The storage rating answers the wrong question. A -22 °F storage limit covers a switched-off panel; a standard panel running at -10 °F sits about 42 °F below its 32 °F operating floor. Measure the enclosure, reload the project once, and fit a thermostat-controlled heater that holds the cabinet above the floor.
An HMI project that vanishes after a cold weekend has one thing to check before anything else: the operating temperature rating. The storage figure printed on the datasheet covers a panel that is switched off. A screen that sat powered through a -10 °F control-room weekend ran about 42 °F below the 32 °F operating floor that standard industrial panels are built around, whatever the storage rating says. Reloading the project puts the line back to work, and the same fault returns the next time the room freezes. This page shows how to read the two ratings, what to measure before the reload, and how to heat the cabinet so the panel never sees that temperature again.
Storage rating and operating rating are different numbers
A datasheet carries two cold numbers and they answer different questions. The storage limit is the temperature a switched-off unit survives with its data intact; for industrial electronics that range commonly runs from -40 to 85 °C (-40 to 185 °F). The operating range is the temperature the unit is built to run in, and on typical HMI datasheets it is 0 to 50 °C (32 to 122 °F). Extended-range models push the operating floor down to -20 °C (-4 °F), and a smaller group of hardened panels reaches -40 °C (-40 °F).
The numbers differ because a running panel does what a stored panel does not: it boots from memory devices that carry their own temperature windows, and it drives an LCD whose liquid crystal slows and can misbehave in cold. Below the operating floor the manufacturer promises nothing about booting, display, touch behavior or data retention.
Read the startup lines too. Some panels list a startup limit separate from the running range; your model's manual carries them, and they decide how long a cold cabinet must warm before a safe start.
Symptoms and what each points to
Match what you saw to the row, and the row to the check:
- Project gone after a restart in a room that fell below 32 °F. The panel ran below the standard operating floor, and the enclosure temperature is the first measurement to take.
- Screen slow to boot, dim, or touch that lags while the room is cold, then normal behavior once warm. That is the out-of-spec condition showing up in the LCD and touch layer before it takes the project down.
- Project gone after a restart at a normal room temperature. Cold is closed as a cause and the meter takes over: measure the panel's supply through the next three starts, because a sag or spike at power-up and a failing memory device are the suspects left.
- Fog or droplets on the inside of the enclosure after the room warms. That is condensation off the cold cabinet surfaces; dry the cabinet before re-energizing anything and seal the moisture path.
Checks to make before the reload
- Find the exact panel model and read three numbers from its manual: operating floor, storage floor, and any startup limit. Write the operating floor in °F on a label next to the panel.
- Measure the air inside the enclosure, beside the panel, during the cold spell. Cold air pools near floors and outside walls, so a room average from a hallway thermostat overstates the panel's temperature. The probe method used to check signage enclosure temperature applies here, with the readings judged against the cold limit.
- Photograph the screen before cycling power: error text, boot logo, alarm history. Those frames are the record a repeat fault gets compared against.
- Read the supply voltage at the panel's terminals through a cold start and write it down.
- Open the enclosure and look for condensation once the room has warmed.
- Save the known-good project from the engineering PC with a revision note before sending it back down.
Example: a -10 °F holiday weekend
The heat goes off over a long holiday weekend. Monday morning the control room reads -10 °F, the line will not run, and a power cycle brings the screen up with an empty project. A reload has the line back by mid-morning.
Work the numbers:
- Storage rating on the datasheet: -22 °F. Converted: (-22 - 32) / 1.8 = -30 °C. That figure covers the panel switched off at -30 °C.
- Room temperature: (-10 - 32) / 1.8 = -23 °C. The room sits 7 °C above the storage limit, which is why the storage line looks like clearance for the weekend.
- Operating floor on a standard panel: 0 °C, which is 32 °F. The powered panel spent three days 23 °C (42 °F) below it.
- Nearest extended-range alternative: a -20 °C (-4 °F) floor still misses by 3 °C (6 °F).
Decision: the weekend ran the panel outside every operating limit on its datasheet, and the storage rating never came into play because the panel ran powered the whole time. The reload was a restore. The cabinet gets heat, and the setpoint decision follows in the next section.
The fix: restore once, then heat the cabinet
Download the project once, using the panel's documented procedure, and let it boot. Repeat power cycles and repeat downloads reset the symptom and erase the evidence. If the download itself stops partway, treat that as a separate fault; a project restore that fails after reboot covers it.
The temperature fix is an enclosure heater with a thermostat. Set it to hold the air at the panel above the operating floor: a 40 °F setpoint over a 32 °F floor gives margin for door openings. Size the heater for the rise it must hold, which is 50 °F between a -10 °F room and a 40 °F cabinet, using the wall loss factor and sizing method the cabinet maker publishes. Give the ceiling the same respect: the panel's upper limit sits at 122 °F (50 °C), so the heater must cycle under thermostat control and the cabinet must stay clear of heat sources that push past it.
Recommendation: if the enclosure can reach the panel's operating floor or below, fit a thermostat-controlled enclosure heater that holds 40 °F at the panel, sized from the cabinet maker's loss calculation for the coldest measured room temperature; if the coldest realistic room temperature stays above 32 °F, skip the heater and put a supply monitor on the panel's feed instead. Either way, treat the reload as temporary until one deliberate shutdown and restart inside the finished cabinet proves the project boots and the PLC link comes back.
File that restart's enclosure temperature and supply readings with the maintenance record. They are the baseline the next cold event gets compared against.
What about the PLC?
Check it from its own datasheet, because the panel's numbers do not transfer. The HMI project and the controller's program and retained registers are separate memories in separate devices with separate operating ranges. Read the controller's operating floor from its manual, then confirm after a cold event that the program is present and the retained values hold. A controller that passes needs nothing else; one that reports faults or lost values after a cold weekend joins the panel in the same temperature review, and its cabinet shares the heater.
When the room never dropped below the limit
If the enclosure measurement stayed above the panel's operating floor through the whole event, the temperature question is closed and the suspects are the supply and the memory. Starts coincide with the day's biggest load switching, so record the panel's supply through three starts before settling on an explanation. A project that disappears on restarts in a warm room is the broader case, and an HMI panel losing its project after a power cycle works through it.
If the panel drops its project a second time while every reading sits inside its ratings, stop reloading it. Repeated downloads destroy the evidence you have been collecting, and a line screen that randomly forgets its project should not run unattended on a production line. Take the panel out of service, and send the maker's support the model, the project revision, and the temperature and supply readings you have on file.
Questions
Will a -22 °F storage rating protect a panel that runs at -10 °F?
No. That rating covers the panel switched off. A standard panel running at -10 °F (-23 °C) sits about 42 °F below a 32 °F operating floor, and the datasheet makes no promises about booting or data retention outside that range. If the machine can be shut down anyway, switching the panel off in a below-floor room is a stopgap that brings the storage rating back into play.
Should the PLC get the same check?
Yes, but from its own datasheet. The HMI project and the PLC program live in separate memories with separate operating ranges, and the numbers do not transfer between them. Read the controller's operating floor from its manual, verify its program and retained values after a cold event, and check the battery as well if retained registers are battery-backed, because battery capacity drops in cold.
What size enclosure heater does a cabinet like this need?
Size it from the cabinet and the temperature rise, and use the sizing method the cabinet maker publishes: exposed wall area multiplied by the wall loss factor multiplied by the rise. For the worked example the rise is 50 °F, from a -10 °F room to a 40 °F setpoint, so add the warm-up load and pick the next standard size above the result. The panel only sets the setpoint.
Is an extended-temperature HMI a substitute for the heater?
Only when its operating floor sits below the lowest temperature the cabinet will actually reach. A panel rated to -20 °C (-4 °F) still misses by 6 °F in a -10 °F room, so match the rating to a measured worst case with margin. Units rated to -40 °C (-40 °F) exist and cover the room, but every other device in the cabinet still needs its own rating confirmed, which is why heating the cabinet usually stays the simpler fix.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.