In cold weather, an outdoor condensing unit can run with pressures too low for the system to feed the evaporator properly. Head pressure controls, extra winter charge and heated drains exist to prevent that. In Hattiesburg's occasional cold snaps, symptoms include a flashing sight glass, low-pressure trips and frozen freezer drains, even if the box is fine in summer.
Most people think of walk-ins as a summer problem, and for Hattiesburg, they mostly are. But winter cold snaps arrive, and equipment set up for summer conditions may behave oddly on the first freezing night. Knowing what changes helps you tell a real fault from a seasonal quirk.
What cold weather does
| Effect | Why |
|---|---|
| Lower head pressure | Cold air removes heat from the condenser very effectively, so pressure falls |
| Poor refrigerant feed | The expansion valve needs a certain pressure difference to work well |
| Flashing sight glass | Charge that was correct in summer can be low for cold conditions |
| Low-pressure trips | Suction pressure drops with the outdoor temperature |
| Frozen drain lines | Outdoor or unheated sections of drain freeze |
| Slow starts | Cold oil and low pressures make the compressor work harder to start |
How equipment handles the cold
- Head pressure control. A valve or fan-cycling system holds pressure up by backing refrigerant into the condenser. It needs extra refrigerant charge to work.
- Crankcase heater. Keeps compressor oil warm.
- Drain line heaters. Keep freezer drains from freezing.
- Insulation and covers. Protect lines and sensors.
If these were never set up, or they have worn out, the first cold snap exposes it.
Symptoms to watch for
- The sight glass bubbles on cold mornings and clears as the day warms.
- The compressor stops on low pressure on cold nights.
- Frost on the suction line or an unusual coil pattern.
- Ice near the drain outlet or in the pan.
- Temperature drifts up in the coldest hours.
What you can do
- Note when it happens. Date, temperature outside and time of day.
- Check that heaters are warm. Drain and door heaters on a freezer.
- Keep the area clear of snow, ice and debris.
- Do not add refrigerant yourself. Winter charging is a technician's job.
- Tell the technician at your annual visit, and ask them to check winter settings.
Why it matters for food
A cold snap does not usually threaten the box directly, but a unit that shuts off on low pressure overnight can let the temperature climb before anyone notices. Check the log the next morning after any freeze, and take a probe reading of the food rather than trusting the display alone.
What a technician does
- Checks the head pressure control operation and settings.
- Confirms the charge is correct for winter conditions.
- Tests crankcase and drain heaters.
- Reviews low-pressure and other control settings.
- Adjusts or replaces worn parts before the next cold spell.
For the sight glass, see sight glass bubbling. For low-pressure trips, read low pressure cutout. The systems guide explains why pressure matters, and the maintenance checklist lists a winter check.
Questions about winter operation
Do Mississippi units need winter charging?
It depends on the design. Units with head pressure controls need the right amount of charge for the coldest conditions they will face. Ask your technician.
Why is the sight glass clear in the afternoon but bubbling at dawn?
Colder outdoor air lowers pressure and charge requirements shift. It can be normal, or a sign of a marginal charge.
Can cold weather hurt an idle unit?
A unit left off in freezing weather can have oil and water issues. Keep crankcase and drain heaters powered where they are fitted.