What Evidence Proves That a Boiler Is Broken or Inefficient?

What counts as a broken boiler?

In everyday language, a boiler may be called broken when heating or hot water is unreliable.

For formal ECO4 assessment, the definition is narrower: the boiler is broken when it is connected to the necessary fuel and electrical supplies but does not respond appropriately to a demand for heat from the central-heating or hot-water system. The installer must record the symptoms and steps used to reach that conclusion.

Relevant evidence may include:

• No ignition or repeated ignition failure
• Unstable firing or repeated lockouts
• A failed pump, fan, gas valve, circuit board or heat exchanger
• Flow temperature or flow rate outside manufacturer tolerances
• Inadequate combi-boiler hot-water flow or temperature
• Corrosion, poor flue condition or an unsafe classification
• Flue-gas analyser results outside the manufacturer’s tolerance

Ofgem’s checklist asks the operative to explain how the failure was identified and notes that sludge alone may not be enough to classify a boiler as broken.

A fault code is therefore evidence of a symptom, not a complete diagnosis. Photograph the display and note when it occurred, but ask the engineer to identify the underlying fault.

What proves that a boiler is inefficient?

“Inefficient” can mean two different things.

Rated inefficiency

Rated inefficiency describes the boiler’s original design.

Under current ECO4 guidance, non-condensing boilers, or systems with manufactured efficiency no better than a non-condensing boiler, are treated as inefficient. Condensing boilers are treated as efficient for this scheme, although an individual condensing boiler may still be poorly set up or underperforming.

Rated efficiency can normally be established from:

• The boiler make and model
• The serial-number plate
• Installation certificates
• Warranty documentation
• Manufacturer information
• The Product Characteristics Database

For ECO4, Ofgem expects documentary or pictorial evidence of boiler age rather than a customer estimate.

Operational inefficiency

Operational inefficiency means the boiler or wider heating system is using more fuel than reasonably necessary.

Evidence can include poor combustion results, incorrect flow settings, excessive cycling, weak controls, sludge, scale, unbalanced radiators or a fault preventing normal operation.

Modern condensing boilers work more efficiently when return-water temperatures are lower. Excessively high settings can therefore reduce real-world performance without proving that the boiler itself is defective.

Warning signs are not proof

Rising bills justify investigation, particularly when the tariff, weather, occupancy and thermostat settings have not changed.

GOV.UK lists a boiler not working properly as one possible cause of unexpectedly high gas consumption. However, bills can also rise because of colder weather, estimated meter readings, heat loss or changes in household behaviour.

Other warning signs include:

• Rooms taking longer to warm up
• Fluctuating hot-water temperatures
• Radiators heating unevenly
• Frequent pressure loss
• Banging, whistling or kettling
• Repeated resets or lockouts
• Water leaks
• Increasing repair frequency

These symptoms justify professional diagnosis, but age and symptoms alone do not prove failure.

An EPC is also supporting evidence rather than a live boiler test. It rates the energy performance of the whole property from A to G and is generally valid for ten years. It may therefore fail to reflect later boiler faults, repairs or heating-control changes.

How to obtain reliable evidence

1. Record the symptoms

Photograph fault codes, leaks, corrosion and warning labels. Record dates, pressure readings and whether heating, hot water or both are affected.

A short video can also help demonstrate unusual noises, repeated ignition attempts or fluctuating display readings.

2. Check basic external causes

Confirm that the boiler has power, fuel, appropriate programmer settings and a genuine demand from the thermostat.

Never remove the boiler casing or attempt work on gas components yourself.

3. Book the right engineer

Work on a gas boiler must be carried out by a Gas Safe registered engineer. Check that the engineer is registered for the relevant type of appliance rather than relying solely on a company logo or advertisement.

4. Request a written diagnostic report

Ask the engineer to record:

• Make, model and serial number
• Approximate installation date
• Symptoms observed
• Tests and measurements completed
• Fault diagnosis
• Combustion or performance readings
• Safety classification
• Recommended repair or replacement

A verbal opinion is less useful when dealing with a landlord, warranty provider or funding application.

5. Request a repair-and-replace comparison

The quotation should identify parts, labour, VAT, availability, repair warranty and any secondary work.

Ask whether replacement parts are readily available and whether other major components are showing signs of deterioration.

6. Collect supporting records

Keep service sheets, repair invoices, warranty documents, EPCs and actual meter readings. A pattern of repeated failures is more persuasive than an unsupported statement that the boiler has “always been unreliable”.

7. Seek a second opinion where necessary

A second assessment is sensible when:

• The original diagnosis is vague
• The proposed repair is expensive
• Replacement is presented as the only option
• The engineer has not supplied test results
• The boiler is still covered by a warranty

When is a boiler beyond economical repair?

A boiler is not automatically beyond economical repair because it is old or because one component has failed.

The decision should consider:

• Parts availability
• Repair cost
• General boiler condition
• Expected reliability
• Remaining warranty
• Previous repair history
• Cost of a suitable replacement

For ECO4, the installer must check whether the necessary parts are available and compare the actual repair cost with the replacement cost and scheme-specific repair thresholds.

If a broken efficient boiler can be repaired economically, the scheme generally expects it to be repaired. If it cannot be repaired economically, replacement may be considered, subject to the wider scheme rules.

Typical costs and potential savings