Mixing boxes, stratification and freeze protection

In much of Canada, outdoor air in winter can be colder than -30 C (-22 F). When that air enters an air handler and is not properly mixed with warmer return air, cold streams can travel straight to heating and cooling coils. The result is frozen and burst coils, freezestat trips that shut down units on the coldest nights, and expensive water damage.

Frozen coils are one of the most common and costly air handler failures in Canadian buildings. Most are preventable. This guide explains why stratification happens, how freezing occurs and what designers, contractors and operators can do about it.

What a mixing box does

A mixing box, or mixing plenum, is the section of an air handler where outdoor air and return air meet. The goal is to produce a uniform mixed air temperature across the full face of the downstream filters and coils. If the outdoor air fraction is 30% and the outdoor air is -20 C while return air is 22 C, the ideal mixed temperature is about 9 C. If mixing is poor, parts of the coil face may see air much colder than that average.

Why air streams stratify

Cold outdoor air is denser than warm return air. Left to itself, cold air tends to sink and flow along the bottom of the plenum while warm air stays at the top. Several design features make this worse:

• Outdoor and return dampers on adjacent faces at right angles that do not direct the streams into each other.
• Short mixing distance between the dampers and the first coil or filter.
• Parallel blade dampers that direct air in one direction, sending streams along a wall.
• Low velocity through the mixing section, which reduces turbulence.
• Large outdoor air fractions, especially in economizer mode or 100% outdoor air units.
• Oversized dampers operating barely open, creating jets.

Stratification can produce temperature differences of many degrees across a coil face. An average mixed air reading that looks safe can hide a band of sub-zero air along the bottom of the coil.

How coils freeze

Water expands as it freezes. In a coil tube, ice forms first where air is coldest and water flow is lowest. Trapped water between ice plugs can generate enough pressure to split tubes or return bends. Freezing risk is highest when:

• The control valve is closed or nearly closed, so water in the coil is stagnant.
• Water flow is low at part load.
• The coil is not fully drained during shutdown.
• Steam coils have condensate backing up because of poor trapping or vacuum conditions.
• Chilled water coils are left full in winter without glycol and without flow.

Heating coils with low flow on a mild sunny afternoon, followed by a sudden temperature drop overnight, are a classic failure scenario.

Design measures

Improve mixing

Static mixers add some pressure drop but are often the most reliable way to achieve good mixing in a short section. Manufacturers publish mixing effectiveness at given distances and velocities.

Preheat and glycol

• Preheat coils upstream of the mixing section raise outdoor air to a safe temperature before it mixes. This is common on 100% outdoor air units and units with high minimum outdoor air.
• Glycol in heating or cooling coils provides freeze protection when water flow stops. It reduces heat transfer and raises pumping energy, so concentration should be chosen deliberately. See glycol systems, concentration and freeze protection.
• Face and bypass coils keep full water flow through the coil while modulating air through a bypass, reducing freeze risk on steam and hot water preheat coils.
• Integral face and bypass steam coils are often used for 100% outdoor air applications in very cold climates.

Circulation pumps and minimum flow

A small coil circulation pump that runs whenever outdoor temperatures are below a threshold keeps water moving through the coil regardless of control valve position. This is a common and effective measure on hot water heating coils.

Chilled water coils in winter

Chilled water coils exposed to winter outdoor air must be drained, protected with glycol, or supplied with continuous flow. Draining often leaves residual water in tubes, so full blow-out with compressed air is needed if relying on draining.

Control measures

Freezestats and low-limit sensors

A low-limit thermostat (freezestat) with an averaging capillary element is mounted across the coil face, typically on the leaving air side of the preheat or heating coil. Common practice is to trip when any section of the element senses about 2 to 4 C (35 to 40 F). On trip, it typically:

1. Stops the supply fan or closes the outdoor air damper.
2. Opens heating valves fully.
3. Starts coil circulation pumps.
4. Raises an alarm.

The capillary must be serpentined across the full coil face, because a single-point sensor will miss cold bands. Many freezestats require manual reset, and operators should investigate the cause before resetting.

Mixed air low-limit control

The BAS should limit the outdoor air damper from opening further when mixed air temperature drops below a low limit, typically several degrees above freezing. The sensor must be an averaging type located where it represents the full face, not a single point.

Sequences

ASHRAE Guideline 36 includes freeze protection logic with staged responses. Designers can adapt these to Canadian conditions. Important points include keeping heating valves at a minimum position in cold weather, running circulation pumps below a set outdoor temperature, and maintaining minimum hot water supply temperature.

Operating practices

• Check averaging sensors and freezestats each fall as part of seasonal changeover.
• Confirm damper blades seal and linkages are tight, so outdoor dampers close fully on shutdown.
• Verify coil circulation pumps run when commanded.
• Inspect steam traps before winter.
• Measure glycol concentration annually.
• After a freezestat trip, find the cause: stratification, damper failure, valve failure, sensor fault or loss of heating water.

Diagnosing stratification in an existing unit

1. Measure temperatures at multiple points across the coil face, for example a grid of thermocouples or a thermal camera on the coil face during cold weather.
2. Compare the spread with the average mixed air temperature.
3. Check damper blade type and orientation, and damper positions during the measurement.
4. Consider adding static mixers, changing damper arrangement or adding preheat.

Infrared thermography can quickly show cold bands across a coil or filter bank.

Key takeaways

• Cold outdoor air and warm return air tend to stratify, leaving bands of sub-zero air on coil faces.
• Opposed blade dampers, good damper arrangement, mixing length and static mixers improve mixing.
• Preheat, glycol, face and bypass coils and circulation pumps reduce freeze risk.
• Averaging freezestats and mixed air low limits must cover the full coil face.
• Seasonal checks and investigating every trip prevent repeated freeze damage.