Summary

"No hot water" is one of the most frequent callouts in UK domestic heating. The fault path diverges sharply depending on whether the property has a combi boiler (instantaneous DHW), a system boiler (sealed system with unvented cylinder), or a regular/conventional boiler (open-vent with vented cylinder). On combi systems the fault almost always lies inside the boiler itself -- diverter valve, flow sensor, NTC thermistor, or secondary plate heat exchanger. On cylinder-based systems the fault is far more likely to be in the external controls and pipework -- motorised valves, cylinder thermostat, wiring, or the cylinder coil itself. This guide provides a structured decision tree for on-site diagnosis across all three boiler types, with specific component references and test procedures.

Key Facts

  • Combi boilers heat DHW instantaneously via a secondary plate heat exchanger; the diverter valve directs flow away from the heating circuit on a DHW demand
  • System boilers use a sealed pressurised system with an unvented hot water cylinder; a motorised zone valve controls flow to the cylinder
  • Regular/conventional boilers use an open-vent system with a feed-and-expansion tank and a vented cylinder; they may use gravity or pumped DHW circulation
  • The Honeywell V4043H (2-port zone valve) is the standard valve in S-plan systems; the Honeywell V4073A (3-port mid-position valve) is used in Y-plan systems
  • A cylinder thermostat (typically Honeywell L641A or equivalent) must click audibly when turned up and down; no click = replace
  • On a combi, if the boiler fires for heating but not DHW, the flow sensor (turbine or Hall-effect type) is failing to signal a DHW demand to the PCB
  • Plate heat exchangers in combi boilers scale up in hard water areas; symptoms include fluctuating DHW temperature and progressively lower flow rates
  • All gas work must be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998

Diagnostic Decision Tree

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Step 1: Identify the system type

Is there a hot water cylinder?
  |
  +-- NO  --> COMBI BOILER (go to Step 2A)
  |
  +-- YES --> Is the system sealed (pressurised, no tanks in loft)?
                |
                +-- YES --> SYSTEM BOILER with unvented cylinder (go to Step 2B)
                |
                +-- NO  --> REGULAR/CONVENTIONAL BOILER with vented cylinder (go to Step 2C)

Step 2A: Combi boiler -- initial checks

Does the boiler fire at all (heating or DHW)?
  |
  +-- NO  --> General boiler fault. Check error code on display, gas supply,
  |           condensate trap, ignition sequence. NOT a DHW-specific fault.
  |
  +-- YES (heating works, DHW does not) -->
        |
        Open a hot tap. Does the boiler display switch to DHW mode?
          |
          +-- NO  --> FLOW SENSOR fault (Step 3A-1)
          |
          +-- YES --> Does the boiler fire/attempt to ignite for DHW?
                |
                +-- NO  --> PCB or wiring fault between flow sensor and PCB
                |
                +-- YES --> Water runs but stays cold or lukewarm -->
                      |
                      Is heating also running when only DHW is demanded?
                        |
                        +-- YES --> DIVERTER VALVE stuck in mid/heating position (Step 3A-2)
                        |
                        +-- NO  --> NTC THERMISTOR or PLATE HEAT EXCHANGER fault (Step 3A-3)

Step 2B: System boiler with unvented cylinder -- initial checks

Does the boiler fire for central heating?
  |
  +-- NO  --> General boiler fault (not DHW-specific)
  |
  +-- YES -->
        |
        Is the programmer/timer calling for hot water?
          |
          +-- NO  --> Programmer fault or incorrect schedule. Test by setting HW to constant/override.
          |
          +-- YES -->
                |
                Is the cylinder thermostat calling for heat? (Turn it up to max; listen for click)
                  |
                  +-- NO click --> CYLINDER THERMOSTAT faulty (Step 3B-1)
                  |
                  +-- YES (clicks, stat is calling) -->
                        |
                        Is voltage reaching the motorised valve? (Test with multimeter at valve terminals)
                          |
                          +-- NO  --> Wiring fault between programmer/stat and valve. Check junction box.
                          |
                          +-- YES -->
                                |
                                Does the valve motor operate? (Listen for motor hum; check manual lever moves)
                                  |
                                  +-- NO  --> MOTORISED VALVE fault -- motor or actuator head (Step 3B-2)
                                  |
                                  +-- YES (valve opens) -->
                                        |
                                        Does the valve end-switch close? (Test grey-to-orange continuity on V4043H)
                                          |
                                          +-- NO  --> End-switch fault. Replace actuator head.
                                          |
                                          +-- YES --> Valve and controls OK. Check pump, boiler response,
                                                      and cylinder coil for blockage/airlock (Step 3B-3)

Step 2C: Regular/conventional boiler with vented cylinder -- initial checks

Same diagnostic path as Step 2B, with these additional checks:
  |
  +-- Is the feed-and-expansion (F&E) tank in the loft full?
  |     +-- NO --> System has lost water. Refill. Check for leaks.
  |     +-- YES --> Continue with motorised valve / cylinder stat checks as per 2B
  |
  +-- Is the system gravity-fed DHW (no motorised valve on HW circuit)?
        +-- YES --> Check for airlocks in gravity circuit. Check that the
        |           vent pipe rises continuously. Check cylinder coil for blockage.
        |           Check pump-overrun is not back-feeding through gravity circuit.
        +-- NO  --> Follow S-plan/Y-plan motorised valve path as per 2B

Detailed Guidance

No hot water from a combi boiler -- what should I check?

Step 3A-1: Flow sensor diagnosis

The flow sensor (also called a flow switch, flow turbine, or water flow detector) signals to the PCB that a DHW tap has been opened. It is typically a turbine/Hall-effect sensor mounted on the DHW water inlet inside the boiler.

Test procedure:

  1. Open a hot tap and observe the boiler display. If it does not switch to DHW mode, the PCB is not receiving a flow signal.
  2. Locate the flow sensor (consult manufacturer parts diagram). It is usually clipped onto the DHW inlet pipe inside the boiler.
  3. Check the wiring connector for corrosion or disconnection.
  4. With the tap running, check for voltage output from the sensor (typically a pulsed signal; manufacturer-specific).
  5. Remove the sensor and inspect for debris or limescale on the turbine. Clean or replace.
  6. On some boilers (e.g. Vaillant ecoTEC, Worcester Greenstar), the flow sensor is integrated into the water valve assembly and must be replaced as a unit.

Common part numbers:

  • Vaillant ecoTEC: 178988 (flow sensor)
  • Worcester Greenstar: 87161461600 (water valve with integrated sensor)
  • Baxi/Potterton: 5114710 (flow sensor)
  • Ideal Logic/Vogue: 175590 (flow sensor)

Step 3A-2: Diverter valve diagnosis

The diverter valve (3-way valve) inside the combi boiler directs heated water from the primary heat exchanger to either the heating circuit or the secondary plate heat exchanger (for DHW). When a DHW demand is detected, the valve should move fully to the DHW position.

Symptoms of a faulty diverter valve:

  • Heating works, DHW does not (valve stuck in heating position)
  • DHW works, heating does not (valve stuck in DHW position)
  • Radiators get warm when only DHW is demanded (valve stuck mid-position or not fully diverting)
  • Lukewarm DHW that never reaches temperature

Test procedure:

  1. Set the programmer to DHW only (heating off). Open a hot tap.
  2. Feel the heating flow pipe leaving the boiler. If it gets hot, the diverter valve is not fully closing off the heating circuit.
  3. On boilers with a diverter valve motor accessible from the front (e.g. Vaillant ecoTEC), listen for the motor operating when switching between heating and DHW demands.
  4. Some diverter valves have a manual override or test pin. Consult the manufacturer manual.
  5. If the valve cartridge is removable, inspect for limescale, debris, or a perished seal.
  6. Replace the diverter valve cartridge or complete valve assembly as required.

Common part numbers:

  • Vaillant ecoTEC: 0020020015 (diverter valve cartridge), 178978 (complete valve)
  • Worcester Greenstar: 87161064420 (diverter valve)
  • Baxi/Potterton Duotec: 7224763 (diverter valve)
  • Ideal Logic: 175407 (diverter valve kit)

Step 3A-3: NTC thermistor and plate heat exchanger diagnosis

NTC thermistor (DHW outlet sensor):

The DHW NTC thermistor measures the outgoing DHW temperature and feeds back to the PCB to modulate the burner. If it reads incorrectly, the boiler may not fire for DHW or may overshoot/undershoot.

  1. Locate the DHW NTC thermistor (usually clipped to the DHW outlet pipe or the plate heat exchanger).
  2. Disconnect at the PCB end to avoid stray resistance readings.
  3. Measure resistance with a multimeter. At 25 degrees C, a typical 10k NTC should read approximately 10k ohms. At 50 degrees C, approximately 3.5k ohms. Cross-reference with manufacturer specifications.
  4. If the reading is open-circuit (infinite resistance) or short-circuit (near zero), replace the thermistor.
  5. Some boilers have two NTC sensors (flow and return on the DHW circuit); test both.

Plate heat exchanger (secondary heat exchanger):

The plate heat exchanger transfers heat from the primary circuit to the mains-pressure DHW. It is highly susceptible to limescale in hard water areas (above 200 ppm CaCO3).

Symptoms of a blocked plate heat exchanger:

  • DHW temperature fluctuates wildly (hot-cold-hot cycling)
  • DHW flow rate has progressively decreased over months/years
  • Boiler fires for DHW but water is only lukewarm at full flow
  • Water temperature improves at reduced flow (partially close the tap)

Diagnosis:

  1. Measure the temperature differential across the plate heat exchanger (primary in/out vs DHW in/out). If the primary side is reaching temperature but the DHW side is not, the exchanger is blocked.
  2. Check DHW flow rate at the tap. Compare against the boiler's rated DHW flow rate (typically 9-13 l/min at 35 degrees C rise for a 24-30 kW combi).
  3. If flow rate is significantly reduced, the exchanger is likely scaled.
  4. The plate heat exchanger can be removed and descaled with a suitable acid solution (e.g. Sentinel X400 or Fernox DS40), or replaced. Prevention: fit a scale reducer on the mains cold inlet.

No hot water from a system boiler -- what should I check?

System boilers heat an unvented (pressurised) hot water cylinder via an internal coil. The cylinder is typically controlled by a motorised zone valve and a cylinder thermostat.

Step 3B-1: Cylinder thermostat diagnosis

The cylinder thermostat (commonly Honeywell L641A, L641B, or Drayton HTS3) is strapped to the cylinder at approximately one-third height from the bottom. It breaks the circuit to the motorised valve when the cylinder reaches the set temperature.

Test procedure:

  1. Turn the cylinder thermostat dial from minimum to maximum. You should hear a distinct click as it passes through the current water temperature. No click = stat is faulty; replace.
  2. With the stat turned to maximum (above the current cylinder temperature), check for 230V at the stat output terminals with a multimeter.
  3. If 230V is present at the input but not the output, the stat contacts are failed open. Replace.
  4. Check the stat is making good thermal contact with the cylinder surface. On a foam-lagged cylinder, the insulation must be cut away to allow direct contact with the copper or stainless steel shell.
  5. Standard replacement: Honeywell L641A1039 (surface-mount, 40-80 degrees C range).

Step 3B-2: Motorised valve diagnosis (S-plan: Honeywell V4043H)

The Honeywell V4043H is a 2-port zone valve used in S-plan systems. One valve controls the heating zone, another controls the hot water zone. The valve is spring-return: with no power, the valve is closed (fail-safe).

Wiring (standard colours):

  • Brown -- Live in (from programmer/thermostat via wiring centre)
  • Blue -- Neutral
  • Green/yellow -- Earth
  • Grey -- One side of the end-switch (connected to orange when valve is fully open)
  • Orange -- Other side of end-switch (carries the "boiler on" signal to the wiring centre)

Test procedure:

  1. Set programmer to HW on, cylinder stat to maximum. Check for 230V on the brown wire at the valve. No voltage = wiring or control fault upstream.
  2. If 230V is present on brown, listen for motor hum. No hum = motor/actuator failure.
  3. If the motor hums but the valve does not open, the valve spindle is seized. Try the manual lever -- push it to the fully open position. If it moves freely, the motor coupling may be stripped.
  4. With the valve fully open (manually or electrically), test continuity between grey and orange wires. If open-circuit, the micro-switch inside the actuator head has failed. The boiler will not fire because the end-switch signal is absent.
  5. Quick test: Remove the actuator head (two screws or a clip), manually open the valve body with pliers on the spindle, and jumper grey to orange at the wiring centre. If the boiler fires and the cylinder heats, the actuator head is confirmed faulty.
  6. Replacement actuator head: Honeywell 40003916-001 (direct replacement for V4043H). The valve body rarely fails and does not normally need replacement.

Step 3B-3: Cylinder coil and circulation faults

If the controls and valve are confirmed working but the cylinder is not heating:

  1. Check the pump is running. With the HW zone valve open and boiler firing, the pump should be circulating water through the cylinder coil. Feel the flow and return pipes at the cylinder -- the flow pipe should be hot, the return warm.
  2. Airlock in the coil. If the flow pipe to the cylinder is hot but the return is cold, there may be an airlock in the coil or the coil is blocked. On an unvented cylinder, bleed any air via the air vent at the top of the coil circuit (if fitted). On older systems, slacken the compression fitting at the top coil connection by half a turn to release trapped air, then retighten.
  3. Blocked coil. If descaling and bleeding do not restore circulation, the coil may be internally blocked with limescale or magnetite sludge. A powerflush of the primary circuit may clear it. In severe cases the cylinder must be replaced.
  4. Pump failure. If the pump runs on the heating circuit but not on the HW circuit, check that the pump speed is adequate and that there are no isolation valves closed on the HW primary circuit.

No hot water from a regular/conventional boiler -- what should I check?

Regular (conventional, heat-only) boilers operate with an open-vent system -- a feed-and-expansion (F&E) tank in the loft and a vented hot water cylinder. The system may use S-plan (two 2-port valves), Y-plan (one 3-port mid-position valve), or gravity DHW circulation.

Y-plan systems: Honeywell V4073A mid-position valve diagnosis

The V4073A is a 3-port motorised mid-position valve. It has three positions:

  • Port A only (hot water): Spring-return default position (power off = hot water only)
  • Mid-position (A + B): Both hot water and heating
  • Port B only (heating): Motor-driven fully clockwise

Wiring (standard colours):

  • Grey -- Live from heating demand (room thermostat via wiring centre)
  • White -- Live from hot water demand (cylinder thermostat via wiring centre)
  • Orange -- Switched live output to boiler/pump (end-switch signal)
  • Blue -- Neutral
  • Green/yellow -- Earth

Test procedure for no hot water:

  1. Switch off all power. Check the manual lever has spring resistance when pulled. If the lever is completely loose with no spring return, the internal spring has failed -- the valve cannot return to the HW-only (port A) position. Replace the valve.
  2. Power on. Set programmer to HW only, cylinder stat to max, room stat to minimum. The valve should spring-return to port A (hot water). Check for 230V on the white wire at the valve.
  3. With the valve in the HW-only position, feel the pipe approximately 300mm from port A. It should get hot. If the pipe from port B also gets hot, the valve is not fully closing off the heating port -- the valve body or ball seal is worn. Replace.
  4. If the valve motor hums but does not move, or moves erratically, replace the actuator head: Honeywell 40003916-002 (V4073A head).
  5. Check orange wire for 230V when the valve is in any demand position. No voltage = end-switch failure; boiler will not fire.

Gravity DHW systems (no motorised valve)

Some older conventional systems use gravity circulation for DHW (no valve, no pump on the HW circuit). The hot water rises naturally from the boiler to the cylinder coil.

Common faults:

  1. Airlock in the gravity circuit. Air trapped at the highest point of the circuit prevents circulation. Bleed from the highest accessible point. Ensure the vent pipe rises continuously from the boiler to above the F&E tank without any dips that could trap air.
  2. Pump overrun back-feeding. If the central heating pump is oversized or positioned incorrectly, it can create a pressure differential that reverses flow through the gravity DHW circuit, pushing hot water away from the cylinder. Fit a non-return valve or check valve on the gravity circuit, or convert to a fully pumped system with a motorised valve.
  3. F&E tank empty. Check the tank in the loft. If empty, the ballvalve may be stuck or the float arm fouled. Refill the system and check for leaks.

Immersion heater diagnosis (backup DHW)

Most vented and unvented cylinders have one or two immersion heaters as a backup or boost heat source. If the boiler-fed coil is not heating the cylinder, the immersion heater can confirm whether the cylinder itself is functional.

Test procedure:

  1. Locate the immersion heater(s). Top-mounted immersions heat the upper portion of the cylinder only (boost). Side-mounted or bottom-mounted immersions heat the full cylinder.
  2. Check the dedicated immersion heater circuit at the consumer unit. The MCB should be on, and the RCD should not have tripped. Reset if necessary.
  3. Check the immersion heater switch (usually a double-pole isolator near the cylinder). Switch on.
  4. Check for 230V at the immersion heater terminals (live and neutral on the thermostat).
  5. If voltage is present but the element does not heat:
    • Turn the immersion thermostat up and listen for a click. No click = thermostat faulty.
    • Check the thermal cut-out (reset button, usually red, on the thermostat). Press to reset. If it trips again immediately, the element or thermostat is faulty.
    • Measure the element resistance between the two heating element terminals (disconnect wires first). A typical 3kW immersion at 230V should read approximately 17-18 ohms. Open circuit = element has burned out. Very low resistance or reading to earth = element has shorted/split. Replace.
  6. Standard immersion heater thermostat: 7-inch or 11-inch rod stat, typically rated to 75 degrees C with an 90 degrees C safety cut-out.

Frequently Asked Questions

I have heating but no hot water -- is it always the diverter valve on a combi?

On a combi boiler, the diverter valve is the single most common cause of "heating works, hot water does not." However, it could also be the flow sensor (boiler does not detect the DHW demand at all), the DHW NTC thermistor (boiler fires but cannot regulate temperature), or a blocked plate heat exchanger (insufficient heat transfer). Check whether the boiler display switches to DHW mode when a hot tap is opened -- if it does not, the fault is upstream of the diverter valve (flow sensor or PCB).

My hot water is lukewarm but the boiler fires -- what is the most likely cause?

On a combi: a partially blocked plate heat exchanger, especially in a hard water area. Reduce the flow at the tap -- if the temperature improves, the exchanger cannot transfer heat fast enough at full flow. On a cylinder system: the motorised valve may be opening but not fully (sticking spindle), the cylinder thermostat may be set too low, or the cylinder coil may be partially blocked with limescale reducing heat transfer.

How do I tell if my motorised valve is faulty without dismantling it?

Set the system to demand hot water (programmer to HW on, cylinder thermostat to max). At the valve, check for 230V on the brown (live-in) wire. If voltage is present, listen for a motor hum. Then check for 230V on the orange wire (end-switch output). If you have 230V in but no 230V on the orange, the valve is not reaching the fully open position. You can confirm by removing the actuator head and manually opening the valve body -- if the boiler then fires, the actuator head is the failed component.

Can I replace just the motorised valve actuator head, or do I need to drain down?

In almost all cases, you can replace just the actuator head without draining the system. The V4043H and V4073A valve bodies are designed for head-only replacement. Remove two screws (or unclip), disconnect the wiring, and fit the new head. The valve body stays in situ on the pipework. Only replace the full valve body if the valve seat is worn, the spindle is corroded, or the body is leaking.

My immersion heater trips the RCD -- what does that mean?

A tripping RCD on the immersion heater circuit almost always indicates the heating element has split, allowing water ingress into the element sheath. This creates an earth fault. Disconnect the element wires and test insulation resistance between each terminal and earth -- it should read above 1 megohm. If low, the element must be replaced. Do not simply reset the RCD and leave a faulty element connected.

Regulations & Standards