A new boiler may reduce gas use, but the percentage on its product label is not the percentage your bill will fall. Your result depends on the efficiency and condition of the old system, the new boiler’s real operating conditions, controls, heat loss, hot-water use, weather, comfort choices and the share of gas used for cooking.

The honest method is to show a range with visible assumptions, then compare it with actual meter readings after installation.

Why headline efficiency is not a savings promise

A quoted seasonal efficiency describes the appliance under a defined test or calculation. Your bill records the whole household’s metered gas, standing charges and taxes. The two measures are not interchangeable.

Consumption can also change because the new controls alter schedules, radiators or pipework are changed, the household uses more heat because the system is more reliable, the winter differs, or hot-water habits change. Standing charges do not fall when you use fewer units, so bill savings are normally smaller than fuel-cost savings expressed as a percentage.

Use your own annual consumption

Find at least 12 months of actual gas consumption in kWh from bills or your supplier account. Longer history is better because one winter can be unusual. Do not use the direct-debit amount: it can include credit, debt and supplier estimates.

Next, estimate what proportion serves space heating and hot water. If gas also fuels a hob or fire, not all consumption is available for boiler savings. Where possible, compare summer use with winter use and label the resulting split as an estimate.

A transparent calculation

Let G be annual metered gas consumption, S the estimated boiler share, E-old the old seasonal efficiency, E-new the cautious new in-use efficiency and P the unit price.

• Useful heat currently delivered = G × S × E-old
• New gas needed for the same useful heat = (G × S × E-old) ÷ E-new
• Estimated gas reduction = (G × S) minus new gas needed
• Estimated fuel-cost reduction = estimated gas reduction × P

Add household gas not attributed to the boiler back into the projected total. Keep standing charges unchanged unless the tariff changes.

Use a range, not one optimistic answer

Run at least three scenarios:

These are not forecasts unless the assumptions are supported. The calculator should never invent default savings or imply that a large percentage is typical.

Factors outside the boiler

Heat loss can dominate consumption. Insulation, glazing, draughts, ventilation and chosen room temperatures affect the useful heat required. Controls affect when and where heat is delivered. System design affects whether a condensing boiler can operate at suitably low return temperatures.

A new appliance cannot compensate for every building or emitter problem. Ask whether the proposal includes heat-loss assessment, control setup, radiator checks, balancing and commissioning.

How to test the result after installation

Record meter readings and heating settings. Compare similar periods using weather-normalised data if available, or at least note outdoor conditions and occupancy. Also compare comfort. Lower gas use accompanied by colder rooms is not the same outcome as delivering the same comfort more efficiently.

Frequently asked questions

How much of my gas use is actually for heating?

There is no single household percentage. Use bills, summer baseline consumption and appliance information to create a labelled estimate. A smart meter can improve the record but does not identify the appliance using each unit of gas.

Why will the quoted efficiency not translate directly into bill savings?

Because it is not the old-to-new efficiency difference applied to the whole bill. The bill includes non-boiler gas and standing charges, while real performance depends on system temperatures, controls, weather and behaviour.

Suggested on-page component

Embed the New-Boiler Savings Scenario Calculator here. Require user-entered consumption, boiler share, old/new efficiency range and unit price. Show formulae and assumptions, exclude standing-charge savings, and label every result an estimate rather than a promise.

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