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

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

• Diverter valve: switches primary flow between CH circuit and DHW heat exchanger; most common DHW failure cause
• Diverter valve motor resistance: typically 500–2,000 ohms between terminals (varies by manufacturer; check service manual)
• Diverter valve mechanical test: operate actuator manually (boiler isolated) to confirm valve body moves freely — sticky cartridge is common
• Flow sensor (flowmeter): detects DHW demand; turbine generates pulses (typically 1–4 pulses per litre); PCB uses pulses to initiate DHW mode
• Flow sensor resistance at rest: typically 3–5 kΩ (Hall-effect sensor) or as per manufacturer
• Flow sensor fault code: typically E5 or DHW flow fault depending on manufacturer
• DHW thermistor: NTC thermistor reads DHW flow temperature; resistance decreases with increasing temperature (typically 10 kΩ at 25°C)
• Minimum hot water flow rate: most combi boilers require minimum 2–3 l/min flow to activate DHW mode — below this, no hot water produced
• Scale threshold: >200 ppm TDS (total dissolved solids) water hardness increases scaling risk significantly; >350 ppm = high risk
• Scale inhibitor dosing: automatic inline scale inhibitors (e.g., Fernox Superscale) or polyphosphate dosing required above 200 ppm
• Warranty implications: many manufacturers void DHW heat exchanger warranty if water hardness exceeds threshold and no scale inhibitor was fitted
• DHW heat exchanger blockage: manifests as low hot water flow rate, intermittent hot water, or cold water after initial burst

Diagnostic Decision Tree

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:

• Diverter valve cartridge replacement: on most modern boilers (Worcester Bosch, Vaillant, Baxi, Ideal), the cartridge is a replaceable part — the motor and body remain; only the moving valve insert is replaced. Cost: £15–50 for cartridge, 30–60 minutes labour.
• Complete diverter valve assembly: where the motor and body are integrated, the whole assembly must be replaced. Cost: £50–150 for the assembly.
• Boiler hydraulic block replacement: on older boilers where the diverter valve is integrated into the hydraulic manifold, replacement involves the entire block — less common, higher cost.

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:

• Reduced DHW flow rate (scale partially blocks water passages)
• Intermittent hot water — water heats then cools rapidly as it passes through the scaled sections
• Boiler runs for longer to achieve target DHW temperature
• Fault codes related to overheat (PCB detects higher primary temperature required to achieve DHW setpoint, triggers overheat protection)

Scale inhibition:

• Polyphosphate inline dosing pot (e.g., BWT Genie, Fernox Superscale): a small cartridge fitted to the cold water supply to the boiler; releases polyphosphate that coats pipe walls and prevents scale adhesion. Replace cartridge annually.
• Electromagnetic water conditioner: prevents scale crystallisation; less effective than polyphosphate in hard water areas
• Water softener: the most effective approach for whole-house scale prevention; ensure a hard water bypass for the drinking water supply

DHW heat exchanger descaling:

• Specialist descaling units (e.g., Adey MagnaClean Pro) circulate citric acid or a specialist descaling solution through the heat exchanger
• This is a service-engineer procedure; requires boiler isolation, connection of chemical circulation circuit, and flushing
• For heavily scaled heat exchangers, descaling may not fully restore performance — heat exchanger replacement may be more cost-effective

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:

• Fault code on boiler display (NTC open circuit or short circuit)
• DHW temperature too high or too low if thermistor is reading incorrectly
• No DHW if PCB goes into safety lockout due to thermistor fault

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

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

• Gas Safety (Installation and Use) Regulations 1998 — Gas Safe registration required for gas-side work

• Building Regulations Approved Document G — hot water temperatures; scalding prevention (Part G4)

• BS EN 677 — gas-fired condensing boilers; efficiency and safety requirements

• Water Supply (Water Fittings) Regulations 1999 — water fittings in contact with potable water

• HHIC: Combi Boiler Best Practice Installation Guide — Heating and Hotwater Industry Council

• Worcester Bosch Service Manuals — fault codes and component resistance values

• Vaillant Service Documentation — ecoTEC diagnostic guides

• Gas Safe Register — engineer registration verification

• HHIC: Boiler Technical Guidance — Heating and Hotwater Industry Council

• boiler not firing — boiler not firing for either CH or DHW

• no hot water — hot water system diagnosis (general)

• boiler losing pressure — pressure-related boiler faults

• no heating — heating system not working