Heating on but no hot water: how an engineer approaches it

What the customer says

“Heating’s lovely and warm but there’s no hot water. The cylinder’s cold.”

That is a good start. It tells you which zone to look at and that the other one works. It is not a diagnosis. Ask a few short questions before you open anything. Since when? Has anyone changed a setting, fitted a new control or had other work done? Is the hot water set to come on at this time of day?

This page is about a system with a hot water CYLINDER, wired with a programmer, a cylinder thermostat and a motorised valve. A combi boiler makes hot water inside itself, with no wiring from the controls. On a combi, heating but no hot water points inside the boiler, and that is a job for a Gas Safe registered engineer.

See the fault for yourself

Before you touch anything, see it happen. Ask for the hot water on its own: hot water on at the programmer, heating off. Then watch the hot water zone with nothing else running.

Look at the simple things first:

• Is the programmer’s hot water channel actually on, or is it waiting for its next time, on holiday or switched off?
• Is the cylinder thermostat’s dial at a sensible setting?
• Does the hot water valve move? Does the boiler fire?

Many “faults” are settings. Finding that out costs a minute and no parts.

What the working zone already proves

On an S-Plan, the heating zone and the hot water zone share a great deal: the supply, the wiring centre’s supply blocks, the boiler, the pump, and the block where the valves’ orange cores meet the boiler’s switched live.

The heating works. So everything the two zones share works. That narrows the search before you take a single reading.

Write down what works and what does not. On a confusing job, that list is often what finds the fault.

On a Y-Plan the chain is different. Its single valve rests at the hot water port, so for hot water alone the valve does nothing, and the cylinder thermostat’s call fires the boiler itself. The approach below still holds: follow that chain instead.

The hot water path: programmer, cylinder stat, valve

On an S-Plan wired with the common numbering, the hot water DEMAND travels like this:

1. The programmer’s hot water ON (HW ON) terminal.
2. The wiring centre’s block 6, and on to the cylinder thermostat’s common.
3. The cylinder thermostat’s call contact, closed while the water is cool.
4. Block 8, and on to the hot water valve’s brown core, which drives its motor.
5. Once the valve is fully open, its orange core.
6. The block where the orange cores meet, and the boiler’s switched live.

Every link is a wire, a screw or a part. Know the cylinder thermostat’s own map before you test it: which terminal is its common, which is its call. Makers number them differently.

Follow the live to neutral

With the hot water on alone and the cylinder cool, take the readings in the order the demand travels. These are live tests, made only because the fault shows only while the controls work. Set the meter to AC volts (V~). Use test leads made to the HSE’s guidance note GS38: finger guards, metal tips of 4 mm or less, fused leads. Keep the black probe on a neutral: block 2 at the wiring centre, the part’s own neutral at a part.

Each point should read about 230 volts (230 V). The fault lies between the last point with 230 V and the first with 0 V. See last 230 V, first 0 V.

Two traps on this job:

• The block where the orange cores meet is shared. If the heating is calling too, that block reads live from the heating valve. Ask for the hot water alone.
• The cylinder thermostat only calls while the water is cooler than its setting. A cylinder heated by an immersion heater may already be warm enough to stop it calling. Know what state it is in before you read its output.

Prove the part before you condemn it

If the readings stop at a part, do not reach for a new one yet. A valve whose brown reads 230 V to earth but will not move may have no neutral. A thermostat with 230 V in and nothing out may simply be satisfied.

Read across the part’s own live and neutral. Full supply and still nothing happens: then the part has failed. See prove a part with two readings.

Before any repair, isolate the circuit, lock it off and prove it dead, the switched lives included. After the repair, prove it: hot water on alone, the valve opens, the boiler fires, and the cylinder starts to warm. Never leave until you have seen the repair work.