Outdoor Condensing Units in Winter: Low-Ambient Walk-In Problems

Cold weather sounds like a gift for a walk-in, but an outdoor condensing unit that is not set up for it can stop cooling in January. The cause is usually pressure, not temperature: the system needs a minimum head pressure to work, and cold air takes it away.

Why cold air causes trouble

The condensing unit sheds heat from the refrigerant into the outdoor air. In winter the air is so cold that the refrigerant condenses at a very low pressure. That pressure is what pushes liquid refrigerant through the expansion valve (TXV) into the evaporator in the box. Too little pressure, and the valve can't feed enough refrigerant.

The result is a starved evaporator. Common symptoms:

• the box drifts warm even though the unit is running;
• the evaporator coil frosts or ices unevenly, often only on part of the coil;
• the compressor cycles on and off on the low-pressure control (short cycling);
• frost on the suction line or oil-return problems on some systems.

Each of these has its own symptom page: not cooling, frozen evaporator coil and short cycling.

Low-ambient controls

Outdoor condensing units in cold climates are normally fitted with some form of head pressure control to keep pressure up in cold weather. The two you will hear about most:

Some units use variable-speed condenser fans for the same purpose, and some electronic controllers read an outdoor temperature sensor. Heatcraft's Beacon II controller, for example, has an outdoor temperature sensor and reports an E5 error if it is shorted. Which method your unit uses, and at what settings, is on its nameplate, wiring diagram and manual.

A flooded head pressure system is a good example of why charge problems are technician work. The correct charge in a flooded system depends on the lowest outdoor temperature it has to handle, so a charge that looks fine in summer can come up short in winter. Topping off without a leak search is not a fix; see refrigerant leaks and recharging.

Crankcase heaters

Refrigerant migrates to the coldest part of a system when it is off. In cold weather that can be the compressor's crankcase, where liquid refrigerant mixes with the oil. On start-up, that liquid can foam the oil out and wash out the bearings.

A crankcase heater keeps the compressor oil warm to prevent this. Heatcraft's condensing unit manual calls it "required" to have the crankcase heater on for 24 hours before first start-up. For operators the lessons are simple:

• Don't cut power to an outdoor condensing unit in winter "to save energy" and then switch it straight back on. If a unit has been off, ask a technician how to restart it.
• A technician should confirm the crankcase heater works as part of pre-winter service.

Snow, ice and water

• Keep the unit clear. Snow drifts or ice over the coil, fan guard or cabinet block airflow and can bend fan blades. Clear snow by hand or with a soft brush, with the unit's disconnect turned off. Never chip ice off the coil or fins.
• Mind roof drainage. Ice dams and dripping from gutters above a unit can encase it in ice. Report it rather than hacking at it.
• Freezer drain lines. A freezer's condensate drain must be heated. Heatcraft specifies a continuously powered drain-line heater wherever the trap or line runs outside, and says traps on low-temperature units must be outside the refrigerated space. A heater that fails in winter freezes the drain, and water then backs up and freezes in the evaporator pan. See drain line heater.

Safety: Turn the condensing unit's disconnect off before clearing snow near the fan, and confirm the fan has stopped. Do not touch refrigerant lines; frosted lines can cause frostbite.

A cooler that gets too cold

Walk-ins built outdoors or against an exterior wall can see the opposite problem. In a hard freeze, a cooler's panels and door side can get cold enough on their own that product near the outside wall freezes, even when the refrigeration is off. The fixes range from moving sensitive product away from exterior walls to adding controls. If product is freezing on a cold night, read walk-in cooler too cold and have a technician look at the controls before assuming the thermostat has failed.

Pre-winter checklist

The preventive maintenance schedule folds these into the rest of the year's service. If you are weighing where to put a new unit, the site's indoor vs outdoor walk-in cooler guide covers the tradeoffs, and condensing unit explains the parts inside the box on the roof.