The 01DTS-504 is a Belimo low-temperature detection sensor (freezestat) for protecting water and air heaters in HVAC systems against frost damage to cooling and heating coils. A 10 ft (3 m) vapor-filled copper capillary tube senses the lowest temperature along its length — even a short cold spot from outdoor-air bypass will trip the switch. The 1× SPDT (4 A @ 24 VAC/VDC) changeover output reverses state when the coldest point on the capillary falls below the adjustable setpoint (15 to 60°F, factory-set to 41°F (5°C)). The switch resets automatically when the sensed temperature rises approximately 2°F above the setpoint. Auto reset is preferred where the freezestat is used as a control input (cycling a heating coil) rather than as a hard safety lockout.
Frost protection on hot-water heating coils, chilled-water cooling coils, and any AHU coil exposed to outdoor air through an economizer or mixing damper. The capillary trips on the coldest point along its length — not the average — so a narrow stratified cold pocket near the coil edge is detected immediately. Code-mandated on most commercial AHUs with outdoor-air capability and the first-line interlock to shut off the supply fan, close the OA damper, and call for full heating recovery.
Key Features
If the vapor-filled capillary develops a leak, the switch automatically moves to the FROST (tripped) position rather than failing in the normal-run position. This fail-safe behavior is the reason vapor-filled freezestats remain specified over electronic alternatives in life-safety AHU applications.
The 10 ft capillary trips on the coldest spot along its entire length — not the average. A narrow cold pocket from outdoor-air bypass at one corner of the coil is enough to trip the switch and protect the coil from freezing damage.
Switch returns to normal automatically when the sensed temperature rises approximately 2°F above the setpoint. Use as a cycling control input for heating coil staging or mixed-air low-limit reset.
15 to 60°F setpoint range, factory-set to 41°F (5°C) (the most common AHU freeze-protection setpoint). Field-adjustable via the internal screw on the switch body.
Sealed enclosure suitable for indoor mechanical room mounting. Transparent ABS cover allows visual inspection of switch state and reset button without removing the cover.
Industry-leading factory warranty backed by Belimo's worldwide service network.
Technical Specifications
| Specification | Detail |
|---|---|
| Sensor Style | Low-temperature detection switch (freezestat) with vapor-filled capillary |
| Measured Value | Temperature (coldest point along capillary length) |
| Sensing Method | Vapor-pressure capillary — temperature drop reduces vapor pressure, actuating the switch |
| Output | 1× SPDT (4 A @ 24 VAC/VDC) changeover (single-pole double-throw) |
| Reset | Auto reset — switch returns to normal automatically when temperature rises ~2°F above setpoint |
| Setpoint Range | 15...60°F [-10...15°C], adjustable |
| Factory Setpoint | 41°F (5°C) |
| Switch Accuracy | ±0.9°F (±0.5°C) |
| Capillary Length | 10 ft [3 m] |
| Probe Material | Copper, vapor-filled with R507 refrigerant |
| Application | Air — across coil face for AHU freeze protection |
| Mounting | A-22D-A08 mounting kit (6 brackets, included) for serpentine routing of the capillary across the coil face |
| Cable Entry | Cable gland with strain relief, M16×1.5 mm, for cable ø4–10 mm |
| Housing | Bottom: PA grey; Cover: transparent ABS (allows visual inspection of switch state) |
| Enclosure Rating | NEMA 4 / IP65 (UL Enclosure Type 4) |
| Protection Class | IEC/EN III, Protective Extra-Low Voltage (PELV) |
| Certification | IEC/EN 60730-1, CE Marking |
| Quality Standard | ISO 9001 |
| Ambient Temperature | -30 to 160°F (-35 to 70°C) — must exceed switching point by ≥2 K (do not mount where housing is exposed to outdoor air) |
| Fluid Temperature | -30 to 160°F (-35 to 70°C) |
| Power Required | None — switch is electromechanical (the SPDT is wired into your control circuit power) |
| Weight | 0.89 lb [0.4 kg] |
| Manufacturer | Belimo |
| Manufacturer Part Number | 01DTS-504 |
| Warranty | 5-year factory warranty |
What's In the Box
- 01DTS freezestat with 10 ft (3 m) vapor-filled copper capillary in NEMA 4 housing
- A-22D-A08 mounting kit (6 brackets for serpentine capillary routing across coil face)
Conduit fitting is not included — see the EXT-TTS-M1612 cable conduit connector in Compatible Accessories below if you need to terminate the freezestat into rigid conduit instead of through the M16 cable gland.
Compatible Accessories
Documentation
Frequently Asked Questions
What is a freezestat and what does it do?
A freezestat is a low-temperature safety switch that protects water and steam coils in air handlers from freezing damage. The capillary tube is strung serpentine across the coil face; if any point on the capillary detects a temperature below the setpoint (typically 38–42°F), the switch trips and signals the BAS to shut off the supply fan, close the outdoor-air damper, and call for full heating. Code-mandated on most commercial AHUs with outdoor-air capability.
How does the 01DTS-504 reset after a trip?
Auto reset — the switch automatically returns to the normal position when the sensed temperature rises approximately 2°F above the setpoint. Auto reset is preferred when the freezestat is used as a cycling control input (e.g., cycling a heating coil to maintain low-limit) rather than as a hard safety lockout. For hard-safety freeze protection on critical AHUs, use the 01DTS-50..X manual-reset variant.
How does the SPDT switch wire into my control panel?
The 1× SPDT (single-pole double-throw) provides three wire connections: COM (common), NO (normally open), and NC (normally closed). In normal/run condition (above setpoint), COM is connected to NC. When the switch trips on low temperature, COM moves to NO. Most freeze-protection schemes use the NC contact in series with the supply fan motor starter — when the switch trips, the contact opens, killing fan power. Rated 4 A at 24 VAC/VDC; do not exceed this for line-voltage applications.
What happens if the capillary tube leaks or breaks?
If the capillary loses vapor charge (cut, kinked, or corroded through), the switch automatically moves to the FROST (tripped) position. This fail-safe behavior — fail to safety, not fail to run — is the primary reason vapor-filled capillary freezestats remain specified over solid-state alternatives in life-safety AHU applications. A broken cap tube triggers the same shutdown sequence as a real freeze event, ensuring the coil is protected even with sensor failure.
What is the setpoint range and where is it factory-set?
Adjustable 15...60°F [-10...15°C]. Factory setting is 41°F (5°C) — the most common AHU freeze-protection setpoint. Adjust the setpoint via the screw on the internal switch body; setpoint accuracy is ±0.9°F (±0.5°C).
How long is the capillary and how do I install it?
This SKU has a 10 ft (3 m) capillary. Install by stringing the capillary in a serpentine pattern across the entire face of the coil being protected, using the included A-22D-A08 brackets (6 supplied) to clip it to the coil casing. Aim for one foot of capillary per square foot of coil face area, with the serpentine covering the full coil evenly. Do not kink the cap tube — kinks disable the sensing function on that section.
Where should I mount the housing?
Mount the housing on the AHU casing or in the adjacent mechanical room — never expose the housing to outdoor air. Belimo specifies that the housing ambient must exceed the switching setpoint by at least 2 K. If the housing itself sees outdoor temperatures, the switch can trip even when the coil being protected is warm. Best practice: mount downstream of the coil being monitored, inside the AHU casing or on the protected-side wall.
Why use a vapor-filled capillary freezestat instead of an electronic low-limit sensor?
Three reasons: (1) Fail-safe — capillary leaks trip the switch to FROST, while electronic sensor failures can leave the coil unprotected. (2) Coldest-point detection — the capillary trips on the lowest temperature anywhere along its length, while a single-point sensor reads only one location and can miss stratified cold spots. (3) Power-independent — the switch works mechanically; no 24V needed for the sensing function (only for whatever load the SPDT is switching). Vapor-filled freezestats remain the spec'd device on most life-safety AHU freeze-protection circuits for these reasons.
- Sensor Type:
- Freezestat / Low Temp Detection
- Switch:
- (1)SPDT
- Switch Rating:
- 4 A @ 24 VAC/VDC
- Reset Functionality:
- Auto Reset
- Capillary Length:
- 10'
- Setpoint Range:
- 15 to 60°F adjustable
- Factory Setpoint:
- 41°F (5°C)
- Probe Material:
- Copper, vapor-filled R507
- Output Signal:
- SPDT Switch (no analog signal)
- Temperature Element:
- None (vapor-pressure switch)
- Humidity Element:
- None
- Display:
- No Display
- Setpoint:
- Adjustable Setpoint (15-60F)
- Override:
- No Override
- Supported Protocols:
- None (dry contact)
- Enclosure:
- NEMA 4 Enclosure
