Boiler Replacement Cost Calculator

If your gas boiler is over 10 to 15 years old, replacing it with a modern condensing boiler or a heat pump can cut your energy bills sharply. A new A-rated condensing gas boiler costs £2,000 to £4,000 installed and operates at 90 to 94% efficiency compared to 60 to 80% for many older boilers. In a house still running a 25-year-old unit, that difference is worth 15 to 25% of the heating bill, or £200 to £500 a year.

The Boiler Upgrade Scheme (BUS) pays £7,500 towards an air source or ground source heat pump, £2,500 towards an air-to-air heat pump and £5,000 towards a biomass boiler in a rural, off-grid property. Until March 2027 an air or ground source heat pump earns an extra £1,500 where the home is heated by oil or LPG and has no mains gas connection. The £7,500 rate dates from October 2023, when it rose from £5,000, and the scheme covers England and Wales only. There is no EPC or insulation condition: you need to own the property, whether it is your home, a second home, a let or business premises, and to be replacing fossil fuel or electric heating such as gas, oil or LPG.

Heat pump economics work differently from a boiler. Installation typically runs £8,000 to £14,000 before the grant, leaving £500 to £6,500 to find afterwards, but the unit delivers 3 to 4 kWh of heat for every 1 kWh of electricity it draws. The homes that do best have good insulation, underfloor heating or large radiators, and no restrictive planning to work around. This calculator covers the gas route: it prices a new condensing boiler against your current one, applies the efficiency gain to the unit-rate part of your gas bill rather than the standing charge, and returns the annual saving, payback and 10-year net figure. The worked example below runs the same house through an air source heat pump by hand, and the heat pump calculator does that with your own figures.

Prices vary more by boiler type than by badge. A combi, the most common choice, comes in at £2,000 to £4,500 installed, a system boiler with a hot water cylinder at £2,500 to £5,000, and a heat-only or regular boiler at £1,800 to £3,500. A mid-range Worcester Bosch Greenstar 30kW combi lands around £2,800 to £3,500 fitted, a premium Vaillant ecoTEC plus at £3,000 to £4,500, and budget Baxi or Ideal units at £2,000 to £2,800. The boiler on its own retails for £600 to £1,800, so much of the gap is the work itself, meaning pipework alteration, a system flush, the gas safety check and removal of the old boiler. A magnetic system filter from Adey or Spirotech adds £150 to £300 and extends boiler life.

Choosing the type is a question of hot water demand rather than price. A combi heats water on demand with no tank and suits homes with one or two bathrooms and small to medium households. A system boiler keeps heated water in a cylinder and gives mains-pressure showers, which counts once two or more bathrooms might run at the same time. A heat-only or regular boiler uses a cylinder plus a cold water tank in the loft, and still earns its place in low water pressure areas, large old houses and Victorian conversions. Output sizing follows the same logic, with combis at 24 to 40 kW, system boilers at 18 to 30 kW and heat-only boilers at 12 to 25 kW.

Regulation sets a floor under all of this. A SAP assessment is mandatory on new builds and major refurbishments, and Boiler Plus 2018 requires every new gas boiler to have time and temperature controls. Combi boilers must carry one additional efficiency feature on top, chosen from weather compensation, load compensation, smart controls or flue gas heat recovery. Those controls are part of why a modern installation beats the raw efficiency figure printed on the case.

Knowing when to act is worth money. Past 15 years a replacement is usually worth costing out, and two or more breakdowns in a year turns repair against replace into a real decision. Fuel bills rising while usage stays flat point to efficiency falling away. Loud banging, kettling or gurgling suggests scale, limescale or a failing pump, and a yellow flame where it should be blue means incomplete combustion and a carbon monoxide risk, so call an engineer immediately. Water pooling under the boiler almost always means replacement. Cold radiator tops over hot bottoms is usually trapped air or sludge buildup, which a system flush may fix rather than a new boiler. Plan ahead, because an emergency replacement takes 1 to 2 days in cold weather and costs more.