A sealed stainless enclosure protects gear from rain and snow, but it does not add heat. If the radio or router inside is rated to 0 degrees C (32 degrees F) and your site sees -20 degrees F, something has to make up the difference. Most of the time that something is the gear's own waste heat, plus a small thermostatic heater for the coldest weeks.
The rule of thumb before anything else: check the manufacturer's minimum operating temperature for every device in the box, not just the one you worry about. Power supplies, batteries and LCDs often have lower limits than the main board, and the weakest device sets the requirement for the whole enclosure.
What you need
- The datasheet minimum operating temperature for each device, including the power supply and any battery
- The power draw of each device in watts, at idle and at full load
- The outside dimensions of the enclosure, to work out surface area
- A small enclosure heater, 20-50 W, with an adjustable thermostat
- Cable glands sized to the cable actually used, plus a spare gland plug
Work out how much heat the box needs
An unheated enclosure in still air loses heat through its surface. A rough steady-state figure for a painted or brushed steel box in wind under 10 mph is 3-5 W per square foot of surface per 10 degrees F of inside-outside difference. A 16 x 12 x 8 in (400 x 300 x 200 mm) wall-mount box has about 7.5 sq ft of surface, so holding the interior 20 degrees F above ambient takes roughly 30-45 W.
Add what the equipment already makes. A router and two radios drawing a combined 25 W contribute most of that as heat. So at -10 degrees F ambient, the same box settles around 10-25 degrees F with no heater at all. That is why some installations survive: the gear heats its own box. It is also why gear that draws 5 W does not survive in the same box.
Size the heater to cover the gap: the difference between what the equipment makes and what the box needs at your coldest design temperature. A 30 W heater with a thermostat set to switch on at 40 degrees F is a common starting point for small stainless boxes.
Choose and place the heater
- Pick a resistive enclosure heater, 20-50 W, with a built-in or separate adjustable thermostat. Sizes and shapes vary, so check its dimensions against the enclosure before ordering.
- Mount it low on a side wall or the floor of the box, never directly under a battery or above the electronics. Warm air rises; you want circulation, not a hot spot on one board.
- Keep the heater clear of the door gasket and any foam insulation by at least an inch (25 mm).
- Feed the heater circuit through its own gland, and fuse it separately from the radio circuit so a heater fault does not take down the link.
- Set the thermostat to switch on around 40 degrees F (5 degrees C) and off around 55 degrees F (13 degrees C). That band keeps the box warm without draining the power system all day.
Seal the enclosure so the heat stays in
Heat lost through leaky glands and a compressed door gasket never reaches the thermostat's setpoint. Stainless wall-mount enclosures with a hinged gasketed door hold heat well once the penetrations are right; the basics are in the condensation guide, and the same sealing discipline applies in reverse for cold.
- Use correctly sized glands for every cable. A gland sized for the actual cable jacket seals; an oversized one leaks air and, in spring, condenses water. The gland-plate guide covers when a plate beats individual holes.
- Plug every unused cutout. An open knockout is an open window in winter.
- Check the door gasket seats evenly along its full length. A hard latching door in freezing weather is often a gasket that has taken a set.
- Consider thin closed-cell foam on the inside walls. Half an inch (12 mm) of insulation roughly halves the heater budget on a small box.
- Skip vents in winter deployments. Every air exchange brings in cold air that the heater must pay for.
Budget the power
The heater becomes the biggest load on a solar or wind-fed site in winter. A 30 W heater cycling 50% of the time draws 360 Wh over a 24-hour day, which can dwarf the radios it protects. Check that against the battery bank and the shortest winter charging day before you commit.
Two ways to cut the load:
- Insulate the box, which lowers the watt-hour cost directly.
- Put the radio on the mountaintop and the router somewhere accessible and warmer, so only the low-draw radio sees the deep cold. That split also cuts the solar budget, since you size the wire run instead of the heater.
Checks when the cold arrives
- On the coldest forecast night, log the enclosure interior temperature, not just ambient. A cheap sensor inside the box confirms the thermostat band and the heater duty cycle.
- Watch for packet loss or reboots starting when the inside temperature approaches the lowest device rating. That is the signature of a heater that is undersized or has failed.
- Feel the door and lower walls early in the morning. If they are only a degree or two above ambient and frost forms inside, the heater is losing the fight and the insulation or wattage must go up.
Mistakes that cost the rating
- Drilling the heater's mounting holes without sealing them. Every hole needs a gland, a plug or a sealed fastener, or the box is no longer hose-water tight; the manufacturer rating for the enclosure assumes its penetrations are sealed as designed.
- Mounting the heater against a battery. Cold already hurts battery capacity; local heating accelerates it further.
- Running a heater with no thermostat. It wastes the power budget and dries out nothing you needed dried.
- Trusting the enclosure's rating to imply a temperature rating. NEMA 4X / IP66 describes water and dust ingress, not the minimum temperature the contents will tolerate; ours are manufacturer-rated, not UL listed, and the cold limit comes from your devices, not the box.
What to put on the order
Specify the enclosure with the gland layout you have drawn: one gland per cable, sized to the cable, plus a plugged spare for the heater feed. If the heater is a bolt-on plate or bracket, put its cutout or mounting pattern on the drawing. For deep-cold sites, order the box slightly deeper than the electronics need so insulation and the heater fit without crowding the backplate, and check the size against the sizing guide before you commit. If you are unsure whether the layout holds the rating, the custom enclosure page walks through what to send with the drawing.
Keep the condensation guide and the heat calculation guide as your references for the other two seasons. A box that is sealed for cold, heated to a thermostat band, and glanded to the cable is the same box that stays dry in July.
Questions
Can electronics keep themselves warm in a sealed outdoor enclosure?
Sometimes. A router or small switch drawing 15-30 W acts like a small heater, and a well-sealed, insulated box can hold the inside air 10-20 degrees F above ambient at steady state. That works down to roughly 0 to -20 degrees F. Below that, or with low-draw gear, you need a dedicated heater with a thermostat.
What size heater do I need for a small stainless enclosure?
For a 16 x 12 x 8 in (400 x 300 x 200 mm) steel box, a 20-30 W heater with a thermostat usually holds the interior above 32 degrees F at -20 degrees F ambient. Add about 10 W per additional 25% of surface area, and add the equipment's own dissipation to the budget. Undersizing by half is common; oversizing only costs a little standby power.
Does a heater cause condensation in a sealed enclosure?
A thermostat-controlled heater actually reduces condensation, because it keeps the enclosure's interior surfaces above the dew point of the trapped air. The condensation problems come from heaters that run without a thermostat, or from enclosures that breathe humid air through leaky glands. Pair the heater with good glands and, if in doubt, a small desiccant pack.
Do I need a vent in an unheated winter enclosure?
Usually no. A vent trades cold air exchange for pressure equalization, and in winter the cold air costs you heat. A sealed box with proper glands and a thermostatic heater holds temperature better. If solar gain in summer drives large pressure swings, use a membrane vent and recalculate the winter heat budget with the vent's leakage included.
