JUMO frostTHERM-AT/-DR Frost Protection Thermostat for Air Handling, Type 604100

A frost protection thermostat with a long averaging probe line

The JUMO frostTHERM-AT/-DR (type 604100) monitors temperature in ventilation and air-conditioning systems to prevent frost damage, for example at heating or cooling coils. It has a low switching differential and high reproducibility. Reset is automatic for the safety temperature monitor (STW) and manual for the safety temperature limiter (STB). The surface-mounted frostTHERM-AT suits wall and pipe mounting, while the frostTHERM-DR has a special case bottom for TH35 rail to DIN EN 60715 or wall mounting in control cabinets.

The probe is a flexible capillary line that senses along its length: if the line falls below the setpoint over 15 cm of a 3 m line, 30 cm of a 6 m line or 40 cm of a 12 m line, the changeover contact operates. PUSH IN terminals let the installer insert wires or ferrules to the stop without a tool, and the contacts carry up to 16 A at 230 V AC, so the thermostat can often switch a fan or damper circuit directly without a contactor.

Technical specifications

Order code, probe lengths and stocked versions

The order code is 604100 / function – design – range – probe line – probe diameter – material / extra code. Function 20 is the STW and 70 the STB; design 5 is the surface-mounted thermostat (standard) and design 7 the DIN rail thermostat with probe line; control range 140 is -10 to +15 degC; probe line options are 1800, 3000, 6000 and 12000 mm; probe diameter is 0 (none) or 9.5 mm; probe material 40 is copper. Extra code 000 is none and 404 selects IP65. The example in the datasheet is 604100 / 20 – 5 – 140 – 3000 – 0 – 40 / 404.

Brackets for fixing the probe line are available as accessories in sets of six (00558730) or three (00558731).

Safety behaviour and installation in air-handling units

If the measuring system is damaged and the expansion fluid escapes, the diaphragm pressure drops and circuit 1-2 opens permanently while 1-4 closes, which drives the system to its safe state. The STB locks automatically on a temperature drop and can only be unlocked manually once the temperature has risen again. The setpoint is adjusted with a screwdriver after the case lid has been removed, and can be checked through the inspection window.

Route the probe line in a meander across the coil face and secure it with the bracket sets, keeping the minimum bending radius of 20 mm. The ambient temperature at the switching head must stay at least 2 degC above the selected switching point. Cable entry on the surface-mounted version uses an M20 x 1.5 gland, while the DIN-rail version connects directly to the PUSH IN terminals. The datasheet notes that certificates for approved device versions can be downloaded from the manufacturer website.

Environmental information is printed for the refrigerant filling: the capillary system contains F-gas R507 with a global warming potential of 3985, and the filling weight rises with probe length from 0.20 g for 1500 mm to 1.43 g for 12,000 mm, equal to 0.797 to 5.699 kg CO2 equivalent. The probe line diameter is about 2.4 mm, or 9.5 mm for the 1.8 m version, and the line must not exceed 120 degC. The admissible storage temperature is -30 to +80 degC, and the factory switching point is 5 degC.

Documents and quotation

The JUMO data sheet 604100 (V3.01, 2026-02-12) is available below. Seven Star LLC supplies JUMO thermostats from Muscat; tell us the function, design, probe length and extra codes and we will prepare a quotation. Further temperature switching devices are listed under https://omansevenstar.com/product-category/sensors-transmitters/temperature/.