Worcester Bosch Greenstar 4000 Review: A Sensible Mid-Range Combi

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The Worcester Bosch Greenstar 4000 is one of the most-fitted combi boilers in the UK and the model that replaced the Greenstar i, the boiler it was designed to drop straight onto. It is the one your installer will most likely suggest if you ask for “a Worcester for around two and a half grand”, and it is genuinely the cheapest Greenstar that reaches a ten-year guarantee. It is also built in Worcester, which is rarer in this industry than the marketing suggests.

Two things about it are worth knowing before you buy, and neither gets said often. The modulation depends on which output you pick: the 30kW modulates 1:10, the 25kW only 1:8. And the ten-year guarantee is conditional on a genuine Greenstar System Filter being fitted, which is the single most common reason a Worcester claim gets refused.

This review covers what the 4000 is good at, where it falls short of the pricier 8000, what you will actually pay, and the easiest way to buy it.

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Worcester Greenstar 4000 specs at a glance

Who the Greenstar 4000 is for

The 4000 is the right boiler if you:

• Live in a small to medium home with one bathroom, or two on the 30kW
• Want the most-installed boiler brand in the UK so any Gas Safe engineer can service it
• Are replacing an older Greenstar i, where SimpleSwitch means the pipework and fixings line up
• Have low mains water pressure, where the 4000’s full flow from 1 bar is a genuine advantage
• Want a ten-year guarantee at the lowest price Worcester offers one

Look elsewhere if you:

• Have three or more bathrooms needing more than 12.3 L/min, where the Greenstar 8000 or a Vaillant ecoTEC Plus 838 is the answer
• Want a stainless steel heat exchanger, which only the cheaper Greenstar 1000 offers in Worcester’s range
• Want the deepest modulation, where the Vaillant ecoTEC Plus runs 12:1
• Want a long guarantee with no filter requirement and no accreditation strings, where Alpha gives you ten as standard

Heating performance

Worcester’s own specifier guide is clear on modulation, and it is worth quoting properly because almost every review gets it wrong: the 25kW combi modulates 1:8 and the 30kW modulates 1:10. Both can throttle down to 3kW of central heating output; the 30kW simply has a wider range above that.

What that buys you is fewer short cycles. On a mild spring morning when your home barely needs heat, a boiler that cannot turn down far enough fires, satisfies the thermostat almost immediately, shuts off and repeats. Every cycle costs a little efficiency and a little wear. At 1:10 the 30kW keeps ticking over gently instead. The Vaillant ecoTEC Plus goes further at 12:1, and in practice that difference shows up as maybe 2% on an annual gas bill, real but small.

Both outputs are ErP A rated for heating and hot water, the same as every modern condensing combi in this class. Where the 4000 stands apart is siting: it achieves full flow rates from just 1 bar of mains pressure, which makes it genuinely useful in flats, high-rise installations and anywhere with a weak incoming supply. That is a practical advantage most spec sheets bury.

It is also Quiet Mark accredited at 45 dB(A) on the combi, around 10% quieter than the Greenstar i it replaced, though the Greenstar 2000 is quieter still if noise is your priority.

What about the Greenstar 4000 system boiler?

The 4000 also comes as a system boiler in 12kW, 15kW, 18kW, 21kW and 24kW, for homes with a hot water cylinder rather than a combi setup. The 18kW and 24kW are the most-fitted, typically going into larger homes where simultaneous hot water demand makes a combi impractical. Note the system variants modulate 1:6 to 1:8 rather than 1:10, and run quieter still at 42 to 45 dB(A).

The system unit costs roughly £100 to £200 less than the equivalent combi, but you will need a hot water cylinder, which adds £500 to £900 to a fresh install. Both iHeat and Heatable quote system installs through their online tools.

Hot water performance

The 30kW produces 12.3 litres per minute at a 35°C rise, the standard industry comparison figure. At the 40°C rise that reflects real operational use, it delivers 10.8 L/min. That is enough for one strong shower at full flow, or a shower running while a kitchen tap fills. It is not enough for two strong simultaneous showers.

The 25kW drops to 10.2 L/min (9 L/min at 40°C), which is fine for a one-bathroom flat or small terrace but starts to feel weak in a 3-bed semi where two adults shower in series. Combined with its shallower 1:8 modulation, the 25kW is the weaker boiler in two ways at once. Most homes should default to the 30kW.

Worcester rates the 25kW for around 10 radiators and one bathroom, and the 30kW for up to 16 radiators and two bathrooms. Compared with the Greenstar 8000, the 4000 30kW is only marginally behind on hot water, so you are not paying the 8000 premium for flow alone.

Smart controls and voice assistant compatibility

Worcester’s EasyControl smart thermostat is an optional add-on at around £250 fitted, and works with both outputs. It brings remote scheduling, holiday mode, energy tracking, room-by-room control through Bosch TRVs, and voice control via Google Assistant or Amazon Alexa. The “Greenstar 4000 with voice control by Google” listings you will see on installer sites are the EasyControl bundle rather than a different boiler.

The 4000 also supports OpenTherm, which lets third-party thermostats such as tado°, Honeywell Evohome and Drayton Wiser modulate the boiler properly rather than just switching it on and off. Worcester’s OpenTherm implementation is basic compared with Vaillant’s, so you will capture some but not all of the benefit.

Worth asking your installer about either way: a modulating control is the cheapest way to actually use the 1:10 modulation you have paid for. Without one, the boiler spends more of its life cycling than it needs to.

Build quality and reliability

The 4000 is built at Worcester Bosch’s Worcester factory, the same site that produces the 8000 series. Worcester consistently tops Which? and Trustpilot reliability surveys among UK manufacturers, with Vaillant the closest competitor.

The heat exchanger is cast aluminium, an evolution of Worcester’s WB3, and it carries its own 10-year guarantee even where the boiler’s own cover is shorter. It is worth being straight about this: the cheaper Greenstar 1000 is the only Greenstar with a stainless steel primary, and paying up to the Greenstar 8000 does not get you one either. Aluminium conducts better and is cheaper to cast, but is more sensitive to system water quality. In its favour, aluminium has wider waterways than stainless steel, so it copes better with debris from old pipework, which makes the 4000 a safer bet than the 1000 on a conversion from a sludgy gravity-fed system.

Servicing is a genuine strength. Access to the burner and heat exchanger baffles takes one screw and one bolt, there is a bottom access hatch for cleaning, and a digital pressure gauge on the display alongside a traditional one near the hydraulics so you can check pressure with the power off. Spare parts availability is excellent; any UK Gas Safe engineer can source 4000 parts within 24 to 48 hours, often same day.

The guarantee, and the filter that decides it

The Greenstar 4000 carries a 7-year guarantee as standard, extending to up to 10 years when three conditions are met: a genuine Greenstar System Filter is fitted, the boiler is installed by a Worcester Accredited Installer, and it is registered within 30 days.

Worcester’s guarantee is parts and labour including the call-out, not parts only. If a heat exchanger fails in year six, Worcester sends an engineer at no cost. Some rival warranties cover parts alone, leaving you a labour bill on a supposedly free repair.

What the Worcester 4000 actually costs

Boiler-only prices sit at around £1,250 to £1,500 for the 25kW and £1,500 to £1,750 for the 30kW from major merchants. Those are not the prices you pay through an installer, who quotes a single fitted figure.

• Greenstar 4000 25kW, like-for-like swap: ~£2,000–£2,400 fitted
• Greenstar 4000 30kW, like-for-like swap: ~£2,200–£2,600 fitted
• Greenstar 4000 30kW, conversion (system to combi): ~£2,800–£3,400 fitted

For a typical 30kW like-for-like swap at £2,400 fitted, finance breaks down roughly as follows:

The 0% deal is cheapest in absolute terms but has the highest monthly figure. The 10-year plan looks attractive monthly but costs roughly £1,800 more in interest across the term. The 5-year plan is usually the sensible middle. Both iHeat and Heatable publish your exact figure inside the quote tool.

Worcester 4000 vs the alternatives

vs Greenstar 8000. The premium tier, but it uses the same cast aluminium heat exchanger. What it buys is a 12-year guarantee ceiling against the 4000’s 10, far higher hot water flow at the top outputs, EasyControl included, and a more refined cabinet. Roughly £400 to £600 more fitted. Worth it if you have real simultaneous shower demand; at 30kW the two flow identically and the 4000 is the smarter buy.

vs Greenstar 2000. Roughly £200 to £300 cheaper fitted. Both carry a 7-year standard guarantee and both use aluminium, but the 2000’s seven years is a hard ceiling that Worcester will not extend, where the 4000 reaches ten with a filter. The 2000 is quieter and takes LPG. If you want the ten years, you have to buy the 4000; there is no cheaper route to it in the Worcester range.

vs Greenstar 1000. The odd one. The cheapest Greenstar has a stainless steel heat exchanger the 4000 does not, but only 5:1 modulation, weaker hot water and a five-year guarantee that cannot be extended. Better component, worse everything else.

vs Vaillant ecoTEC Plus. The closest competitor at this price. The Vaillant has better modulation at 12:1 and a stainless steel heat exchanger; Worcester has the larger service network and the UK factory. Pricing is essentially the same.

vs Viessmann Vitodens 100-W. The Viessmann gives you a stainless steel heat exchanger and built-in WiFi as standard, usually for less. The Worcester counters with the biggest service network in the country and a guarantee that does not hinge on registering inside 30 days.

vs Ideal Logic Combi 30. Ideal sits a tier below on build and guarantee length but is roughly £200 to £300 cheaper fitted. The right choice if budget is tight and you are not staying long.

Pros and cons

Frequently asked questions

Is the Worcester Greenstar 4000 a good boiler?

Yes, for most UK homes it is one of the safest mid-range choices on the market. It is reliable, genuinely built in Worcester, easy to service, Quiet Mark accredited, well supported by spare parts, and the cheapest Greenstar reaching a ten-year guarantee. It is not the most technically advanced boiler, and it uses an aluminium heat exchanger where the cheaper Greenstar 1000 uses stainless steel, but it is the one most engineers would happily fit in their own home.

What is the modulation ratio of the Greenstar 4000?

It depends on the output, which most reviews miss. Worcester’s own specifier guide gives the 25kW combi a 1:8 modulation ratio and the 30kW combi 1:10. Both throttle down to a 3kW minimum central heating output; the 30kW simply has a wider range above it. The system variants modulate 1:6 to 1:8. For deeper modulation than any of them, the Vaillant ecoTEC Plus runs 12:1.

What is the guarantee on a Worcester 4000?

Seven years as standard, extending to up to ten years when a genuine Greenstar System Filter is fitted, the boiler is installed by a Worcester Accredited Installer, and it is registered within 30 days. The guarantee covers parts and labour including the call-out. Annual servicing by a Gas Safe registered engineer is required throughout. Installers on Worcester’s Excelerate scheme who are not accredited can still reach the extended length by paying a fee after registration.

Does the Greenstar 4000 have a stainless steel heat exchanger?

No. The Greenstar 4000 uses a cast aluminium primary heat exchanger, an evolution of Worcester’s WB3, which carries its own 10-year guarantee. Oddly, the cheaper Greenstar 1000 is the only Greenstar with a stainless steel primary, and the flagship 8000 is aluminium too. Aluminium is more sensitive to water quality but has wider waterways, which cope better with debris from old pipework.

Should I buy the 25kW or 30kW Greenstar 4000?

The 30kW for most homes. It flows 12.3 litres per minute against the 25kW’s 10.2, and modulates 1:10 against 1:8, for roughly £150 more fitted. Worcester rates the 25kW for around 10 radiators and one bathroom, and the 30kW for up to 16 radiators and two bathrooms. Only choose the 25kW if you have a genuinely small home, since it is the weaker boiler on both flow and modulation.

How much does a Worcester 4000 cost installed?

For a typical 30kW like-for-like swap, expect £2,200 to £2,600 fitted. Conversions, such as system to combi or back-boiler to combi, are typically £2,800 to £3,400. Fixed-price online installers such as iHeat and Heatable quote the exact figure for your home in under two minutes.

What is the difference between the Greenstar 4000 and the 8000?

The 8000 uses the same cast aluminium heat exchanger as the 4000. What it buys is a 12-year guarantee ceiling against the 4000’s 10, far higher hot water flow at the top outputs, EasyControl included, and a more refined cabinet. At the 30kW output the two flow identically, so the 4000 covers most use cases for £400 to £600 less.

Will the Greenstar 4000 fit in a kitchen cupboard?

Usually. It measures 724mm high, 400mm wide and 310mm deep, so it fits a standard kitchen cupboard, though it needs 5mm at the sides, 186mm below and clearance at the front for servicing. It also achieves full flow from 1 bar of mains pressure, which makes it a common choice for flats and high-rise installations.

Does the Worcester 4000 come with smart controls?

No, smart controls are an optional add-on. The standard install includes a basic programmable thermostat. Worcester’s EasyControl smart thermostat, which adds app control, energy tracking and voice control via Google Assistant or Alexa, is around £250 fitted. The 4000 also supports OpenTherm for third-party thermostats such as tado° and Honeywell Evohome, though in a basic form.

Final verdict

The Greenstar 4000 is the boiler we would recommend to most UK homeowners by default. A sensible, mid-range combi genuinely built in Worcester, from the country’s most-installed brand, Quiet Mark accredited, easy to service, and the cheapest route to a ten-year Worcester guarantee. For the typical one-bathroom home it gives you the fewest things to worry about across a decade of ownership.

Two honest caveats. It uses a cast aluminium heat exchanger where the cheaper Greenstar 1000 uses stainless steel, and paying up to the 8000 does not fix that, since Worcester uses aluminium there too. And the ten-year guarantee is not automatic: it needs a genuine Greenstar filter and an accredited install, or you are on seven.

Our recommendation: buy the 30kW rather than the 25kW, insist on the filter, and register it. If you want a stainless steel exchanger at this money, the Viessmann Vitodens 100-W is the alternative. For where it sits across the market, see our best combi boiler UK guide.

Don’t forget your EPC

A new boiler usually improves your home’s energy rating, but your existing EPC won’t show it. If you’re selling, letting, or your certificate is coming up to ten years old, it’s worth getting a fresh assessment booked.

Get an EPC from £65 → Accredited assessors, nationwide

This review was last updated in July 2026. Specifications are taken from Worcester Bosch’s own Greenstar 4000 specifier guide. Prices and finance terms are indicative and accurate at time of publishing, and guarantee terms change over time, so always confirm the details for your specific quote before committing. We are not a credit broker, lender or installer. iHeat and Heatable handle quoting, finance and installation directly.