The Worcester Bosch Greenstar Heatslave II 18/25 is a floor-standing oil combi boiler for homes that aren’t on the mains gas network. It heats your radiators and gives hot water on demand without a separate cylinder. There’s an internal version for a kitchen or utility room, and an external version that sits outside the house.
It’s still on sale. Worcester updated the range in 2022, and the current 18/25 models are listed as “2022+” versions. We’ll cover what the 18/25 does well, where it falls short, what size home it suits and what an installation typically costs.
What Are The Heatslave II 18/25’s Key Specs?
It’s a 25kW floor-standing oil combi with hot water flow of up to 18 l/min, sold in internal and external versions.
| Specification | Greenstar Heatslave II 18/25 |
| Boiler type | Oil combi, floor-standing |
| Versions | Internal and external |
| Maximum central heating output | 25kW |
| Hot water flow rate | Up to 18 l/min |
| ErP rating | A or B depending on version (check the label on the model quoted) |
| Internal dimensions (H x W x D) | 855 x 520 x 600mm |
| External dimensions (H x W x D) | 950 x 640 x 770mm |
| Fuel | Heating oil (kerosene) |
| Other sizes in range | 12/18 and 25/32 |
| Status | Current (2022+ versions) |
What Is The Worcester Greenstar Heatslave II?
It’s Worcester’s range of oil-fired combi boilers, which Worcester describes as the UK’s best-selling oil boiler.
The name tells you the size. The 18/25 has a maximum central heating output of 25kW, with the smaller 12/18 and larger 25/32 either side of it.
A gas combi hangs on the wall, but the Heatslave II stands on the floor. The internal model is built to fit under a standard kitchen worktop, so it can sit alongside your units.
The external model lives in a weatherproof casing outside. That frees up indoor space and keeps the burner noise out of the house.
Inside, the boiler keeps a small store of water hot so it reaches the tap quickly. Worcester calls this “rapid comfort”. There are standard and eco settings, so you can trade faster hot water for lower oil use.
What Are The Heatslave II’s Key Features?
Its big selling point is combi convenience on oil, with no cylinder or loft tanks to find room for.
- Combi convenience on oil. Heating and mains-pressure hot water from one appliance.
- Internal or external. Choose the version that suits your space. The external model is frost-protected for winter.
- Fits under a worktop. The internal 18/25 is sized to slot in beneath a kitchen worktop.
- Eco and standard hot water modes. Eco mode cuts the time the boiler spends keeping water hot, which saves oil but means a slightly slower first draw.
- Concealed controls. The control panel sits behind a front cover, which keeps the front tidy.
- Smart control support. It works with Worcester and Bosch smart controls, and it’s also listed as solar compatible.
- Flexible flueing. Room-sealed flues, conventional flue kits and plume management kits help on awkward sites.
How Efficient Is The Heatslave II 18/25?
It’s a condensing boiler, so it uses less oil than older non-condensing models by recovering heat from the flue gases.
Oil boilers over 8 years old are usually less efficient than when they were new, so replacing one often brings a drop in oil use. That assumes the system is set up properly.
Oil prices move around more than gas, so running costs depend heavily on when you fill the tank.
A few habits help whatever the price. Get the boiler serviced every year and use eco mode if you don’t need instant hot water. A programmable or smart thermostat also stops the heating running when nobody needs it.
What Are The Pros And Cons Of The Heatslave II 18/25?
It’s a proven, well-supported oil combi, but it costs more to fit than gas and relies on decent mains pressure.
Pros
- Combi convenience for homes off the gas grid
- Strong hot water flow for an oil combi of this size
- External version saves indoor space and keeps noise outside
- Internal version fits under a standard worktop
- Widely fitted, so engineers and parts are easy to find
Cons
- More expensive to buy and fit than a gas combi
- Hot water performance still depends on your mains pressure and flow
- Needs an oil tank, oil deliveries and annual servicing by an OFTEC-registered engineer
- Not ideal for homes with three or more bathrooms used at once
What Size Home Does The Heatslave II 18/25 Suit?
It suits most two to four-bedroom homes with one or two bathrooms.
Its 25kW output covers the heat demand of a typical detached rural house with reasonable insulation. The hot water flow is enough for one shower at a time, with some left over for a tap.
If your home is small or very well insulated, the 12/18 may be the better match. For a larger, older property with high heat loss, look at the 25/32.
Where several bathrooms are used together, any combi will struggle. An oil system or regular boiler with a hot water cylinder copes better, and our guide to the different types of boilers explains the trade-offs.
How Much Does It Cost To Install In 2026?
A like-for-like Heatslave II 18/25 swap typically costs around £4,500 to £6,500, depending on the version.
Oil installations vary more than gas jobs because of the tank, flue route and pipework. The ranges below are a guide only, for an 18/25 fitted by a registered installer. They assume the existing oil tank is in good condition, and your quote will depend on your property.
| Job | Typical installed price guide |
| Like-for-like swap, internal 18/25 | £4,500 to £6,000 |
| Like-for-like swap, external 18/25 | £5,000 to £6,500 |
| Conversion from regular oil boiler to combi | Add £800 to £2,000 |
| New bunded oil tank (if needed) | Add £2,000 to £3,500 |
To see the oil and LPG boilers we fit, visit our LPG and oil boilers page.
What Are The Best Alternatives To The Heatslave II 18/25?
The 12/18 or 25/32 Heatslave II is the simplest switch if you just need a different output.
- Worcester Greenstar Heatslave II 12/18 or 25/32. The same boiler in a smaller or larger output.
- Worcester Greenstar Danesmoor. Worcester’s oil system and regular boilers, better for homes with several bathrooms and a hot water cylinder.
- Grant Vortex combi. A popular oil combi from another established UK oil boiler maker.
- An air source heat pump. For some off-gas homes a heat pump is now a realistic option, and government grants can reduce the upfront cost. It depends on insulation, radiator sizes and space for the outdoor unit.
Is The Worcester Greenstar Heatslave II 18/25 Worth It?
Yes, for an off-gas home with one or two bathrooms that wants to stay on oil, it’s one of the safest choices you can make.
It’s well proven, widely supported and gives you combi convenience without a cylinder. The external version is a good option if indoor space is tight.
Choose it if
- Your home is off the gas grid and you already have an oil tank
- You want hot water on demand without a cylinder
- You have one or two bathrooms and decent mains water pressure
Look elsewhere if
- Several showers often run at the same time
- Your mains water pressure is low
- You’re ready to move away from oil and your home suits a heat pump
Get A Free Boiler Quote From Clever Energy Boilers
Clever Energy Boilers are Worcester Bosch accredited installers. We help homeowners and landlords across the North West, Yorkshire and the Midlands choose the right oil or gas boiler.
Before we recommend a model and size, we’ll check your heat demand, hot water use and the condition of your tank.
Our Worcester boiler installations are carried out by experienced, professional and registered installers. Long warranty cover is available through Clever Energy on qualifying installs.
Get your fixed price online with a free boiler quote.
Frequently Asked Questions
What Does 18/25 Mean On A Worcester Heatslave?
It describes the boiler’s output. The 18/25 has a maximum central heating output of 25kW and sits between the smaller 12/18 and the larger 25/32 in the Heatslave II range.
Is The Worcester Heatslave II Still Available?
Yes. The Heatslave II 18/25 is still sold in internal and external versions, and the current models are the updated 2022+ versions.
What Is The Hot Water Flow Rate Of The Heatslave II 18/25?
Up to 18 litres per minute, according to Worcester. The flow you actually get depends on your incoming mains pressure and flow rate.
Should I Choose The Internal Or External Heatslave II?
Go external if you want to save indoor space or keep boiler noise out of the house. The internal version is the better fit if you’ve already got room in a kitchen or utility and a simple flue route.
How Often Does An Oil Combi Need Servicing?
Once a year, by an OFTEC-registered engineer. Regular servicing keeps the burner clean and running efficiently.