Enclosure heat exchanger not cooling: why the coils stay room temp

Short answer: room-temperature coils are the normal state of an air-to-air enclosure heat exchanger, and there is no gas inside one to leak. The fins are a sealed plate or heat-pipe core, not an evaporator, so nothing can boil away. If your machine only survives with the doors open, the heat load has outrun the core's capacity in watts per degree. Restore power to both fans, clean the core through its full depth, then re-size with one division: load in watts ÷ allowed rise in °C. For a 500 W example cabinet in a 31 °C (88 °F) shop with 40 °C (104 °F) rated electronics, that is about 56 W/°C, near the top of the usual 7.2 to 71.6 W/°C (4 to 40 W/°F) catalog range.

What that unit actually is

An air-to-air heat exchanger is two air streams and a metal wall between them. An internal fan pulls hot cabinet air across one face of the finned core, an external fan pulls shop air across the other face, and heat passes through the metal while the two airs never mix. Many models do part of that job with heat pipes: sealed tubes holding a fixed charge of working fluid that evaporates at the hot end and condenses at the cool end. The fluid never leaves the welded walls, so there is nothing to top up and nothing that gets used up. You will find no compressor, no expansion device and no service port on one.

That is why the touch test misleads you. The core metal has no reason to run hot like a condenser or cold like an evaporator; it floats somewhere between the two air streams. Coils at room temperature only tell you something when the cabinet air is hot at the same moment. A cool inside face on a hot cabinet means little or no heat is reaching the core.

One look at the nameplate settles the refrigerant question. A plate that lists a refrigerant type and a charge weight belongs to a small air conditioner, and the diagnosis changes: listen for the compressor, feel the discharge line, clean the condenser. A plate that gives capacity in W/°C or W/°F and nothing else is an exchanger, and there is no leak to chase.

Symptoms and what each one points to

• The machine faults after roughly half an hour with the doors shut and runs all day with them open. That is cabinet air overtemperature, and the 30-minute figure is thermal mass running out of margin, not a part dying. Estimating an enclosure's time to overheat shows how the trip time scales with the shortfall.
• Coils and discharge air sit at room temperature while the cabinet interior is hot. Little or no heat is reaching or leaving the core.
• One grille is dead or both fans are silent. The exchanger has no supply or a failed fan, and its capacity collapses to a trickle.
• The fans roar but you feel almost no draft through the fin pack. The core is blocked through its depth, not just on the visible face.
• Trouble arrives only on hot afternoons or when neighbouring machines run. Hot discharge is re-entering the intake, or another fan is blowing warm air at it.
• It ran cool for years and now trips. The load has grown with every drive and power supply added since the exchanger was sized.

Causes, most likely first

1. No power at the exchanger's fans. A blown fuse, a tripped breaker, a dead small transformer or a unit that was never wired at all. Nothing else matches room-temperature coils on a hot cabinet as well as this one.
2. A core blocked through its depth. Machine-shop oil mist and metallic dust pack the passages. Wiping the face behind the fan clears a fraction of the flow.
3. Recirculation. The discharge dumps into a corner, the unit sits hard against a wall, or the machine's own fan bank blows warm air at the intake, so the ambient that counts climbs above shop temperature.
4. Undersized for the present load. The unit works exactly as designed, but its capacity times the available rise is less than the watts going in.
5. A lost refrigerant charge. Set this one aside at the nameplate: an exchanger has no charge to lose.

How to check each cause

Lock out the machine and verify absence of voltage before you touch any wiring in the panel. Then work down the list:

1. Air at both grilles. With the cabinet hot, hold a paper strip at each grille. Both sides must move air. A dead side points at the exchanger's fused supply, its transformer or a stalled motor.
2. Cabinet temperature at the trip. Hang a thermometer near the top of the cabinet and read it when the machine faults. Drives and PLCs commonly carry 40 °C (104 °F) ratings and some datasheets stretch to 55 °C (131 °F), so a 31 °C (88 °F) shop leaves a 9 °C (16 °F) margin at the low rating.
3. Temperature drop across the core. Probe the internal air 50 mm [2 in] before and after the core while the cabinet is hot. A working unit drops several °C across the core. Less than about 2 °C on a hot cabinet means heat is barely moving.
4. Intake temperature, not room temperature. Read the air at the outside intake with the machine running. Every extra kelvin there eats capacity. At a 500 W load, an intake at 36 °C instead of 31 °C cuts the allowed rise from 9 to 4 °C. The capacity you need then climbs from 56 to 125 W/°C.
5. Nameplate against measured watts. Convert W/°F ratings by multiplying by 1.8, so 4 W/°F is 7.2 W/°C. Read your cabinet's input power with a clamp meter; in a control cabinet almost every watt in becomes heat. If capacity × allowed rise sits below the measured watts, the unit is simply too small.

The sizing check that decides it

Capacity comes from one division: required W/°C = heat load in W ÷ allowed rise in °C above the intake air. Take the worked case behind the symptoms: a cabinet shedding an assumed 500 W at idle, intake air at 31 °C (88 °F), drives and PLCs rated to 40 °C (104 °F).

• Allowed rise: 40 - 31 = 9 °C
• Required capacity: 500 ÷ 9 ≈ 56 W/°C

Vendor catalogs rate these units from about 7.2 W/°C (4 W/°F) in small frames to 71.6 W/°C (40 W/°F) in the largest. On this cabinet the two ends of that range give:

The walls lend a hand. Stainless sheet passes roughly 3.7 W/m²K in panel heat calculations, so a cabinet near 1.0 × 1.0 × 0.25 m with about 2.75 m² of exposed skin sheds 3.7 × 2.75 × 9 ≈ 92 W at a 9 K rise. That trims the exchanger's share to 408 W, about 45 W/°C. Size against 56 W/°C anyway; the wall credit shrinks when the box stands in a warm corner or against a warm frame.

An open door hides all of this. A 30 × 30 in [760 × 760 mm] doorway with a 0.4 m/s chimney draft moves about 500 CFM [0.23 m³/s]. At a 9 K rise that is 0.23 × 1.2 × 1005 × 9 ≈ 2.5 kW of free cooling. That is why the door-open test always passes. For the full path from load to cooler size, see the enclosure heat calculation guide.

The fix, in the order the checks point

1. Fans silent or one side dead. Find the exchanger's supply, usually a small fused transformer tap inside the panel, restore it, and test each fan. A motor that hums without turning fails the paper-strip test just like a dead fuse, so replace it.
2. Core blocked. Pull the unit and wash or back-blow the passages from both faces until you can see daylight through them. Warm water and mild detergent handle an oil-misted core; dry it fully before re-fitting. Wiping the visible face is not enough. The enclosure heat exchanger cleaning guide covers what to inspect and how to confirm the airflow came back.
3. Recirculation. Give the discharge open air, pull the unit out of any alcove, and re-aim any fan bank that blows warm air at the intake. Re-measure the intake temperature afterwards and put that number into the sizing formula.
4. Undersized. Buy capacity by the formula, not by cabinet volume. An exchanger can only pull the interior toward the intake temperature. When you need air below shop temperature, a closed-loop air conditioner is the tool, and the fan vs air conditioner vs heat exchanger comparison lays out the choice.

Keep it from coming back

• Add a measured trigger to the maintenance list: log the temperature drop across the core each month and deep-clean when it halves. The drop is the honest signal; a calendar guess cleans too early or too late.
• Keep the doors shut. In a 31 °C humid shop an open door feeds dust and moisture into the cabinet while the chimney effect masks the fault. Closed-loop enclosure cooling where the air carries fumes covers the sealed approach for exactly this air.
• Wire a warning. A thermostat inside the cabinet set at 40 °C (104 °F) gives you weeks of notice instead of a 30-minute shutdown.
• Re-run the sizing check each time you add a drive or a supply. Adding 200 W of load at a 9 K rise asks for another 22 W/°C of capacity.

When to re-specify the enclosure

If the cabinet itself is tired, with leaking gaskets, rust or no wall space for a bigger core, replace it with the end state in mind: a sealed stainless box carrying a gasketed closed-loop cooler. Our 304 and 316L wall-mount and HMI enclosures are made to order in sizes from 6 × 4 × 4 in to 60 × 36 × 16 in (H × W × D). Every cutout lands where your exchanger needs it, and the box is delivered in 10 business days. The boxes are manufacturer-rated NEMA 4X-type / IP66; not UL listed. NEMA 250 defines the Type 4X tests but never certifies boxes, and the rating belongs to the finished assembly. The exchanger's gasketed frame, fitted as its instructions show, is what keeps the box sealed after you cut the wall. Pick the shell on the stainless enclosure size finder.

The decision, by finding:

• Fans silent or one grille dead: restore power or replace the motor; the core is fine.
• Core clean, both fans running, interior still hot: capacity × rise sits below your measured watts, so buy more W/°C.
• Interior air below shop temperature, or air too oily for a washable core: move to closed-loop air conditioning.
• Cabinet leaking, rusting or out of wall space: order a sealed stainless enclosure with the cutout drawn for the exchanger.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions