The Vaillant ecoTEC plus 835 is a 35kW gas condensing combi that heats your radiators and gives you hot water straight from the mains, with no tank or cylinder. It was built for larger family homes, usually three or four bedrooms with a main bathroom and an en-suite.
It’s no longer made. Vaillant discontinued the ecoTEC plus 825, 832, 835 and 838 and replaced them with a remastered ecoTEC plus range, where the closest match is the 836. If you’ve got an 835 on the wall, this review explains what it does well and what to expect as it ages. It also covers what to look for when it’s time to replace it.
What Are The Vaillant ecoTEC Plus 835’s Key Specs?
It’s a 35kW natural gas combi with A-rated, 94% seasonal efficiency and a hot water flow of 14.3 litres per minute.
| Specification | Vaillant ecoTEC plus 835 |
| Boiler type | Gas condensing combi, wall-hung |
| Hot water output | 35kW |
| Central heating output | 6.4 to 32.2kW (at 50/30°C) |
| Hot water flow rate | 14.3 litres per minute (at a 35°C temperature rise) |
| ErP seasonal heating efficiency | 94%, A-rated |
| Dimensions (H x W x D) | 720 x 440 x 372mm |
| Weight | Approx. 38kg (dry) |
| Fuel | Natural gas only (no LPG version of the 835) |
| Status | Discontinued. Replaced by the remastered ecoTEC plus range (826, 832, 836, 840 and 940) |
What Is The Vaillant ecoTEC Plus 835?
It’s the second-largest combi from Vaillant’s old ecoTEC plus range, which sat above the simpler ecoTEC Pro and below the premium ecoTEC exclusive. Only the 838, and the 938 with its built-in hot water store, were bigger.
The number on the badge is about hot water, not heating. You get 35kW when you run a tap or shower, but the most it sends to the radiators is just over 32kW.
That’s still more heat than most houses need. People pick a 35kW combi for the hot water flow, not because their radiators demand it.
As a combi, it only heats water when you ask for it. There’s no stored hot water, no cold water tank in the loft and nothing losing heat through the day. If you’re not sure a combi still suits your home, our guide to combi, system and conventional boilers sets out the differences.
What Features Does The ecoTEC Plus 835 Have?
The 835’s main features are a stainless steel heat exchanger, a burner that turns down low, a compact case and support for Vaillant’s eBUS controls.
What Heat Exchanger Does The 835 Use?
It uses a stainless steel heat exchanger, like the rest of the ecoTEC plus family. Stainless copes well with the acidic condensate that every condensing boiler produces. It’s one reason the range earned a solid reputation with heating engineers.
How Low Does The Burner Turn Down?
On heating, the burner turns down to around 6.4kW. On mild days it ticks over at a gentle low output instead of firing hard and cutting out.
That saves gas and means less wear on the parts.
Will It Fit In A Kitchen Cupboard?
Usually, yes, as long as the right service clearances are left around it. At 720mm high and 440mm wide, it matches the height of a standard kitchen wall unit. It’s a little deeper than the smaller models at 372mm.
Which Controls Work With The 835?
It works with standard programmers and room thermostats, as well as Vaillant’s own eBUS controls. The eBUS controls can add weather compensation and app control, and both help a condensing boiler run more efficiently. Our smart thermostats page covers the options we fit.
What Does The Display Show?
The digital display shows system pressure, temperatures and fault codes. If your 835 flashes up an F code, our Vaillant boiler error codes guide explains what the common ones mean.
How Efficient Is The ecoTEC Plus 835?
It’s rated 94% for ErP seasonal heating efficiency, which puts it in the A band. If it replaced an old non-condensing boiler, your household will have seen a clear drop in gas use.
Real-world efficiency depends on how the system’s set up, though. A condensing boiler only condenses properly when the water coming back from the radiators is cool enough. A lower flow temperature and good controls make a real difference.
An 835 that’s missed a few services, or has sludge building up in the pipework, will burn more gas than it did when new.
That’s why yearly servicing matters more as the boiler gets older. A blocked condensate trap, a worn fan or a scaled-up plate heat exchanger will all push your bills up.
What Are The Pros And Cons Of The ecoTEC Plus 835?
Its biggest strengths are strong hot water flow and A-rated efficiency, but you can’t buy one new any more.
Pros
- Strong hot water flow of 14.3 litres per minute for a single combi
- A-rated, 94% seasonal efficiency
- Stainless steel heat exchanger with a good reliability record
- Compact casing for a 35kW boiler
- Vaillant parts are easy to get hold of and engineers know the boiler well
Cons
- Discontinued, so you can’t buy a new one
- No LPG version in the 35kW size
- Older units will be out of their original cover and need more upkeep
- Won’t run two showers at full flow at the same time
- Oversized for flats and small homes with one bathroom
What Size Home Does The ecoTEC Plus 835 Suit?
It suits three and four-bedroom homes with a main bathroom, an en-suite and roughly 12 to 18 radiators. At 14.3 litres per minute, it’ll run one shower strongly while someone else uses a basin or the kitchen tap. Run two showers at once, though, and both will lose flow.
A combi can’t deliver more water than your incoming mains supplies. If the mains flow is weak, a bigger boiler won’t fix it, and a system boiler with an unvented cylinder may be the better route.
We measure mains flow and pressure during the survey. Our guide on how to size a combi boiler explains how to match the output to your home.
How Much Does It Cost To Replace An ecoTEC Plus 835 In 2026?
Expect roughly £2,300 to £3,100 for a straight combi swap to its nearest current equivalent, the Vaillant ecoTEC plus 836. Moving the boiler or converting from a system or regular boiler costs more.
The guide ranges below include a standard flue, a magnetic filter, a system flush and basic controls. Your price will depend on the flue position, gas pipe size, the state of the system and the controls you choose.
| Type of job | Guide installed price |
| Combi to combi swap, same position | £2,300 to £3,100 |
| Combi swap with boiler moved or new flue route | £2,900 to £3,800 |
| System or regular boiler converted to combi | £3,200 to £4,300 |
These are guide figures only, and a survey gets you a fixed price. For a fuller breakdown, see how much a new boiler costs. If you’d rather spread the cost, read about boilers on finance.
What Are The Best Alternatives To The ecoTEC Plus 835?
The closest replacement is the Vaillant ecoTEC plus 836, with the 840 for busier homes and the ecoTEC exclusive 835 if you want Vaillant’s premium model.
- Vaillant ecoTEC plus 836: the direct successor in the remastered range. It gives up to 29.9kW to the radiators and 14.6 litres per minute at a 35°C rise, in a slimmer 352mm-deep casing.
- Vaillant ecoTEC plus 840: for busier homes, with 16.3 litres per minute at a 35°C rise.
- Vaillant ecoTEC exclusive 835: Vaillant’s premium combi, with 14.5 litres per minute, wide modulation and Quiet Mark approval. Our ecoTEC exclusive 832 review covers the older version.
- Other brands: our best combi boiler reviews and Vaillant boilers reviewed compare the main options.
Is The Vaillant ecoTEC Plus 835 Worth It?
It was a good boiler in its day, but once an 835 is over 8 years old, replacing it is usually a better option than paying for repairs.
The 835 was a well-built, efficient combi with good hot water flow for its size. Plenty are still running well.
If yours is younger than that, reliable and serviced every year, there’s no rush. If it’s over 8 years old, breaking down regularly or needs an expensive part, a new ecoTEC plus 836 or 840 is the sensible like-for-like move.
Keep Your 835 For Now If
- It’s reliable and has a full service history
- It’s under 8 years old and hasn’t developed any faults
- Your hot water demand hasn’t changed since it was fitted
Look At A Replacement If
- It’s over 8 years old, when efficiency usually drops and faults become more common
- It needs a major part such as a heat exchanger, gas valve or PCB
- Breakdowns are becoming a regular winter event
- Your household now needs more hot water than it can deliver
- You’d like modern controls, app access and a slimmer boiler
Get A Free Boiler Quote From Clever Energy Boilers
Clever Energy Boilers carries out new boiler installations for homeowners and landlords across the North West, Yorkshire and the Midlands. If your ecoTEC plus 835 is nearing the end of its life, we’ll help you pick the right replacement.
Our experienced, professional and registered installers check your hot water use, mains supply and radiator count before recommending a size. You can get long, market-leading warranties through Clever Energy on the boilers we fit. A servicing plan then keeps your new boiler in good shape year after year.
Get your free, no-obligation boiler quote online in a few minutes.
Frequently Asked Questions
Is The Vaillant ecoTEC Plus 835 Discontinued?
Yes. Vaillant discontinued the ecoTEC plus 825, 832, 835 and 838, and the remastered ecoTEC plus range replaced them. The 836 is the closest match to the 835.
What Replaced The Vaillant ecoTEC Plus 835?
The ecoTEC plus 836 replaced it, with 14.6 litres per minute of hot water at a 35°C rise. If you need more flow, the ecoTEC plus 840 delivers 16.3 litres per minute.
Can The ecoTEC Plus 835 Run On LPG?
No. Vaillant didn’t make an LPG version of the 835. If your home runs on LPG, see our page on LPG and oil boilers.
Is 35kW Too Big For My House?
It can be, especially in a flat or a house with one bathroom. A 35kW combi is sized for hot water demand in homes with more than one bathroom. For smaller homes, a 26kW to 32kW combi is usually a better fit and costs less to buy.
How Often Should An ecoTEC Plus 835 Be Serviced?
Once a year, by a Gas Safe registered engineer. Servicing matters more as the boiler gets older, because worn or blocked parts push up gas use and the risk of a breakdown.