sits far below it: slush still flows and absorbs expansion, so tubes survive well past the freeze point and split only near the burst rating. That gap is the whole idea of burst protection — an idle coil with a 30 % mix can sit in 0 °F air all winter without splitting. It is not an operating condition: moving slush plugs passes and kills capacity.

Typical values for inhibited glycols, mixed by volume. Formulations differ by a degree or two, and real loops drift lean as they get topped off with water — the fluid maker’s chart governs, and a refractometer reading of the actual loop is the ground truth. Glycol also carries less heat per gallon: the Waterside Load Calculator stays water-only for exactly that reason.

Why steam coils split

A modulating steam valve throttled back to a light load drops the coil below atmospheric pressure. The vacuum holds condensate up in the tubes — the trap can’t drain what the pressure difference is holding — and sub-freezing air across the face does the rest. The fixes are piping and arrangement, not tuning: a working vacuum breaker so the coil can drain at low pressure, a real drip leg into a generously sized trap, and — for coils that see freezing air — a two-position valve with face-and-bypass dampers, which moves the modulation to the air side and keeps the coil at full pressure and temperature wall to wall.

Why coils split while the trend looks fine

The MAT this page computes is an average. A mixing box that layers instead of mixing runs cold blankets 10–20 °F below that average along the bottom of the plenum — which is why coils split while the trend looks healthy, why freezestat elements are serpentined across the whole coil face (the stat trips on its coldest foot of capillary, not the average), and why a single-point MA-T sensor can lie about all of it. Averaging elements and honest damper arithmetic are the countermeasures.

The protection stack, roughly in order:

1. Freezestat hard trip — manual reset, so a human has to look before the fan comes back. An auto-reset stat cycles the fan all night while the coil keeps icing.
2. Low-limit MA-T override — drive the dampers toward minimum when the mixed air sags, whatever the cooling logic wants. The Economizers lesson calls this the floor under free cooling.
3. Pump-on-freeze / valve-open-on-trip — dump heat into the coil and get the water moving the moment the stat fires.
4. Glycol at burst-protection strength — for chilled-water coils idle through winter, where no heat source exists to rescue them.
5. Drain what’s truly idle — an empty coil can’t split.
6. Winter-safe minimum positions — a minimum-OA share that keeps the calculated MAT warm by arithmetic, so the routine case never gets near the bands above.