Hot Water Not Heating on Combi Boiler: Diverter Valve Test, Flow Sensor, Heat Exchanger Scaling and DHW Thermistor Diagnosis

Quick Answer: When a combi boiler produces no hot water but heating works, the most likely causes are: failed diverter valve (stuck in heating position), blocked or scaled DHW heat exchanger, failed DHW flow sensor (flowmeter), or failed DHW thermistor. Diagnose by checking the boiler fault code, testing diverter valve operation (motor resistance and mechanical movement), measuring flow sensor resistance at rest and in flow, and checking thermistor resistance at known temperature. Hard water areas (above 200 ppm TDS) require annual scale inhibitor dosing to prevent premature heat exchanger failure.

Summary

A combination (combi) boiler that heats radiators but produces no hot water presents a specific diagnostic challenge. The boiler's heating circuit is functioning — confirming that gas supply, ignition, burner, and primary heat exchanger are all working — but the domestic hot water (DHW) function has failed. The fault is in the DHW-specific components.

This is one of the most common boiler faults in the UK, where combi boilers account for approximately 70% of domestic heating installations. The diverter valve, which switches primary hot water flow between the heating circuit and the DHW heat exchanger, is the single most commonly replaced component on combi boilers after 5–10 years. DHW heat exchanger scaling is the dominant cause of poor hot water performance in hard water areas — particularly in the South East, where water hardness regularly exceeds 400 ppm.

Gas Safe registration is required for any work involving the gas valve, burner, or gas-carrying components. The diagnostic procedures described here (diverter valve, flow sensor, thermistor, and heat exchanger) are on the water/control side of the boiler and can be carried out by a qualified heating engineer — but in practice, most boiler diagnostics and repairs are completed by Gas Safe registered engineers who will also ensure the combustion side is safe.

Key Facts

Diagnostic Decision Tree

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COMBI BOILER — HOT WATER NOT HEATING (heating works)
        |
        v
Does the boiler show a fault code?
    YES → note code; refer to manufacturer fault code table
    NO  → continue manual diagnosis
        |
        v
Open a hot water tap — does the boiler fire in DHW mode?
    YES (fires but water stays cold) → goto HEAT EXCHANGER / THERMISTOR
    NO (boiler does not fire at all) → goto FLOW SENSOR / DIVERTER
        |
        +----- FLOW SENSOR / DIVERTER -------+
        |                                     |
        v                                     |
Does the flow LED on the PCB illuminate      |
when tap is opened? (if visible)             |
    YES → PCB receiving flow signal          |
          → fault is diverter or CH side     |
    NO  → flow sensor not signalling         |
          → check minimum flow rate:         |
            test flow rate at tap (litre/30s)|
            < 2 l/min → fit aerator or       |
            increase mains pressure          |
            > 2 l/min → flow sensor failed   |
            → check sensor resistance,       |
            replace sensor                   |
        |                                    |
        v                                    |
DIVERTER VALVE TEST:                         |
1. Isolate boiler (gas and electric)         |
2. Access diverter valve (usually in         |
   hydraulic block or manifold)              |
3. Disconnect motor; check resistance:       |
   typically 500–2,000 ohms on motor coil    |
   (check service manual for exact value)    |
4. High resistance or open circuit →         |
   motor failed; replace motor or assembly   |
5. Correct resistance → motor OK →          |
   manual actuate the valve cartridge:       |
   sticky/stiff → descale or replace         |
   cartridge                                 |
                                             |
        +----- HEAT EXCHANGER / THERMISTOR --+
        |
        v
Boiler fires in DHW mode — water temperature outcome:
    Water is tepid (below 35°C) → scaled DHW heat exchanger
    Water is initially hot then goes cold → partially blocked HX
    Water is correct temperature but fault code present → thermistor suspect
        |
        v
DHW THERMISTOR TEST:
1. Isolate boiler (electric isolation only)
2. Disconnect thermistor — usually a 2-wire clip connector on the DHW outlet
3. Measure resistance at known water temperature:
   At 25°C → expect approx 10 kΩ (varies; check service data)
   At 60°C → expect approx 2.5 kΩ
   At 80°C → expect approx 1.5 kΩ
4. If resistance does not change with temperature or is open/short circuit
   → replace thermistor (usually £8–20)
        |
        v
HEAT EXCHANGER SCALING TEST:
Flow rate test: run DHW tap fully, measure flow rate over 30 seconds
    < 4 l/min at good mains pressure → likely partial blockage
    Good flow but poor temperature → scale insulating HX plates
Options:
    Scale flush (MagnaClean descaler, Fernox F3) through HX — specialist tool
    Replace DHW heat exchanger (see manufacturer instructions)

Detailed Guidance

Diverter Valve — Operation and Failure Modes

The diverter valve is an electrically-actuated 3-way or 2-way valve in the boiler's hydraulic circuit. In 'rest' position (heating demand only), it routes primary water through the CH circuit. When a hot water tap is opened and the flow sensor signals DHW demand to the PCB, the PCB activates the diverter valve motor to switch primary flow through the DHW heat exchanger.

Failure modes:

  1. Stuck in CH position (most common): The valve cartridge is corroded, scaled, or worn and no longer moves when the motor operates. Symptom: heating works, DHW does not fire, no fault code. Diagnosis: motor resistance in spec but valve does not move mechanically.
  2. Stuck in DHW position: Radiators don't heat up fully; hot water works. Less common.
  3. Motor failure: Motor coil open circuit or high resistance. No movement even when PCB signals correctly. Symptom: fault code on some boilers (motor current fault).
  4. Partial movement: Valve moves partially but doesn't fully switch. Results in poor heating performance AND poor hot water — mixture of both circuits.

Repair options:

Flow Sensor (Flowmeter) Testing

The DHW flow sensor detects water flowing in the cold supply to the DHW heat exchanger. Most modern combis use a Hall-effect turbine sensor: a small paddle wheel spins in the flow, interrupting a magnetic field detected by a Hall-effect sensor, generating pulses that the PCB counts.

Testing the flow sensor:

  1. Check minimum flow rate: measure l/min at the tap. Below 2 l/min (varies by boiler — check manufacturer spec), the sensor may not generate enough pulses to trigger DHW mode. If flow is adequate but boiler doesn't fire, proceed to electrical test.
  2. Disconnect sensor connector; measure sensor voltage: on a powered-up boiler (pre-DHW mode), supply voltage should be 5V DC (or 12V on some models) to the sensor.
  3. Open tap slowly and watch for voltage pulses at the sensor output terminal (3rd wire on a 3-wire Hall sensor). Absence of pulses with good supply voltage → sensor turbine is stuck (debris) or Hall effect chip failed.
  4. Remove sensor; check turbine spins freely; clean of debris; retest.

DHW Heat Exchanger Scaling

In hard water areas, calcium carbonate (limescale) deposits on the internal surfaces of the DHW heat exchanger (a plate heat exchanger or coaxial heat exchanger, depending on boiler type). Scale is a thermal insulator — a 1mm scale layer can reduce heat transfer efficiency by 20–30%.

Symptoms of scaling:

Scale inhibition:

DHW heat exchanger descaling:

DHW Thermistor Diagnosis

The DHW NTC thermistor monitors the temperature of the water leaving the DHW heat exchanger. The PCB uses this to modulate the boiler output and prevent scalding (DHW target is typically 55–60°C). A failed thermistor will cause:

NTC (Negative Temperature Coefficient) thermistors follow a predictable resistance-temperature curve. Common domestic boiler thermistor resistance values:

Temperature Approximate Resistance
10°C 20–25 kΩ
25°C 10–12 kΩ
40°C 5–6 kΩ
60°C 2–3 kΩ
80°C 1–1.5 kΩ

Test by disconnecting the thermistor and measuring resistance with a multimeter while the pipe is at a known temperature (use an infrared thermometer on the pipe surface). Compare to the manufacturer's resistance table. Significant deviation or open/short circuit → replace thermistor.

Frequently Asked Questions

The boiler fires for DHW but the water is only warm, not hot — what's the issue?

Likely causes in priority order: (1) DHW heat exchanger partially scaled — primary water cannot transfer heat efficiently; (2) diverter valve partially stuck — not all primary flow is being diverted to DHW; (3) boiler output too low for demand — check boiler output vs simultaneous demand (simultaneous heating + DHW demand); (4) cold water mains temperature very low in winter — boiler has to raise temperature from 5°C instead of 15°C, reducing apparent output. Start with a scale check (flow rate test) and diverter valve mechanical test.

How long should a DHW heat exchanger last?

Typically 10–15 years in a soft water area with good maintenance. In hard water areas without scale inhibition, the DHW heat exchanger can fail within 3–5 years. Annual scale inhibitor cartridge replacement is essential in hard water areas. Many manufacturer warranties (Worcester Bosch, Vaillant) specifically require evidence of scale inhibitor installation in hard water areas — without it, the warranty claim for heat exchanger failure may be rejected.

Does the DHW flow rate affect the hot water temperature output?

Yes — on a combi boiler, the DHW temperature is determined by the boiler output power divided by the flow rate. High flow rate = lower temperature rise (water moves through the heat exchanger quickly). Low flow rate = higher temperature rise. Most combi boilers modulate between minimum and maximum burner output to maintain a target temperature (typically 55°C) across a range of flow rates (typically 6–14 l/min). Below the boiler's minimum flow rate, DHW mode may not activate. Above the boiler's rated maximum flow rate, water will not reach the target temperature.

Regulations & Standards