Immersion heater length vs thermostat length: why the numbers differ

An 11-inch immersion heater does not automatically need an 11-inch thermostat. The two lengths describe different components. Tesla’s current TIH540 product reference, for example, lists an 11-inch element with a 7-inch thermostat. Choosing a spare by copying the biggest number on the heater listing can therefore lead to the wrong part.

Use the heater and cylinder references to identify the specified replacement. If your installer has already specified an item, these are the exact APM pages for the Tesla 11-inch Incoloy/copper immersion heater with thermostat and the separate Tesla thermostat listed as 11-inch, 50–80°C. These links are not a recommendation to pair the two items. Confirm the actual reference, supplied contents and required thermostat before ordering.

Two numbers, two components

The heater assembly and the thermostat are separately identified in a manufacturer’s selection information. The heater’s nominal element length is not a measurement of the replacement thermostat’s sensing rod. The cylinder boss or thread is a third specification; it cannot be inferred from either length.

That example is a reason to check the exact model, not a rule that every 11-inch heater takes any 7-inch thermostat. Even a matching nominal rod length does not establish the correct contacts, safety cut-out, pocket fit or permitted cylinder application.

Read across the exact model row

Tesla’s current Incoloy-with-copper-pocket page gives several different combinations. Three examples show why the model reference matters:

• TIH540: 11-inch element with a 7-inch thermostat.
• TIH547: 18-inch element with an 11-inch thermostat.
• TIH550: 27-inch element with an 18-inch thermostat.

These are manufacturer examples from one product family. They do not establish the correct replacement for another make, a different heater revision or every item with a similar title. Read the instructions and parts information for the actual equipment; where references or published documents disagree, resolve the exact revision before buying.

Are you replacing the heater or just the thermostat?

A complete heater assembly may be supplied with a thermostat, but the exact pack and model determine what is included. Record the full heater reference and check the cylinder’s required boss, element, material and power specification with the installer.

A replacement thermostat must be specified for the heater and cylinder. The installer needs the part reference, sensing-rod and pocket requirements, contacts and rating, retention arrangement, operating range and required over-temperature protection. Similar colour or appearance is not enough.

If the existing heater came with a thermostat, do not assume that buying a longer rod is an upgrade. Ask for the specified replacement. Likewise, a thermostat listing does not mean you are buying an element, a cylinder control kit or every safety component required by the installation.

What to send before ordering

A useful enquiry starts with identification, not dismantling. Send safely visible labels or information already recorded by your installer:

• Cylinder manufacturer, model and serial number.
• Heater manufacturer and complete part reference.
• Existing thermostat reference, where safely available.
• Whether the installer has specified a complete heater or a thermostat-only replacement.
• The exact replacement reference or measurements supplied by the competent installer, plus the quantity and postcode or collection preference.

Do not remove electrical covers or pull out a thermostat to take a photograph or measure it. Electrical and hot-water replacement work needs a competent person. Repeated overheating or operation of a safety cut-out needs investigation; do not bypass a safety control or choose a replacement from the symptom alone.

A copyable parts enquiry

“I need a [complete immersion heater / replacement thermostat]. The cylinder is [make/model] and the heater reference is [reference]. My installer has specified [exact part and length]. Please confirm the supplied model, contents and availability for [quantity], for [postcode / Acton collection]. Photos of the safely visible labels are attached.”

Call 020 8702 8080 or WhatsApp the shop; alternatively call 07737 263641 or WhatsApp the company mobile. You can also use our contact page. Confirm the exact part and collection arrangements before travelling.

Source: Tesla’s Incoloy-with-copper-thermostat-pocket model reference, checked 3 November 2026. Manufacturer examples establish the listed combinations; they do not certify the contents, condition or suitability of an individual supplied item.