Review · Heating
Greenstar 4000 30kW Combi
The gas boiler to fit if a gas boiler is honestly the right answer for the next five to seven years; a modulating combi with the parts network and installer familiarity that a bridging technology needs.
Review · Editorial · Last reviewed 2026-07-17 · Evidence verified 2026-07-17
Review · Editorial
This review is our considered editorial read; the manufacturer and independent references are cited, but some claims are judgement rather than tested measurement.
Right for
Households whose fabric case for a heat pump has not yet been made, or whose emitter set is not yet ready to run at low flow; the pragmatic choice for a boiler that dies before the retrofit is scoped.
Wrong for
Anyone with a fabric-improved home and correctly sized emitters; specifying gas at that point is buying twenty years of higher running cost to avoid a decision that could have been made once.
How we assessed it
Method, not marketing
Assessed as a bridging technology rather than a destination product; read against modulation depth and part-load efficiency, not headline SEDBUK. Running cost indexed to the Ofgem cap. The whole review is framed by the acknowledgement that new gas boiler installations sit on a policy clock that is only running one way.
Scored on four axes
Comfort in the room
4 / 5
Running character
3 / 5
Installation reality
5 / 5
Service life
4 / 5
Every score is defended in the finding it belongs to; a numeral is not an argument.
What we found
- 01
Modulation depth is real and matters.
The 4000 modulates down to roughly a fifth of its rated output, which keeps the boiler firing steadily on a shoulder-season morning rather than short-cycling; short-cycling is where advertised efficiency and actual efficiency diverge in a real UK home.
Evidence · Manufacturer datasheet
- 02
Return-temperature discipline is doing most of the work.
Any modern condensing boiler earns its rated efficiency only when the return water is below the dewpoint; specifying weather compensation and a flow temperature no higher than fifty-five is worth more on the annual bill than any brand choice at this tier.
Evidence · Independent reference
- 03
The bridging window is narrowing.
Regulatory direction of travel is unambiguous; the sensible planning horizon for a gas boiler installed in the current window is five to seven years, and the fabric and emitter improvements made now should be specified to work with the heat pump that follows.
Evidence · Editorial judgement
What we would buy instead
- aroTHERM plus 7 kW
The heat pump specified in place of the boiler when the fabric and emitters are ready; the destination this bridge is heading toward.
- Ecodan PUZ-WM85
The same destination through the strongest local installer network in much of the UK.
The evidence behind this guide
Sources for the factual claims on this page.
Sources support factual claims about Greenstar 4000 30kW Combi. They do not stand in for editorial judgement; the verdict, suitability calls and price stance on this page are our view, labelled as such. If the evidence weakens, so does the confidence rating.
Evidence verified · 2026-07-17
Official register
- Default tariff cap decisions
Ofgem · Accessed 2026-07-17
Supports · Technical claim · Primary
Independent publication
- Boilers: which is the most efficient?
Energy Saving Trust · Accessed 2026-07-17
Supports · Technical claim · Primary
Manufacturer information
- Worcester Bosch Greenstar 4000
Worcester Bosch · Accessed 2026-07-17
Supports · Technical claim · Product range · Secondary
The gas boiler to fit if a gas boiler is honestly the right answer for the next five to seven years; a modulating combi with the parts network and installer familiarity that a bridging technology needs.