Short answer: find the steam or flue-gas leak first — a fuel conversion is almost never the cause — then stop trying to seal saturated air out and keep the heads above the dew point instead. That means a gasketed stainless box of about 12 x 10 x 6 in [300 x 250 x 150 mm] for two control heads, 15-20 W of anti-condensation heat, a gasketed conduit entry, and a VCI emitter you renew every 12 months. Duct seal and a rubber-gasketed threaded cover will not hold this air out: every cool-down pumps room air through the conduit past the seal. The sections below rank the causes, show you how to test each one, and give the heater arithmetic for the box.
What the moisture pattern tells you
Read the pattern before you open anything; each symptom points at one source.
- Film that forms after shutdown and dries during the run → cool-down condensation. The heads sit below the room's dew point once the kettles stop.
- Water inside a gasketed threaded cover with the openings duct-sealed → the vapor arrives through the conduit with air pumping, not through the gasket.
- A warm or wet streak on the wall or the kettle skirt under the lagging → a steam leak under the insulation and the second skin.
- Fog that starts when the boiler fires → flue-gas spillage; combustion gas carries a heavy load of water vapor.
- White powder and corrosion on terminals only → daily humidity peaks; bare steel corrodes measurably once relative humidity holds above about 60%.
- Moisture worst high on the wall beside the 2 x 2 in [50 x 50 mm] vent → humid air rising and leaking past the opening.
Two of these often run together: the leak loads the air and the cycling moves it into the heads.
Causes in order of likelihood
- A steam leak under the lagging. Burner and steam service is the classic trigger: a fitting repacked and not re-lagged, or lagging disturbed and left open. Condensing steam gives up about 2.26 MJ per kilogram [970 Btu per pound], so one leaking joint seasons a whole corner of the room.
- A flue-gas leak or spillage. Flue gas is wet — the CH4 + 2 O2 → CO2 + 2 H2O arithmetic gives 2.25 kg of water vapor per kg of natural gas burned. A sound stack carries it out; a loose joint or a draft changed by the burner swap spills it into the room.
- Daily cool-down cycling. Not a leak: the kettles run hot all shift, the room cools overnight, and the heads on top of the tank fall below the new dew point. An inverted foam cooler helps because it slows exactly this; a heated enclosure does the same job permanently.
- The LP-to-NG conversion itself. Rarely the cause on its own. Both fuels make wet flue gas and the stack carries it away; the conversion matters only where it added joints, changed the draft or disturbed the venting.
How to check each cause
Work down the list; the first test that comes back positive is your answer.
- Scan under load. With the kettles steaming, walk the piping, the kettle skirt and the wall with a thermal camera. A warm streak marks wet lagging or a joint leaking behind it. One safety note: lock the boiler out before you open insulation, because lagging over a live leak holds scalding water.
- Log the air for a week. Put one logging temperature and humidity sensor inside the head and a second beside it. If the inside one rides 90-100% RH during downtime while the outside one sits lower, vapor is entering and the surfaces are dipping below the dew point. Near-saturated air condenses on anything a degree or two cooler: at 30 C [86 F] and 95% RH the dew point is about 29 C [84 F].
- Watch the vent. Stand at the 2 x 2 in opening during a steam cycle. A visible humid plume means the room air is loaded, which points back to the kettles or the flue.
- Check the flue. With the boiler running, look for spillage at the draft hood and joints — a smoke pencil or a mirror shows it — and confirm the termination sits away from the controls.
Condensation lands on the coldest surface first, and the same dew-point logic that decides when checking pipe condensation against the dew point needs attention before adding trays applies to lagging and second skins.
The fix: stop the leak, then hold the box above the dew point
- Fix the leak. Remake the joint, dry the lagging and seal the second skin. No enclosure survives a live steam plume overhead.
- Re-house the heads. A gasketed stainless wall-mount box: 304 stainless for washdown and food areas, 316L beside chlorides or sanitizer mist. For two control heads, something near 12 x 10 x 6 in [300 x 250 x 150 mm] usually does it; we build custom boxes to your size and cutouts and deliver in 10 business days. Mount it out of the direct plume and away from the vent, and pick the door gasket by temperature and chemical exposure, not by hope — the foam or rubber door gasket comparison covers the choices. The box is rated NEMA 4X / IP66 by the manufacturer; not UL listed.
- Seal the entry. Fit a gasketed gland at every entry and seal the conduit run where it climbs from a wet zone; keeping the NEMA 4X rating through a field-drilled entry covers the details. Rate every accessory the same: one IP54 gland or thermostat drops the whole assembly to IP54, and the kettle corner will find it. Do not add an open drain or louver to relieve the box — the opening sets the assembly rating and lets saturated air walk straight in.
- Heat it, sized by the box. The arithmetic for the 12 x 10 x 6 in stainless box: surface area 2 x (0.305 x 0.254) + 2 x (0.305 x 0.152) + 2 x (0.254 x 0.152) = 0.32 m², and stainless sheds heat at k = 3.7 W/m²K, the value our heat-loss guides use. Holding the walls 10 K above a 30 C room takes P = 3.7 x 0.32 x 10 = 12 W; 15 K of margin takes 18 W. Fit 15-20 W on a thermostat set 10 K above the room's hottest hour — about 40 C for a 30 C corner — and run it through downtime, which is when the condensation forms. Always-on at 15 W costs 0.36 kWh a day. Bigger boxes scale the same way:
| Box (in) | Surface (m²) | Watts for +10 K | Watts for +15 K |
|---|---|---|---|
| 10 x 8 x 6 | 0.24 | 9 | 13 |
| 12 x 10 x 6 | 0.32 | 12 | 18 |
| 16 x 14 x 8 | 0.60 | 22 | 33 |
Stainless at k = 3.7 W/m²K, still indoor air; add a few watts if washdown air or a doorway draft washes the box. The full method sits in the anti-condensation heater sizing article, and the condensation guide works the dew-point side for larger cabinets. 5. Inhibit the metal. Drop a VCI emitter sized to the box volume — the smallest standard size covers this 12 L [0.4 cu ft] space. The vapor plates the metal surfaces so an occasional film cannot corrode the terminals. It protects metal; it does not dry the air, which is why steps 2-4 come first.
Why the threaded cover and the duct seal failed
A rubber gasket stops splashes and hose jets; it cannot stop air pumping. When the steam shuts down, the head cools a few degrees, the trapped air contracts, and outside air is drawn in through the conduit — the easiest path. Next shift it warms, the air expands and part of it is pushed out, and the cycle repeats every shift. Cooling from 40 C to 20 C [104 F to 68 F] shrinks air by about 6%, so each litre of head space swallows roughly 60 mL of room air per shutdown; a year of single shifts moves more than 20 L of saturated air past your terminals. Anti-seize helps the threads, the duct seal is a plug against a pressure difference rather than a vapor barrier, and both leave the pumping untouched. That mechanism is exactly what why a rating cannot stop condensation in a sealed enclosure explains.
Keeping it dry after the fix
- Write the VCI emitter's install date inside the door and renew it every 12 months; maker data gives a 12-24 month service life, with the short end above 40 C and in near-saturated air — both describe this spot.
- Glance at the inside logger weekly. Interior RH topping 90% during downtime means the heater tripped off or a new leak has started.
- Inspect the door gasket at each washdown and replace it once it takes a permanent set.
- Re-scan the lagging after every boiler or kettle service; the last thing touched is where the next leak lives.
- Clean existing corrosion off the terminals, then keep the film away — stopping the water comes before any cleaner or coating, as stopping corrosion by fixing the water first sets out.
When to re-specify the enclosure
Decide by the water source and the duty:
- Leak fixed, heads in light covers now: re-house them in a custom stainless enclosure built to your terminal layout and cutouts from one piece, delivered in 10 business days. 304 for washdown, 316L beside chlorides or sanitizer mist.
- The head is a display: stainless HMI housings take panel PCs from 7 to 23.8 in [178 to 605 mm]; the 10.1 in panel PC HMI housing is the common pick, with the IP65-type front seal with the display installed, manufacturer rated. Keep the same gland-and-heater discipline on the housing.
- Vapor you cannot kill at the tank: move the box to a drier wall a few metres away and run only sensor and actuator cable to the kettle through sealed glands.
- Leak still open: no enclosure helps; move the controls out of the plume and remake the joint first.
Get the leak, the heat and the entry right and a modest 15 W box rides out any amount of washdown humidity.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 10.1" HMI enclosure | 304 stainless | 8.72 x 12.66 x 3.48 in | IP65-type front seal with the display installed, manufacturer rated; not UL listed |
Questions
How often should the VCI emitter be replaced?
Every 12 months beside a steam kettle. Emitter makers quote a 12-24 month service life, with the short end above 40 C and in air that sits near saturation — both true here. Write the install date on the emitter, keep a spare on the shelf, and pick the smallest standard size for a 12 L box. The vapor protects the metal surfaces; it does not dry the air, so it backs up the heater instead of replacing it.
Will silica gel packs keep a control head dry beside a steam kettle?
Only briefly. Silica gel holds roughly 30% of its own weight in water in near-saturated air, so a 50 g pack is spent once it has taken about 15 g — days, not months, at kettle humidity. A 15 W heater runs for years on 0.36 kWh a day and holds the whole box above the dew point. Treat desiccant as a top-up in a box you open daily, never as the fix.
What heater size holds the heads dry during daily downtime?
For a 12 x 10 x 6 in [300 x 250 x 150 mm] stainless box, 12 W holds the walls 10 K above a 30 C room (k = 3.7 W/m²K over 0.32 m²). Fit 15-20 W: always-on at 15 W leaves the walls about 13 K above the room, so a 29 C dew point never reaches them. Set a thermostat about 10 K above the room's hottest hour.
Does the LP to NG burner change cause the condensation?
Almost never on its own. Burning natural gas makes about 2.25 kg of water vapor per kg of fuel and LP about 1.6 kg, so the swap alone shifts flue moisture modestly — and a sound flue carries it out of the building either way. When moisture appears after a burner job, suspect what the job touched: a steam fitting under the lagging or a flue joint that no longer seals.
