A capillary thermostat is never really bought off a page — it is specified. Two thermostats with the same headline rating can differ in setpoint range, capillary length, bulb, shaft and terminals, and every one of those differences decides whether the part drops into your appliance or fights it. This guide sets out the specification in the order we work through it with equipment makers, and ends with the checklist that turns a specification into a quotation.
1. Setpoint: a range for controls, a trip point for limiters
Start with the temperature the device actually works at. A control is specified by an adjustable range that brackets the working point with room on both sides — accuracy and dial resolution are best away from the ends of the scale, and ranges are configured to order on platforms such as the AG16-K (50–400°C). A limiter is specified by a single fixed trip point, never a range: the AG16-S carries standard points at 380 / 400 / 430°C, and trip points are factory-calibrated to your specification when none of the standards lands in your window. Confusing the two — asking for an adjustable limiter, or a control with one fixed temperature — is the most common specification error we see.
2. Rating, poles and contacts
Convert the heating element’s power to current at your line voltage, then choose a rating with margin — commercial duty should never run a switch at 100% of its rating. The AG16-K covers 16A / 20A at 110V / 250V for most countertop equipment. Heavier loads move to the AG32 family: up to 32A on the single-pole AG32-1K, and the three-pole AG32-3K (16A / 32A / 50A · 110V / 250V / 400V) where a three-phase load must switch all phases together, in one device. Contacts: SPST switches the heater alone; SPDT adds a second circuit for a pilot light or alarm. Gas appliances are a separate case — the control switches a millivolt circuit, not mains, and needs the GAS series (0.67A · DC 5V, 125–300°C) built for that duty.
3. Capillary length
Specify the length along the route the tube will actually take — from the bulb seat, out of the tank or cavity, along the harness path to the panel — plus a hand’s width of service slack. Lengths are configured to order. Too short is worse than too long: a taut capillary feeds every vibration and panel flex into the bulb seal. The measuring and routing rules are covered in full in our installation guide.
4. Bulb: size it to the well
The bulb is the only part that senses, and it is specified by diameter and length against the thermowell it must fill. Fully inserted, in firm metal-to-metal contact, it reads the process; half-seated or undersized, it reads a lagging blend of process and ambient, and the control overshoots and hunts no matter how good the switch is. On tanks, the bulb belongs immersed in the medium or hard against the tank wall — never resting where sediment or scale insulates it.
5. Shaft, terminals and mounting
The last step decides whether the configured thermostat is a drop-in part or a panel redesign. Shaft length and profile, knob format — round, square or custom profile, with the scale printed to match your range and your brand — terminals and mounting are all confirmed against your panel and harness design before samples are made, as set out in our OEM and private label guide.
The checklist: what to send us
Five items turn an enquiry into a configured quotation: (1) the appliance and the device’s job — control or limiter; (2) the working temperature, and the required trip point if it is a limiter; (3) the element power and the supply voltage and phases; (4) the bulb-to-panel distance measured along the real route; (5) the thermowell diameter and depth — or, if you are replacing an existing thermostat, the original part number or a clear photo of its rating label, and we match mounting, ratings and capillary length. With that in hand we confirm the full configuration — ratings, range, capillary, bulb, shaft, terminals and mounting — quote against it, and make configured samples for your bench testing before any production commitment.
Certifications
Our thermostats are produced under a TÜV-certified quality system — certificate R50309054 (EN 60730-1 / EN IEC 60730-2-9), verifiable on Certipedia. Confirm the approvals carried by the specific models you select for the markets you sell into.
FAQ
I only have the old thermostat — can you work from that?
Yes. Send the original part number or a clear photo of the rating label, and we match the mounting, ratings, setpoint range and capillary length of the unit being replaced. A photo of the installed part, showing the bulb and its routing, answers most of the remaining questions.
Do I need a control, a limiter, or both?
Both, doing different jobs. The control regulates temperature in normal service and switches constantly; the limiter is an independent backup that cuts power only on a fault and is manual-reset so the fault gets looked at. Heated commercial equipment normally carries the pair, as separate devices with separate bulbs.
Can one specification cover several appliance models?
Yes — that is the normal OEM pattern. One thermostat family is held constant while ranges, capillary lengths, bulbs and knobs are configured per model. Send us the model matrix with each model’s working temperature and bulb-to-panel distance, and we structure the configurations against it.
Capillary Control Thermostats · Manual Reset Safety Limit Thermostats · OEM Thermostats and Private Labelling