Worcester Bosch Greenstar vs Vaillant ecoTEC for a 100-Square-Metre Bungalow

Start with the heat loss figure, not the boiler catalogue

A well-built 100 m2 bungalow with 270 mm loft insulation and filled cavities loses heat at roughly 4-6 kW at design outdoor temperature. An older bungalow with an uninsulated loft and solid walls can sit at 9-12 kW for the same floor area. That single number decides more about the shortlist than any brand loyalty, because both the Worcester Bosch Greenstar and the Vaillant ecoTEC are sold in output bands that overshoot most bungalows by a wide margin.

The Greenstar 4000 comes in 25 kW and 30 kW combi variants; the ecoTEC plus 832 delivers 32 kW to the domestic hot water side. For space heating, both throttle down. The relevant specification is the minimum modulation output, not the badge figure. A boiler that will not modulate below 6 kW spends a mild October cycling on and off against a house that only wants 3 kW, and short-cycling is where efficiency and component life both erode.

Modulation range is where the two diverge

The Vaillant ecoTEC plus 832 modulates down to roughly 1:10 on the central heating circuit, meaning a 32 kW appliance can trickle heat at around 3 kW. The Worcester Bosch Greenstar 4000 sits closer to 1:6 on the smaller outputs. For a tightly insulated bungalow with a 4-5 kW design load, that difference is not cosmetic. The ecoTEC will hold a steady low flame through a mild afternoon; the Greenstar is more likely to fire, satisfy the flow temperature, and shut off in a repeating pattern.

Short-cycling has measurable consequences. Each ignition cycle draws the fan and pump through a start sequence, and condensing efficiency depends on return water staying below about 55 degrees so the flue gases give up their latent heat. A boiler that fires briefly and stops never settles into the low return-temperature regime where the quoted 94 percent ErP efficiency actually appears. In practice the seasonal figure for an oversized, poorly-modulating install can drift several points below the label.

This is the argument for sizing down. A 25 kW Greenstar in a low-demand bungalow behaves better than a 30 kW one, and both behave better once a weather-compensating or load-compensating control is fitted. Vaillant pairs the ecoTEC with the sensoComfort control that reads outdoor temperature and lowers flow temperature as the day warms, which keeps the flame low and the return cool. Worcester offers comparable behaviour through its EasyControl smart thermostat when wired for open-therm modulation rather than simple on-off switching.

The wiring detail matters more than the thermostat brand. A smart thermostat that only opens and closes a relay gives none of the modulation benefit; it just schedules the same crude firing pattern more conveniently. Load or weather compensation requires the two-wire modulating connection, and whether the installer sets that up correctly varies more than the hardware does.

For domestic hot water, the 832 designation on the ecoTEC and the 30 kW Greenstar both deliver adequate combi flow for a single-bathroom bungalow, around 12-13 litres per minute at a 35 degree rise. A second bathroom pushes the case toward a system boiler with a cylinder, at which point the modulation argument on the heating side stays identical but the hot water sizing changes entirely.

Warranty and parts, the unglamorous decider

Worcester Bosch offers up to 10 or 12 years warranty on the Greenstar range when installed by an accredited fitter and registered, often conditional on fitting a Worcester filter and system treatment. Vaillant offers up to 10 years on the ecoTEC through its Advance installer scheme, with similar registration conditions. Both warranties collapse to two years if the paperwork or the annual service lapses, so the headline number is really a statement about installer accreditation and record-keeping.

The insulation work changes which boiler you need

Spend 1,000 to 2,000 pounds tightening the fabric before choosing an output band and the sums shift. Loft insulation topped from 100 mm to 270 mm of mineral wool cuts roof heat loss by a large fraction, and on a bungalow the roof is the dominant surface because the footprint and the ceiling area are the same. Cavity wall injection, where cavities are unfilled and suitable, runs in the region of 500 to 1,500 pounds for a bungalow-sized envelope and removes a further slice of the design load.

Draught proofing the sash windows addresses the uncontrolled air changes that no boiler sizing accounts for. Brush strips and parting-bead seals on original sashes cut infiltration without sealing the frames shut, and on a single-storey house exposed on all four elevations the wind-driven leakage is proportionally larger than in a mid-terrace. Every one of these measures lowers the design heat load, and each kilowatt removed pushes a 25 kW boiler further into oversized territory.

The sequence has a payoff the boiler choice cannot match on its own. A house that loses 5 kW instead of 10 kW can run its flow temperature at 45-50 degrees instead of 70, which is precisely the regime where a condensing boiler recovers latent heat and where the modulation gap between the two brands stops mattering, because both spend most of the season on a low steady flame.

A worked comparison on annual gas

Take the uninsulated bungalow at a 10 kW design load with an annual space-heating demand near 12,000 kWh at the meter. Fit either boiler at a real seasonal efficiency of 88 percent because oversizing and 70 degree flow temperatures suppress condensing, and the gas input is roughly 13,600 kWh a year.

Now insulate. Loft to 270 mm, cavities filled, sashes draught-proofed, design load down to 5-6 kW and demand down toward 7,500 kWh. Drop the flow temperature to 50 degrees so the boiler condenses properly and seasonal efficiency climbs toward 92-93 percent. Gas input falls to around 8,100 kWh. At a unit rate of 6 to 7 pence per kWh, the fabric work is saving 350 to 400 pounds a year on the gas side, and it does so whether the appliance on the wall is the Greenstar or the ecoTEC.

The boiler-to-boiler difference in the same insulated house is a point or two of seasonal efficiency from the ecoTEC’s wider modulation, worth perhaps 15 to 30 pounds a year. Set against a 200 to 400 pound difference in fitted price, the payback on choosing the more modulating unit runs to a decade or more purely on gas. The case for it rests on comfort steadiness and short-cycling wear, not on the annual bill.

Existing pipework decides the first day of the job

Before either boiler goes on the wall, the state of the existing system sets the workload. A bungalow with 20-year-old radiators and black magnetite sludge in the return needs a power flush of the radiators, typically 300 to 500 pounds, or the debris migrates into the new heat exchanger and voids the warranty. Both Worcester and Vaillant make system cleanliness a warranty condition and require a magnetic filter, the Worcester Greenstar filter or the Vaillant equivalent, fitted on the return.

If the retrofit adds underfloor heating to a rear extension, the manifold setup introduces a second flow temperature. Underfloor circuits run at 35-45 degrees while radiators want 50-60, so the manifold carries a blending valve and its own pump, and the boiler’s low-temperature modulation becomes an advantage the ecoTEC’s 1:10 range exploits more fully. On a mixed system the wider modulation earns more of its keep than it does on radiators alone.

One detail is easy to defer and expensive to ignore: the condensation vapour barrier behind any internally insulated wall. Adding insulation to a solid wall without a continuous vapour control layer on the warm side pushes the dew point into the masonry, and interstitial condensation shows up as damp months later, long after the boiler is signed off and the connection between the two problems has been forgotten.

Which one goes on the wall

On the specification sheet the ecoTEC plus 832 has the edge in modulation and the Greenstar 4000 tends to have the simpler, cheaper spare parts supply through a denser merchant network. Both are reliable enough that installer competence and correct control wiring outweigh the badge. If the bungalow will end up with a mixed radiator and underfloor system and a low design load, the wider Vaillant modulation is the technically cleaner match. If it stays radiator-only and the priority is parts availability and a slightly lower fitted price, the Worcester holds up.

What neither datasheet answers is how far the eventual flow temperature will actually fall once the loft, cavities and sashes are done, because that depends on radiator surface area the current system may not have. A bungalow sized for 70 degree flow often needs larger radiators to run at 50, and whether that upgrade happens is the variable that decides which of these two boilers ever reaches the efficiency printed on its label.