Grant vs Worcester oil boilers

Grant and Worcester Bosch are the two names most off-gas-grid households weigh up when an oil boiler reaches the end of its life. The comparison does not resolve into a single winner, because the two makers publish different things. Grant's Vortex oil boiler range consists of thirty-four models, with outputs ranging from 15kW up to 70kW, and Grant has been designing and manufacturing oil-fired boilers for over 40 years¹. Worcester Bosch sells oil boilers within its Greenstar range, describes them as long-lasting and reliable off-grid home heating, and states that the range is over 90% efficient and A rated².

The clearest published difference is warranty. Grant states that Vortex oil boilers installed by a G1 Oil Boiler Installer are available with a guarantee of up to 10 years, terms and conditions applying¹. Worcester Bosch states that the entire oil boiler range can reach up to a 7 year guarantee when installed by a Worcester Accredited Installer with a Greenstar System Filter, and separately offers a 5 year guarantee on the main primary heat exchanger and a 10 year guarantee on the secondary condensing heat exchanger³.

Price is harder to pin down. Worcester Bosch publishes a range of around £2,500 to about £3,700 for its boilers, while Grant publishes no equivalent list range, so oil boiler prices are installer-quoted in practice⁵. Independent statistics put a heat-only oil boiler at £1,200 to £2,450 for the unit, a system oil boiler at £1,450 to £2,375, and installation across all three oil boiler types at £500 up to £2,000⁶.

Grant or Worcester: what the comparison turns on

The two brands sit in the same market but approach it differently. Grant is an oil specialist: the company has spent over 40 years designing and manufacturing oil-fired boilers, and its Vortex range covers floor standing, wall hung, boiler house and combi boiler models¹. That breadth matters off the gas grid, where a household may need a boiler house unit, a wall hung unit in a utility room, or a combi with no cylinder at all.

Worcester Bosch comes to oil from a much larger gas boiler business. Its gas boilers are made at its headquarters in Warndon, Worcestershire, and the company carries a broad domestic heating range across gas, oil and controls⁷. Its oil boilers sit inside the Greenstar family, which the maker describes as over 90% efficient and A rated for efficiency⁸.

The practical consequence is that Grant's oil range is deeper and more specialised, while Worcester's oil offer is part of a wider brand ecosystem that includes controls, filters and a national accredited installer network. For a household, that shapes what happens at installation and in the years afterwards: which engineer is likely to know the appliance, which parts are held, and how the guarantee is administered.

Both makers publish engineer-facing troubleshooting material, which is a reasonable proxy for how well supported the appliance is in the field. Grant's articles walk through specific combustion faults, including a flame that is slow to stabilise during start-up, a boiler working but with burner pulses, and a boiler working but with visible smoke⁹. Worcester Bosch publishes error symbol guidance for its boilers, explaining what individual key presses and combinations mean¹².

What neither brand changes is the underlying dependence. An oil boiler ties the household to a delivered fuel supply, a storage tank, and an annual service by a competent engineer. Grant's Vortex floor standing oil-fired boilers are HVO compatible, which means they can be converted to using Hydrotreated Vegetable Oil biofuel in a few simple steps, and Grant describes that conversion as involving only a few minor modifications to the boiler and system that an engineer can carry out during an annual boiler service¹³. That is the main route by which either brand reduces, rather than removes, fossil dependence.

Price: Worcester's £730 to £2,500 range and where Grant sits

Worcester Bosch publishes a price band for its boilers of around £2,500 up to about £3,700⁵. Grant does not publish a comparable list range, so the honest position is that Grant oil boiler prices are installer-quoted and no published range exists to set beside Worcester's.

Independent statistics give a fuller picture of what an oil boiler installation costs in the UK. A heat-only oil boiler is listed at £1,200 to £2,450 for the unit, with an average total cost of £3,075 once installation is included. A system oil boiler is listed at £1,450 to £2,375, with an average total cost of £3,163. Installation costs for all three types of oil boiler can vary from £500 up to £2,000⁶.

For context, the same independent source puts a typical gas boiler replacement at £1,500 to £2,300, and another puts gas boilers at around £3,000 to £4,000⁶. Oil sits in a similar band once installation is counted, which is why the brand decision usually turns on warranty and support rather than headline price.

Extended warranty is priced separately by Grant. An extended warranty for an oil boiler, standard or system, is £242.00, and for a combination oil boiler it is £363.00¹⁵. Worcester Bosch prices out-of-guarantee repairs from £381 including VAT, covering the boiler and its Worcester controls for labour, VAT and most parts¹⁶.

Where a household is replacing oil heating altogether, grant funding changes the arithmetic. The Boiler Upgrade Scheme offers a £9,000 grant for oil heated homes, increased from £7,500, and that enhanced uplift is expected to be available until March 2027¹⁷. The Warm Homes: Local Grant, by contrast, does not cover gas and oil boilers as eligible improvements¹⁹.

Documentation and manuals: how readable is each maker's paperwork

Paperwork quality is an underrated part of oil boiler ownership, because an oil appliance needs commissioning data, nozzle and pressure settings, and a service record that satisfies the guarantee. Worcester Bosch's manual readability is measured independently at an average score of 58.82 out of 100 across 18 manuals, ranked sixth among the brands assessed⁶. That is a middling score: readable enough for an engineer, not written as plain English for a householder.

Grant's approach is to publish short troubleshooting articles on its professional support pages and then direct the reader to the manual. Its guidance on a boiler working but with burner pulses ends by telling the reader to refer to the boiler's manual for further support¹⁰. The same pattern runs through its other fault articles, which list checks for a heating engineer rather than instructions for a householder⁹.

Both makers therefore treat the manual as the reference document and the website as a triage tool. For a household, the practical test is whether the installer leaves the commissioning record, the benchmark and the service book in a form the next engineer can use. Worcester Bosch states that, under its oil-fired heat exchanger guarantees, the householder may be asked to prove the date of installation, that the appliance was correctly commissioned, and that the annual service has been completed to the manufacturer's requirements, to the satisfaction of Bosch Thermotechnology Ltd, when requested⁴. That is a documentation requirement as much as a warranty one.

Worcester Bosch also publishes error symbol guidance explaining what the symbols on its boilers mean, including that one key has been pressed and that two keys have been pressed at once¹². Grant's equivalent material is fault-led rather than symbol-led, covering nozzle size and type, fan condition, flue condition, air supply, room ventilation, draught and fuel pressure⁹.

Efficiency and output: what each range offers

Grant's Vortex oil boiler range runs from 15kW up to 70kW across thirty-four models, covering floor standing, wall hung, boiler house and combi formats¹. That spread is wide enough to serve a small cottage and a large rural property from the same family, which is unusual in oil.

Worcester Bosch's oil boilers sit in the Greenstar range, which the maker states is over 90% efficient and A rated for efficiency across both gas and oil models². Worcester describes its Greenstar condensing boilers as A rated for efficiency¹⁷. For comparison, oil-fired range cooker boilers are required to meet a minimum of 80% efficiency under Scottish building standards guidance²⁰.

The efficiency figures are not directly comparable, because Worcester publishes a percentage and Grant publishes an output range. Where an official or independent efficiency figure is needed, the preferred source of boiler efficiency is the Product Characteristics Database, covering space heating and domestic hot water²¹. That is the reference a household or installer should use rather than a maker's headline claim.

Output matters more than efficiency in oil, because an oversized boiler cycles and an undersized one never reaches temperature. Worcester Bosch states that its oil-fired boilers should be sized in accordance with BS5449 and the whole house boiler sizing method, so as to meet the hot water and heating demands of the property⁴. That is the standard to apply to either brand.

Warranty and support compared

This is where the two brands diverge most clearly, and where the terms matter more than the headline number.

Grant states that Vortex oil boilers installed by a G1 Oil Boiler Installer are available with a guarantee of up to 10 years, with terms and conditions applying¹. Its extended warranty for oil boilers covers all electrical and mechanical components, the boiler water jacket, labour and call out costs¹⁵.

Worcester Bosch states that the entire oil boiler range can reach up to a 7 year guarantee when installed by a Worcester Accredited Installer with a Greenstar System Filter³. Separately, the main primary heat exchanger on the Worcester range of oil-fired boilers is eligible for a 5 year guarantee, and the secondary condensing heat exchanger for a 10 year guarantee, both subject to terms and conditions⁴. Worcester describes its oil boiler guarantee as an industry leading, no quibble guarantee³.

Exclusions apply to Worcester's manufacturer guarantee: filters, fire valves, and water and oil storage tanks are not covered¹⁶. That last exclusion is worth noting off the gas grid, where the oil tank is a separate asset with its own inspection and replacement cycle.

Independent guidance notes that Worcester Bosch boilers typically come with seven or ten-year warranties, and that boilers from brands such as Viessmann, Vaillant or Worcester Bosch tend to last longer as they are of higher quality⁷. Independent statistics also record that less than 8% of Vaillant's negative feedback concerned broken boilers, compared with nearly 15% for Worcester Bosch, over the last 12 months⁶. That is a complaint-share figure, not a reliability ranking, and it is included here because it is the only independent comparative data on the two brands' service record in the sources.

Both makers require an annual service to keep the guarantee valid. Worcester Bosch states that the appliance and the system need to be serviced on a regular basis, generally at least once per year⁴. The Chartered Institute of Plumbing and Heating Engineering describes an annual boiler service as essential and a relatively inexpensive piece of maintenance to undergo each year²³.

Which oil boiler fits which home

The choice usually follows the property, not the brand. Grant's range covers floor standing, wall hung, boiler house and combi models, so a household with a dedicated boiler house, a utility room wall, or no space for a cylinder can all be served from the same family¹. Worcester's oil boilers sit within the Greenstar range and are positioned for off-grid home heating³.

Where a cylinder is already in place and working, a system or regular oil boiler keeps the existing hot water arrangement. Where there is no cylinder and no space for one, a combi removes it. Where the property is large or has multiple bathrooms, output becomes the constraint, and Grant's 15kW to 70kW spread gives more headroom at the top end¹.

Sizing should follow BS5449 and the whole house boiler sizing method, which Worcester Bosch names as the standard for meeting a property's hot water and heating demands⁴. That applies to either brand.

Fuel choice is the other variable. Grant's floor standing Vortex oil-fired boilers are HVO compatible, convertible to Hydrotreated Vegetable Oil biofuel in a few simple steps, with the modification carried out during an annual service¹³. That gives a household a route to a lower-carbon liquid fuel without replacing the appliance. Worcester does not state an equivalent HVO position for its oil boilers in the material available.

For a household, the independence question is the same either way. An oil boiler keeps the home warm but leaves it dependent on delivered fuel, a storage tank, an annual service and a competent engineer. Grant's HVO compatibility and Worcester's wider controls ecosystem reduce some of that dependence at the margins; neither removes it. Households comparing oil with other off-grid options can read more in oil boilers and heating oil and heating a home off the gas grid, and the Grant Vortex range and Worcester Bosch Greenstar Heatslave II pages cover each maker's oil models in more detail.