Worcester Bosch Greenstar Heatslave II oil combi

The Worcester Bosch Greenstar Heatslave II is an oil-fired combination boiler: it burns heating oil and heats domestic hot water on demand, with no stored cylinder. Worcester Bosch describes the External variant as "The UK's best-selling oil boiler"¹, and the range is sold in outputs from 12kW to 32kW, which covers most oil-heated homes in the UK².

For a household off the gas grid, this is the mainstream choice. It keeps the home on delivered fuel rather than mains gas, which means a tank in the garden, a delivery schedule and a supplier relationship, but no standing charge to a gas network. Hot water arrives without waiting for a cylinder to recover, and the boiler can be sited indoors under a worktop or outside entirely¹.

The guarantee is where the detail matters. The standard term is 2 years, and the headline of up to 7 years depends on who fits the boiler and what is fitted alongside it⁴. This page sets out the outputs, the siting options, the guarantee tiers, the controls, and what owning an oil combi means for a household's energy position.

What the Greenstar Heatslave II is: an oil combi with instant hot water

The Heatslave II is a combination boiler running on oil. Worcester Bosch lists the fuel type simply as oil, and describes the hot water provision as "Rapid comfort, Hot water without waiting"². The External model is described as "providing instant hot water, however off-grid you are"¹, which is the clearest statement of who the product is for: properties with no mains gas connection.

A combi removes the cylinder and the cold water tank from the system. Water is heated as it is drawn, so there is no stored volume to run cold and no recovery time between showers. The trade-off is flow rate: a combi serves one outlet well at a time, and simultaneous demand from two bathrooms is limited by the boiler's hot water output rather than by a tank's stored volume. Worcester Bosch lists a maximum domestic hot water output of 15 kW on the Heatslave II 12/18 2022 onwards⁴.

The wider oil range from Worcester Bosch includes combi, system and regular models, and the maker states that combi oil boilers "provide instant hot water" as a category⁵. The Heatslave II is the combi in that line-up. Households that need to serve several bathrooms at once, or that want a cylinder for other reasons, sit outside what a combi does, and the oil boilers page sets out the alternatives.

For energy independence, the position is mixed and worth stating plainly. An oil combi takes the household off the gas network entirely, which removes one utility dependence. It replaces it with a tank that must be filled, a distributor who delivers, and a fuel whose price moves with global markets rather than with a regulator's cap. The boiler itself needs electricity for ignition, controls and pumping, so a power cut stops the heating regardless of how full the tank is.

Output range: 12kW to 32kW for oil-heated homes

Worcester Bosch describes the Heatslave II as "Flexible, Suits many homes, with sizes from 12kW-32kW"³, and the same 12kW to 32kW span appears on the product page and on the External model's page². That is the central heating output range across the family, and it is the figure an installer works from when sizing a system.

The range is wide enough to cover a small cottage and a large detached house. At the bottom end, 12kW suits a well-insulated smaller property with modest radiator demand. At the top, 32kW covers larger, less insulated homes or those with high hot water demand. The specific model within the range is chosen after a heat loss assessment, not from floor area alone, and the boiler sizing page explains how that calculation works.

For context on where the Heatslave II sits in the wider Worcester Bosch oil line-up, the Danesmoor Utility and Danesmoor External System both run across the same 12kW to 32kW band⁶, while the Greenstar Utility Regular is a larger, non-combi model spanning 32kW to 70kW for properties needing more raw output³. The Heatslave II is therefore the mid-range combi option rather than the largest oil appliance the company makes.

The output figure that matters most for a combi is not the central heating number but the hot water performance, because that is what limits simultaneous use. Worcester Bosch publishes a maximum domestic hot water output of 15 kW for the 12/18 model⁴, and an 18 kW central heating output for the same unit⁴. Households with more than one bathroom in regular simultaneous use are the ones most likely to find a combi's limits, and that is a design conversation rather than a specification one.

Siting and installation: under a worktop, or outside with the External model

The Heatslave II is described as "Flexible enough to go under a kitchen counter"², and the External model carries the same under-counter description alongside its outdoor role¹. The External variant is the one designed for outdoor installation: Worcester Bosch lists it as "Space saving, Choose to fit outside for extra space"³, which removes the appliance from the internal floor plan altogether.

That matters in smaller properties and in conversions where internal space is tight. An outdoor oil boiler also moves the noise and the flue terminal outside the living space, and it removes the need for an internal flue route through a wall or roof. The trade-off is weather exposure, and Worcester Bosch addresses that directly: the External model is described as "Frost protected, High performance even in harsh weather conditions"¹.

Siting is not a household decision made from a brochure. Worcester Bosch states that "Each heating system needs custom designing" and directs buyers to find and contact a Worcester Accredited Installer for a home visit and free quote². The installer assesses the property, the flue position, the oil supply route from the tank and the clearances before confirming where the boiler goes. The boiler installation page covers the process, and domestic oil tanks covers the tank side of the installation, which is a separate set of rules from the boiler itself.

For a household, the practical point is that the External option widens the number of properties where an oil combi can be fitted at all. A kitchen with no spare floor space, or a utility room already full, is not a barrier if the boiler can go outside. What it does not change is the dependence on a tank, a delivery, and a flue position that satisfies the regulations.

Guarantee: up to 7 years, and how the tiers work

The guarantee on the Heatslave II is not one number. The standard term is 2 years⁴, and the longer terms are conditional on the installer and on what is fitted with the boiler.

Worcester Bosch states "Up to a 7 year guarantee when installed by a Worcester Accredited Installer" on the Heatslave II and on the External model, with terms and conditions applying². The same 7 year term appears across the oil range, including the Danesmoor Utility, the Danesmoor External System and the Greenstar Utility Regular⁶. Separately, Worcester Bosch states that "You can get up to an 7 year guarantee across our entire oil boiler range when you have your boiler installed with a Greenstar System Filter" by a Worcester Accredited Installer⁵, and elsewhere that "Up to 5 Years Guarantee available on all Worcester Bosch oil boilers"¹⁰.

The tier structure on the Heatslave II is set out on the professional product page. With any Greenstar System Filter, the guarantee is 5 years. With any Greenstar System Filter plus a £40 payment, it is 7 years, but Worcester Bosch lists that route as "Non-WAI loyalty members only"⁴. The External model carries the identical structure¹¹.

Two further conditions are worth knowing. Filters are excluded from the manufacturer guarantee¹², so the Greenstar System Filter that unlocks the longer term is not itself covered by it. And a Worcester Bosch control installed with a new boiler "will receive the same guarantee length as your new boiler"¹³, which means the control's cover rises or falls with the boiler's.

The boiler warranties page covers registration and what a guarantee does and does not include. For a household, the practical consequence is that the installer choice determines the cover: an otherwise identical Heatslave II fitted by a non-accredited installer carries the 2 year standard term unless the loyalty scheme route applies.

Controls and solar compatibility: Comfort, EasyControl and solar link-up

The Heatslave II 12/18 2022 onwards is listed as EasyControl compatible and solar compatible², and the External equivalent carries the same two listings¹. Solar compatibility means the boiler can work alongside a solar water heating arrangement rather than generating anything itself, and the solar thermal water heating page explains what that involves.

On controls, Worcester Bosch sells the Greenstar Comfort range. The Comfort II is listed as both wall mounted and boiler mounted¹³, which gives a choice between a control on the wall and one fitted to the appliance itself. The Comfort I is also available, and Worcester Bosch directs buyers to a Worcester Accredited Installer to book a home visit and get a free quote for it¹⁴. The heating controls and thermostats page covers what controls do for running costs generally.

The guarantee link is the point that ties controls to the boiler. A Worcester Bosch control installed with a new boiler receives the same guarantee length as the boiler¹³, so a 7 year boiler guarantee carries a 7 year control guarantee. That is a reason the controls are usually specified at the same time as the appliance rather than added later.

For energy independence, controls are where a household gains the most from an oil system without changing fuel. Zoning and timed control reduce the hours the burner runs, which reduces oil consumption, which reduces the number of deliveries. What controls do not do is change the fuel dependence: the boiler still burns oil, and the household still buys it from a distributor. Solar compatibility adds a renewable input to the hot water side, but it does not remove the tanker from the drive.

Who Worcester Bosch are and what their reputation rests on

Worcester Bosch is part of the Bosch group¹⁵. The company manufactures and sells across gas and oil boilers, controls and cylinders, and its oil range sits alongside a much larger gas range¹⁵. The brand's reputation in the UK rests on that scale, on a manufacturer-run repair and guarantee service, and on the guarantee lengths it attaches to accredited installation.

The guarantee terms themselves show how the company structures that relationship. Worcester Bosch runs its own repair service, and the manufacturer guarantee covers defects in manufacture, parts and labour. It excludes external pumps, filters, fire valves and external electric wiring¹². On Worcester Greenstar gas-fired boilers, the primary heat exchanger "is guaranteed against faulty materials or workmanship for a period of 120 calendar months from the date of installation", and the warranty states that a heat exchanger "which has proved to be faulty or defective in manufacture will be exchanged or repaired free of material charges (and free of labour charges if within the main product warranty period) if repaired directly by Bosch Thermotechnology Ltd"¹².

That clause is the reason installation quality and servicing matter to the guarantee, not just to the boiler. The company's oil range also includes the Greenstar Danesmoor and Greenstar Utility families, and the Worcester Bosch boilers page covers the full line-up. For households comparing oil brands, Grant vs Worcester oil boilers sets the two ranges side by side.

Cost and what owning an oil combi means for energy independence

Worcester Bosch publishes no list price for a supplied and fitted Heatslave II. Prices are installer-quoted, following the home visit and system design that the maker states every installation requires². No published figure for this model is available, so no range is given here.

What can be said is what the fuel choice means. Oil heating keeps a household off the gas network, which is a genuine independence from one utility and from the gas price cap. It substitutes a different set of dependencies: a storage tank on the property, a distributor who delivers, a fuel priced on international markets, and electricity for ignition, controls and pumping.

The wider evidence on fuel switching is mixed and worth reading carefully. Independent guidance on bioLPG states that "switching to biomass from oil would result in a monetised air quality damage equivalent of +£831, compared to -£42 you w" for the alternative being compared¹⁶. An independent consultation on Scottish energy policy states that "More homegrown energy means greater energy independence"¹⁷. These are separate findings on separate questions, and neither is a running cost comparison for the Heatslave II.

The honest position for a household is that an oil combi is a fuel-dependent appliance. It delivers instant hot water and removes the cylinder, and it keeps the home off mains gas. It does not make the home self-sustaining, and the heating and energy independence page sets out what would. Households weighing the fuel itself should read gas vs oil boiler running costs and LPG vs oil boilers, and off-grid home heating covers the wider options.

How to get one: accredited installers and home visits

Worcester Bosch does not sell boilers direct to householders for installation. The route is through a Worcester Accredited Installer, and the maker states that "Each heating system needs custom designing. Find and contact a Worcester Accredited Installer" for a home visit and free quote². The same instruction appears on the controls pages: find and contact a Worcester Accredited Installer in your area to book a home visit and get a free quote¹⁴.

The process is therefore:

1. Find a Worcester Accredited Installer in the area.
2. Book a home visit, at which the existing system is assessed and the heating system is designed.
3. Receive a free quote covering the boiler, the controls and the installation.
4. Confirm the guarantee tier at the point of order, since it depends on the installer's accreditation and on whether a Greenstar System Filter is included⁴.

The installer's accreditation is not a formality. It is the condition on which the 7 year guarantee rests, and the £40 route to the same length is restricted to loyalty scheme members¹¹. A household that wants the longest cover needs to establish the installer's status before the work is booked, not after.

For the wider picture on who may legally work on oil heating and what registration involves, the OFTEC registration page covers the technician side. The new boiler cost page covers what a replacement generally involves, and boiler servicing and maintenance covers what keeps a guarantee valid after installation.