If your Vaillant boiler has heating but no hot water, this is one of the most common faults seen by Gas Safe engineers across Surrey and London, particularly on Vaillant ecoTEC combi boilers.

In many cases, the boiler appears to work normally for central heating. However, when a hot tap is opened, the boiler fails to respond, or hot water never arrives.

This guide explains the most common causes of this problem, how to check Vaillant SE diagnostic codes, and when you should contact a Gas Safe engineer.

For professional diagnostics, see our boiler repair services in Surrey:
https://www.certificates4uk.co.uk/boiler-repair-surrey/

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Why Your Vaillant Boiler Has Heating but No Hot Water

Combi boilers use internal components to switch between heating and domestic hot water. If one of these parts fails, the boiler may continue heating radiators while hot water stops working.

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1. Faulty Diverter Valve (Most Common Cause)

The diverter valve controls whether hot water is sent to:

• Radiators (central heating)
• Taps and showers (domestic hot water)

If the valve becomes stuck or worn, the boiler may stay in heating mode.

Common symptoms

• Radiators heat normally
• Hot taps stay cold
• The boiler fires, but no hot water is produced

This fault typically requires diverter valve replacement by a Gas Safe engineer.

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2. Blocked Plate Heat Exchanger (Limescale)

In hard water areas such as Surrey and London, limescale can gradually block the plate heat exchanger.

When this happens, heat cannot transfer properly into the domestic water supply.

Common symptoms

• Hot water for a few seconds, then cold
• The boiler fires briefly when the tap opens
• The problem gradually worsens over time

Cleaning or replacing the plate heat exchanger may be required. In some systems, a power flush service can also improve circulation.

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3. Low Boiler Pressure

Vaillant boilers must operate within a specific pressure range.

Check the pressure gauge on the boiler.

Normal pressure (when cold)

1.0 – 1.5 bar

If system pressure drops too low, the boiler may fail to detect hot water demand.

Low pressure can also indicate leaks elsewhere in the heating system.

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Vaillant Flow Sensor SE Code Test

Many Vaillant ecoTEC boilers include a built-in diagnostic test that can help identify hot water faults.

How to perform the test

1. Press and hold the top two buttons on the boiler control panel
2. While holding them, open a hot water tap
3. Watch the display for SE diagnostic numbers

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What the SE Codes Mean

SE12 or SE14

Normal water flow detected.

The flow sensor is working correctly.

SE05 or low numbers

No water flow detected.

The boiler cannot detect water movement, so it will not fire for hot water.

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Flow Sensor or Wiring Harness Fault?

If the display shows SE05 when a tap is opened, the cause is usually either a faulty flow sensor or damaged wiring.

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Step 1: Check the Wiring Harness

Inspect the cable connected to the flow sensor.

Look for:

• Broken wires
• Loose connectors
• Burnt insulation
• Stretched or damaged cables

If the wiring is damaged, the fault lies in the harness rather than the boiler itself.

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Step 2: Test the Flow Sensor

If the wiring appears intact, the flow sensor itself may have failed.

Without accurate flow detection, the boiler cannot activate hot water mode even though central heating continues to work.

In this situation, flow sensor replacement is required.

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Common Misdiagnoses to Avoid

Many homeowners are incorrectly advised that:

• The PCB needs replacing
• The boiler must be replaced entirely

In reality, a simple flow sensor or wiring harness fault can prevent hot water from working while heating still operates normally.

Correct diagnosis is essential before replacing expensive components.

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When to Call a Gas Safe Engineer

You should contact a professional if:

• Hot water remains cold
• The boiler does not fire when a tap opens
• SE codes remain low during testing
• The problem repeatedly returns

Boiler covers should never be removed by unqualified persons.

A Gas Safe-registered engineer must perform all internal diagnostics and component replacements.

Gas safety guidance can also be found on the official website:
https://www.gassaferegister.co.uk/

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Vaillant Boiler Repair in Surrey and London

Certificates4UK Ltd specialises in diagnosing and repairing common faults on Vaillant boilers, including:

• Vaillant ecoTEC combi boilers
• Flow sensor replacement
• Diverter valve repair
• Plate heat exchanger cleaning
• Power flushing and boiler servicing

Our engineers regularly repair boilers across Surrey and London, including Kingston, Richmond, Wimbledon, and surrounding areas.

You can also learn more about Gas Safety Certificates (Gas Safe with CP12) here:
https://www.certificates4uk.co.uk/gas-safety-certificate/

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Frequently Asked Questions

Why does my Vaillant boiler heat radiators but not water?

This usually indicates a diverter valve problem, a blocked plate heat exchanger, or a faulty flow sensor preventing the boiler from detecting water demand.

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Can low pressure stop hot water?

Yes. Low pressure can prevent correct circulation and may stop the boiler from responding to hot water demand.

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Can I fix this myself?

Homeowners can check pressure and perform the SE diagnostic test, but internal components must only be repaired by a Gas Safe engineer.

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