Immersion Heater Elements — Fault Finding, Testing & Readings
This guide covers how to fault find an immersion heater element on a "no hot water" or "keeps tripping" complaint — what order to test things in, what readings mean, and what to look out for physically. It applies to any immersion element on any hot water cylinder (vented or unvented).
Scope Note — Electrical vs Plumbing
- Electrician's scope: safe isolation, diagnosing the stat and element electrically, confirming whether the fault is the element, the stat, or the wiring, and reporting findings.
- Physically removing/replacing the element itself requires draining the cylinder down. On an unvented cylinder this is plumbing work and should be done by someone G3 qualified (Building Regs Part G) — it also needs re-commissioning afterwards (pressure and temperature checks, safety valve tests). Don't attempt to pull an element from an unvented cylinder as an electrician.
- On a vented cylinder (fed from a cold water tank) draining down is simpler, but confirm company policy on whether this is done in-house or handed to a plumber before starting.
Step-by-Step Fault Finding Order
1. Safe Isolation
Isolate the supply and prove dead with an approved voltage indicator before opening the immersion terminal box lid.
2. Visual Inspection First
- Corrosion or staining on the terminals — greening on brass spade terminals usually means electrolysis/moisture ingress over time.
- Damp or staining around the boss/flange — sign of a leak at the gasket or thread.
- Condition of the flex/tails and grommet.
3. Check the Thermal Overload (Manual Reset High-Limit Stat) Before Testing the Element
This is the step most often skipped, and it changes what the rest of the test means.
Most immersion heaters have a combined thermostat/energy cut-out (ECO), or a separate high-limit safety stat, with a manual reset button (often a small button or pin under a cap on the stat body — it does not reset itself).
- Check whether it has tripped before touching anything else.
- If it has tripped, think about why before just resetting it and walking away:
- Genuine one-off nuisance trip (rare) — reset and monitor.
- Element starting to fail — scale build-up or an internal fault can create a local hot spot on the sheath that trips the safety stat well before the bulk water is actually overheated. If it keeps re-tripping, suspect the element, not just the stat.
- Main working thermostat stuck/failed closed — lets the water overheat and the safety cut-out does its job correctly. Here the stat is fine and the element may be fine too — the working thermostat is the actual fault.
- If it trips again shortly after being reset, that's a real fault — don't just keep resetting it.
4. Isolate the Stat from the Element Before Testing Resistance
Disconnect the leads between the stat and the element terminals before putting a meter across the element. If the stat is left wired in, you can read through a parallel path and get a false result.
5. Test Element Resistance (Ohms) Across the Two Element Terminals
Set the multimeter to the Ω (resistance) range and probe across the element's two spade terminals.
Expected resistance is roughly R = V² ÷ P at 240V:
| Element rating | Expected resistance (approx., 240V) |
|---|---|
| 1 kW | ~57 Ω |
| 2 kW | ~29 Ω |
| 3 kW | ~19 Ω |
| 3.6 kW | ~16 Ω |
| 4.5 kW | ~13 Ω |
Reading interpretation:
- In the expected range for its rating → winding intact, element is healthy.
- Very high — hundreds of kΩ up into MΩ, or "OL"/open circuit → winding has broken internally. Condemn the element.
- At or near 0 Ω → internal short. Condemn the element.
Real worked examples (Gledhill StainlessLite+ PLUDR250, 3kW Incoloy 14" elements):
- Bottom element (no hot water complaint): 6.83 MΩ — open circuit, confirmed failed, condemned.
- Top element (tested for comparison, left in service): 19.6 Ω — matches the ~19.2 Ω expected for a 3kW element almost exactly. Confirmed healthy.
6. Test Element to Earth (Insulation Check)
With the element still isolated, probe from one terminal to the metal sheath/earth.
- Healthy = effectively infinite resistance / no continuity.
- Any continuity or low reading = insulation breakdown inside the sheath — often a pinhole or crack letting water into the element. This is frequently the real cause behind an RCD that's been "nuisance tripping," not an actual nuisance at all. Condemn the element.
7. Record and Report
Note both readings (element resistance + insulation to earth) with photos in the job file. If the element or stat is condemned on an unvented cylinder, flag to the office/customer that a G3 plumber is needed for the physical swap and re-commissioning — this is not a same-visit electrical fix.
Physical Signs Worth Checking, Even If It Tests Healthy
- Pitting, bulging, or visible scale on the sheath if the element is ever pulled.
- Elements in hard water areas fail younger — scale insulates the sheath, makes it run hotter, and it effectively cooks itself over time, often showing up first as repeated stat trips before it fully fails electrically.
- Worth flagging a scale reducer/inhibitor to the customer on any cylinder with a history of repeat element failures.
Quick Reference Summary
- Test the thermal overload stat first. Repeated re-trips after reset point to the element or the main thermostat, not just the stat.
- Element ohms in the expected range for its wattage = healthy.
- Element ohms very high (MΩ / OL) = open circuit, condemn.
- Element ohms near 0 Ω = shorted, condemn.
- Continuity from element terminal to earth = insulation breakdown, condemn — often the real cause of RCD trips blamed on other things.
- Electrician's job stops at diagnosis and isolation on unvented cylinders — the physical element swap needs a G3 plumber and re-commissioning afterwards.
Reference
- Gledhill technical support (StainlessLite+ range manufacturer, Blackpool): 01253 474550