No Hot Water from the Boiler: Step-by-Step Diagnosis for Combi, System and Regular Boilers

Quick Answer: No hot water from a UK domestic boiler points to one of five common faults: diverter valve failure (combi boilers — heating works, hot water doesn't), faulty flow sensor or temperature sensor, blocked or seized DHW (domestic hot water) plate heat exchanger, faulty cylinder thermostat (system / regular boilers), or programmer / time clock fault. Diagnosis follows boiler type — combi, system or regular. Always start by confirming the boiler has power, gas, and adequate system pressure (1.0-1.5 bar cold for combi/system) before investigating component faults. Gas Safe registration is mandatory for any boiler component work.

Summary

"No hot water" is one of the top three reasons a homeowner calls a heating engineer. The diagnosis depends entirely on boiler type — a combi boiler that heats radiators but won't heat the tap is a different problem from a system boiler with cold cylinder. Quote stage on a fault call: ask what TYPE of boiler, what symptoms (heating working? cold water working? any error code displayed?), and how old the boiler is (a 12-year-old boiler near end-of-life leans toward replacement; a 4-year-old boiler is worth diagnostic time).

The three boiler types behave differently:

This guide gives a structured fault tree for each. The principle: cheapest checks first (power, gas, pressure, programmer), most expensive checks last (component swaps inside the boiler).

Key Facts

Decision Tree: First Steps for All Boilers

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                        START
                          |
               Hot tap turned on, no hot water
                          |
              Is the boiler displaying an error?
              /                              \
            YES                              NO
            |                                 |
   Note error code,                Check: heating working?
   look up specific                  /                  \
   code in section                YES                   NO
   below                          |                     |
                              Combi:                 General system
                          DHW-side fault             fault — check power,
                          (diverter, exchanger,      gas, pressure first
                          flow sensor)
                          
                          System/Regular:
                          Cylinder-side
                          (thermostat,
                          valve, pump)

Detailed Guidance

Combi Boilers: Diagnosis Sequence

Step 1 — Confirm fundamentals:

Step 2 — Open hot tap and listen:

Step 3 — Check heating side for cross-reference:

Step 4 — Common combi DHW faults:

Diverter valve seized closed (heating only)

Plate heat exchanger blocked/scaled

Flow sensor fault

DHW thermistor fault

Pump fault (combi internal)

System Boilers: Diagnosis Sequence

System boilers heat a separate cylinder. The fault path adds cylinder-side components.

Step 1 — Confirm boiler operation:

Step 2 — Check cylinder side:

Step 3 — System-boiler-specific faults:

Cylinder thermostat failed (open circuit)

Motorised valve failed (Y-plan or S-plan)

Programmer / time clock failure

Pump failure (in heating circuit)

Cylinder coil scaling

Regular (Heat-Only) Boilers: Same As System, Plus Cold-Feed Issues

Regular boilers add the loft cold-feed cistern complexity. Fault path is essentially the same as system boilers, with these additions:

Cold-feed cistern empty or low

Air lock in primary circuit

Reading Error Codes

Most modern boilers display error codes. Common patterns:

Combi boilers (typical brand interpretations):

System boilers generally use the same codes as combi, but cylinder-side faults may appear differently — sometimes as "no demand" rather than "no flame."

Brand-specific codes vary considerably. Always look up the specific brand and model for accurate interpretation.

Programmer / Wireless Stat Issues

A frequently misdiagnosed cause of "no hot water" is the programmer or wireless thermostat:

Always check the programmer first on system / regular boiler faults. £20 of investigation can save £200 of unnecessary part replacement.

When to Recommend Replacement

A 60-minute diagnostic visit on an old boiler often reveals:

Recommendation criteria:

Programme on a Typical Fault Call

For a combi boiler "no hot water" call:

Typical first-visit fee: £80-£140 diagnostic. Parts and additional labour added.

Frequently Asked Questions

My combi boiler heating works fine but no hot water — what's most likely?

The diverter valve. It's stuck in the heating position, so when the hot tap demands DHW, the diverter doesn't switch. Most common combi fault on boilers 5-15 years old. Replacement £180-£350.

Why does my hot water start hot then go cold?

Two common causes. First, low DHW flow rate from limescale in the plate heat exchanger — boiler can't keep up with demand. Second, an oversized hot water demand for the boiler's output (multiple taps open, low boiler kW rating) — boiler can't sustain output. First case is fixed by descale/replacement; second case is a system sizing issue.

Can I check the diverter valve without replacing it?

A diagnostic test: with the boiler on, manually move the diverter valve lever (or operate the motorised override) — if water flows correctly with the lever moved, the actuator/motor is at fault. If water still doesn't flow, the valve body is seized. Either way, replacement is usually the answer.

Why does the boiler keep losing pressure — and the hot water gets cold when this happens?

Pressure loss usually means a leak — visible (wet patch on floor) or hidden (corroded radiator, leaking pipework, leaking automatic air vent). When pressure drops below 0.5 bar, the boiler shuts down, including DHW. Find and fix the leak; refill to 1.0-1.5 bar.

Does servicing prevent these faults?

Yes — annual servicing identifies aging components before they fail completely, descales the plate heat exchanger before it's fully blocked, and replaces wear parts (sealing washers, gaskets) before they leak. £80-£150 annual service. Reduces emergency call-outs significantly.

Regulations & Standards