Last 230 V, first 0 V: the rule that finds a fault with three readings

A demand travels from point to point

When the heating is timed on and the room thermostat calls, 230 V travels along a chain. On an S-Plan it goes like this:

1. The programmer’s central heating ON (CH ON) terminal.
2. The wiring centre’s block 4, and on to the room thermostat’s live in.
3. The room thermostat’s switched live out, and block 5.
4. The heating valve’s brown core, which drives its motor.
5. Once the valve is fully open, its orange core.
6. Block 10, where the orange cores meet.
7. The boiler’s switched live, which fires the burner.

That chain is the heating DEMAND. Every link in it is a wire, a screw or a part. Between any two points there is nothing else to blame.

That is why THE RULE works on every system. A Y-Plan has a different chain, and a combi a much shorter one: the supply, the timer and thermostat, the boiler’s room-thermostat terminals. The rule does not change. Only the voltage can: where a boiler runs its thermostat input at an extra-low voltage, such as 24 V, read its manual for what each point should show.

Confirm the fault before you pick up the meter

See the fault for yourself first. The customer’s words tell you where to look. They are not a diagnosis.

Ask for one thing at a time. Set the heating on its own, then the hot water on its own. Watch what moves, what clicks and what fires.

Note what works as carefully as what does not. On an S-Plan, a hot water zone that works has already proved the supply, the boiler and the pump. So a heating fault is on the heating chain, and you know which chain to trace.

Keep the black probe on a neutral

Set the meter to AC volts (V~). Put the black probe on a neutral and leave it there. Use block 2 when you read at the wiring centre, and a part’s own neutral when you read at a part. Only the red probe moves.

Why a neutral and not earth? A reading to earth shows that a live has arrived. It does not show that the part has the neutral it needs to work. A reading across a part’s own live and neutral is the voltage the part actually gets. See prove a part with two readings.

The trace, and how to read it

With the demand set, read each point in the order the demand reaches it. A working chain reads about 230 V at every point.

A broken chain changes once, from 230 V to 0 V. Say block 5 reads 230 V and the heating valve’s brown reads 0 V. The last point with 230 V is block 5. The first point with 0 V is the brown. Between them is the brown core from block 5 to the valve, and its two screws. Nothing else.

Now you know where to look, and you have not touched a single part.

Between two points there is only a wire, a screw or a part

Once the gap is found, stop the live testing. Isolate the circuit, lock it off and prove it dead. Then look at what lies between the two points: both ends of the wire, both screws, and the core between them. A CONTINUITY test, made dead, beeps along a good wire and stays silent across a broken one. Never test continuity on a live circuit.

If the gap spans a part, it has 230 V going in and 0 V coming out at a moment when it should be sending the live on. Check first that it should be: is the thermostat actually calling, is the channel actually on? Then prove the part before you condemn it.

Why three readings beat testing every part

You do not have to start at one end. On a long chain, read the middle. If the middle reads 230 V, the fault is further on. If it reads 0 V, it is before. Each reading halves what is left.

Often three readings are enough: one where the demand arrives, one where it should leave a part, one where it should arrive next. Compare that with changing parts on a hunch. A new valve on a broken wire still does not open, and the customer pays for both.

What a reading can hide

On site, readings are not always a clean 230 V or 0 V.

• A wire joined to nothing can show tens of volts on a digital meter, picked up from the cables beside it.
• A neutral can break on the far side of a part. Then the part’s own neutral can read 230 V to earth, through the part itself. The part has a live but cannot work.

So 0 V on a meter never proves a circuit dead: only a proved two-pole tester, after isolation, does that. And an odd reading needs thought before it needs a new part.

Isolate before you touch anything

These are LIVE TESTS. Regulation 14 of the Electricity at Work Regulations 1989 allows work on or near live conductors only where it is unreasonable for them to be dead, it is reasonable to work live, and suitable precautions are taken. The HSE’s guidance note GS38 counts testing and fault finding on live equipment as live working. Tracing a demand can meet that test: a demand only exists while the switches work.

So test live only to find the fault, with leads made to GS38: finger guards, metal tips of 4 mm or less (2 mm where practicable), fused leads. Keep your fingers behind the guards and the probes on only as long as the reading takes. Before any repair, isolate, lock off and prove dead with a proved two-pole tester. See safe isolation in five steps.