For our case study today, we were called to inspect a large house in Surrey, completely refurbished around 7 years ago, with multiple underfloor and radiator heating zones.

The house had a basement boiler room, containing a pair of 46Kw Vaillant EcoTec 46Kw light commercial boilers, using a cascaded flue system and a header.

Vaillant 466 cascade

A maintenance company had paid a number of visits to keep the boilers running, but they were plagued with frequent reliability problems. The last inspection had also raised issues with the flue header and chimney lining installation, even though they had been maintaining it for years previously.

We are primarily a boiler installer rather than a repair company, and explained to the owners that our inspection visit would be chargeable; we were being asked primarily for advice on the safety of the flue system.

The installation was in many ways, perfectly acceptable. The pipework had been laid out well and the controls, which provided variable temperature heating, had been well installed. The strange thing was, the actual boiler rig was really grim, so that it was incongruous. We wondered if two different installers had been responsible; the customer had left the original job in the hands of their builder, in 2013.

So, what was wrong?

The first thing one looks for in a commercial grade boiler room is ventilation – the boilers in this application were ‘open flue’, meaning the air for safe combustion has to be derived from the room. For this application High and Low level ventilation is required, to ensure safe combustion and to keep air temperatures within a safe level.

This was the boiler plant room ventilation, including combustion, as we found it.

These vents have to be sized appropriately for the boiler output and the type of application the boilers are intended for. The ventilation cannot be blocked accidently, and has to be fit for purpose.

In this boiler room we had 92Kw of boilers and a closable window leading into a light well. So that’s a non compliance for starters.

Then we look at the flue system. Vaillant make a flue header for their commercial boilers when installed in a cascade side by side. This links the two boiler outlets and commons them, for onward connection to a flue system that would normally be vertical. In our case the system was a chimney liner, leading up three floors to a rooftop.

Most boiler manufacturers don’t make flexible liners, expecting the installation company to design and select suitable materials to complete the job. On this example neither had been done, the flue liner was slightly larger diameter than the socket on the boiler flue cascade. The two items were never going to fit, so some enterprising individual cut slots in the stainless flue chimney liner, and then wiped copious quantities of sealant around the mess resulting. And of course, this was leaking. Condensate water was running over the floor, which meant that flue gases were leaking too. There was no form of mechanical fixing between the two parts.

Procuring a flue liner that is incompatible with the boiler cascade spigot isn’t unknown, it is a complete pain because by the time your mistake has been spotted, the liner has been cut to length by the supplier and it is snaking all over the place and won’t be returnable. It is an expensive error.

Cascade header to flue liner. The two parts have been glued together in a misaligned butt joint with silicon

However this isn’t an excuse for blundering on regardless and trying to butcher two incompatible parts together which form a safety critical role. This would be classed as dangerous.

When we opened one of the Vaillant 46Kw units, it was leaking internally from the rear where the plastic flue collector mounts onto the stainless Giannioni heat exchanger. The rust inside was at an advanced stage from leaking combustion fumes, and it was game over. The other unit had a dicky pump.

We considered making one reasonable unit out of two broken ones, but investigations revealed that Vaillant had since ceased production of the 466 and replaced it with a new smaller packaged model. This would be more labour intensive to swap and align.

We start today

Our customer decided that given the many callouts both boilers had needed over the preceding 7 years, and the fact that the flue system needed replacement, they would cut their losses and start afresh.

Lots of boxes to break into
New Viessmann 200W cascade now installed. That was quick!

We suggested a new Viessmann cascade, using 2 x 49Kw Viessmann 200W units. These can be supplied with a floor standing mounting frame, a dedicated flue header and hydraulic header.

New Viessmann flue header. Flue joints fit now. The boiler combustion air is sourced from the boiler room itself, also known as ‘open flue’.

We specified a Poujoulat flexible flue system; our customer used their scaffold supplier to get us onto the top of the chimney.

Safety equipment ready
Nice views from the top
New flue liner arrives
This time the flue components are the right ones….rigid stainless Poujoulat to Viessmann connection

This time our new flue system was an excellent fit, once we had removed the remains of a previous flue liner from the chimney, which had partially disintegrated and blocked the annulus.

This is taken from the boiler room, looking up the chimney. The remains of an old liner can be seen blocking our path about 5 metres up. We won’t tell you how it was unblocked, but it involved rope and a big pair of Stilsons.

The Viessmann twin boiler cascade was fitted with the Viessmann 300 series cascade controller, this manages how many boilers are running and also rotates the ‘lead’ boiler, to ensure annual running hours are similar.

Viessmann cascade controller 300K

Finally, we had to sort out the boiler room ventilation. We used Spiroduct tubing and coupled this to a above ground custom made air vent, with the high level and low level grilles at each end, the customer will paint this to match the wall rendering.

High and Low vent ductwork from ground level inlet (see next picture)

The wrought iron metal grille covering the light well was taken off site and reworked by our excellent local blacksmith ( to fit around the new ventilation tubes.

High and Low vents in underground boiler room downstairs pass through lightwell and terminate in this custom box. The existing wrought iron grille was modified to pass around the new ductwork

We arranged commissioning via Viessmann, and used the Viessmann registered installer extended warranty scheme, giving our customer a 10 year parts and labour warranty on their new commercial boiler system, which is unsurpassed, to the best of our knowledge.