The house already had a Glow-worm 24cxi combi. A 3rd generation Nest Learning Thermostat went in first. A few months later the old boiler was short of hot water and looked close to a breakdown. Three quotes later — two Vaillant options and one Intergas — the owners chose the ecoTEC exclusive 835 Green iQ. The family already knew Vaillant from a parent’s ecoTEC, which is a common and honest reason to pick a brand.
This is a project note from that install, not a current product brochure. Vaillant’s exclusive Green iQ combi range at the time was the 835 (35 kW DHW) and 843 (43 kW DHW). Later exclusive models use different badges (824 / 831 / 837 on Vaillant’s professional pages). Treat the figures below as the 835 as published, not a live price list.
Why the Green iQ, and what the claims meant
The extra efficiency on this boiler was Vaillant’s extra-condense stainless-steel heat exchanger: a second condensing stage that recovers more heat from the flue, especially in hot-water mode, and cuts the visible plume. That is a built-in recuperator, not a box you add on the flue later.
- Modulation. Published heating range at 50/30 °C was about 3.9–26.4 kW, a ratio around 1:10. The hub’s “low modulation ratio” meant a wide turndown — the boiler can run at a small fraction of its maximum. That is the useful number for a small heat load.
- Hot water. About 14.5 l/min at a 35 K rise on the 835. That is why it replaced a tired 24 kW-class Glow-worm that was no longer delivering.
- ErP. The boiler itself was A for heating and A for hot water, with a published 94% ErP efficiency. The A+ heating band was not the boiler on its own. Vaillant’s own literature said A+ when a compatible control (they named vSMART at the time) was added. A Nest on switched live does not, by itself, make the label A+.
- Controls and connectivity. The range was designed for Vaillant eBUS / app controls. A Nest can still run the heating — see Nest and a combi boiler — but it is not the path Vaillant used for that A+ package.
Green iQ mode on these boilers was Vaillant’s running-cycle optimisation and a recyclability claim (they published 85%). It is not a heat-pump hybrid and it does not remove the need to set a sensible flow temperature.
What went on the system
The boiler was not hung on dirty pipes. The job included a powerflush / system cleanse and inhibitor. That matters more than the badge on the case. The add-ons were:
- Spirotech Spirovent RV2 micro-bubble de-aerator on the flow. It takes dissolved air out of the circulating water. Less oxygen in the water is less corrosion. It does not replace inhibitor.
- Spirotech Spirotrap MB3 magnetic dirt filter on the return. Magnetic and non-magnetic particles. Spirotech’s 20-year guarantee on that filter is the figure they still publish; see the MB3 review.
- 15 mm inline magnetic scale device (Calmag on the original note) on the cold feed to the boiler. Treat that as a weak extra, not as a water softener. In hard-water London a softener, or at least an honest conversation about scale, is the stronger protection for a combi heat exchanger.
- Remote filling loop with a gauge (Altecnic on the original note). The 835 already had a built-in filling loop. The remote loop is a dosing point and a backup if the integral loop fails.
The hub page had Amazon product widgets on these parts. Those ads are stripped. Buy the current equivalent from a merchant the installer already uses, and keep the serials for any filter guarantee.
What this note is not
It is not an as-built record for a named address, and it is not a current “buy this boiler” recommendation. The 835 Green iQ is an older exclusive. If you are specifying a new Vaillant combi now, start from today’s exclusive / plus range, the live guarantee (Vaillant Advance and a boiler-protection kit were the extra years on the old exclusive), and whether you want eBUS weather compensation rather than an on/off Nest.
The alternative on the original quotes was Intergas. The 2-in-1 heat exchanger case for hard water is on the Intergas Combi Compact ECO RF review.
Related
- Combi boiler installation
- Combi boiler
- Combi boiler cost
- Nest and a combi boiler
- Spirotech Spirotrap MB3 review
- How to service a Vaillant ecoTEC
- HA39NS underfloor heating case study
Project-specific design and install note, not a DIY guide. Gas work in the UK needs a Gas Safe registered engineer. Figures are from Vaillant’s published exclusive Green iQ literature for the 835, not a live catalogue. System water to BS 7593.