Vaillant Boiler & Underfloor Heating Problems Solved | Marylebone Case Study

When Other Engineers Can't Find the Answer

There's a particular kind of frustration that comes with living in a property that isn't working properly — especially when you've already had engineers in, paid for their time, and still have the same problem. That was exactly the situation facing the owners of a five-storey Marylebone townhouse in late 2024: inconsistent heating across multiple floors, a ground-floor underfloor system that had completely stopped working, and a Vaillant boiler that kept throwing fault codes and locking out at random intervals.

Two previous contractors had attended. Neither had been able to identify a definitive cause. One had suggested the boiler may need replacing. Another had mentioned the possibility of a failed UFH pump. Meanwhile, the family was managing with electric plug-in heaters in the lower-ground floor — which, in a property of this calibre, felt as incongruous as it sounds.

This is the kind of case that RARE Plumbing specialises in. Not the straightforward swap-out or the simple service — but the complex, multi-system diagnostic that requires both experience and patience.

About the Property

The townhouse is a substantial Victorian conversion on a quiet street close to Marylebone High Street. Across its five floors, the heating system comprises a single Vaillant ecoTEC boiler, a pressurised cylinder for domestic hot water, wet underfloor heating on the lower-ground and ground floors, and radiators on the upper three floors. The UFH was controlled by a Heatmiser system with individual zone thermostats.

This kind of setup — a single boiler serving both UFH and radiator circuits across multiple floors — requires careful hydraulic design and regular maintenance to perform well. When something goes wrong, the symptoms can present in unexpected ways, which is exactly why previous engineers had struggled to diagnose the root cause.

Our Initial Assessment

When our engineer arrived, the first step was a full system audit before touching anything. This meant: checking the boiler's fault log and service history, measuring flow and return temperatures across the heating circuits, inspecting the manifold and each UFH zone independently, testing actuator function zone by zone, checking system pressure, water quality and inhibitor levels, and reviewing the Heatmiser controller configuration.

What this methodical approach revealed was not one problem, but three — each compounding the others:

Problem One: A Faulty Diverter Valve on the Vaillant Boiler

The Vaillant ecoTEC uses a diverter valve to switch water flow between the domestic hot water circuit and the central heating circuit. When this valve fails — and it is one of the more common failure points on Vaillant combination and system boilers — it can partially stick, causing erratic behaviour: the boiler fires correctly but heat distribution is inconsistent, and in some cases the boiler's internal sensors trigger fault codes and lockouts because the expected temperature rise doesn't occur within the programmed time.

This was exactly what was happening here. The diverter valve was partially jammed — not fully closed, but not fully open either. The boiler was working, but heat wasn't being distributed efficiently to the heating circuits. The fault codes the homeowner had been seeing — and which had led one contractor to suggest boiler replacement — were a symptom of this valve failure, not of a failed boiler.

Problem Two: A Partially Blocked UFH Manifold Zone

The lower-ground floor UFH manifold had a severely restricted flow on the main zone. Flow meters showed almost no circulation through the affected circuits. On stripping and inspecting the manifold, we found significant scale and debris accumulation on the affected ports — almost certainly the result of the system never having been properly flushed or treated since installation.

This was the direct cause of the lower-ground floor having no heat: water simply wasn't circulating through the loops. It was a mechanical problem, not an electrical or controls issue — which is why reconfiguring controllers and thermostats (as one previous engineer had attempted) hadn't resolved it.

Problem Three: Significant Magnetite Contamination Throughout

Water quality testing revealed heavy magnetite contamination throughout the system. This black iron oxide sludge — a byproduct of corrosion within the boiler heat exchanger, pipework, and radiators — was present at levels that would meaningfully reduce system efficiency and accelerate component wear. The absence of any magnetic filter in the system meant there was nothing capturing and removing this contamination before it circulated continuously through every component.

The Solution

With a clear picture of all three root causes, we carried out the following work in a single extended visit:

Diverter valve replacement: We replaced the Vaillant diverter valve with an OEM Vaillant part. This is a job that requires draining the relevant part of the system, careful disassembly, and correct reassembly — it's not a difficult job for a competent engineer, but it does require the right parts and knowledge of the boiler's internals.

Full system power flush: Using professional power flushing equipment, we circulated a chemical cleaner through the entire system to break down and remove magnetite and scale deposits. The water drained from the system at the end of this process was noticeably dark — evidence of the contamination that had been accumulating for years.

Manifold service and zone restoration: We stripped, cleaned, and reassembled the lower-ground UFH manifold, replacing the affected flow meters and flushing the circuits individually to confirm full flow restoration. Each circuit was then rebalanced to ensure correct flow rates across all loops.

Magnetic filter installation: A Spirotech SpiroTrap magnetic filter was installed on the return to the boiler. This captures magnetite before it reaches the heat exchanger, protecting the boiler and maintaining water quality going forward.

System refill and recommissioning: The system was refilled with correctly dosed inhibitor, pressure-tested, and fully recommissioned. Every zone was tested for correct response to thermostat demand. The Heatmiser controller was verified and the heating schedule confirmed with the client.

The Result

Even heat distribution was restored across all five floors. The lower-ground UFH, which had been cold for over two winters, reached operating temperature within 40 minutes of the system being restarted. The Vaillant boiler ran through a full heating and hot water cycle without a single fault code — for the first time in over a year.

The client, who had been prepared for the possibility of a new boiler and extensive remedial work, was understandably relieved. Total cost: a fraction of what either of the replacement quotes had suggested. The system that other engineers had been unable to diagnose was repaired in a single day.

What This Case Illustrates

Complex heating systems in prime London properties require a diagnostic approach, not a parts-replacement approach. The temptation — for some contractors — is to identify the most visible symptom and address it, or to quote for major work when the root cause isn't immediately obvious. At RARE, we take the time to understand the whole system before we recommend anything.

This property now holds an annual PPM contract with us. Every year, we carry out a full system service: boiler service and CP12, UFH balancing and flow check, water quality testing, inhibitor top-up, and a comprehensive written report. It's the kind of structured maintenance programme that prevents the kind of cascading failures we found on that first visit — and gives the homeowner genuine peace of mind.