When a Routine Service Uncovers a Hidden Danger: High CO Readings on a Vaillant ecoTEC Plus in Ealing
The job was booked as a routine annual service. The customer had no complaints — the boiler was firing, the radiators were hot, and nothing looked wrong. But the moment I inserted the probe for the flue gas analysis, the numbers told a very different story. The CO/CO₂ ratio was well outside Vaillant’s acceptable parameters. This appliance was not burning gas safely, and no visual inspection in the world would have found it.
What followed was a full boiler strip down service — the only way to get to the root cause and bring the combustion readings back to where they needed to be.
Job Snapshot | |
Service Type | Boiler service (escalated to full strip down service) |
Location | Ealing, West London |
Boiler Model | Vaillant ecoTEC plus (approx. 8–10 years old) |
Customer Type | Homeowner |
Primary Issue | High CO/CO₂ ratio detected during flue gas analysis |
Resolution | Full strip down: heat exchanger clean, condensate trap flush, gasket replacement |
Installed by | Vincent O’Gara, Gas Safe Registered — Ealing Boiler Services |
The Problem
That assumption lasted until the flue gas analysis. A high CO/CO₂ ratio is not a marginal finding you can note and leave for next year. It is a direct indication of incomplete combustion — the burner is not receiving the correct balance of gas and air, meaning gas is being burned inefficiently and potentially dangerously. Carbon monoxide, a by-product of incomplete combustion, is odourless, colourless, and lethal in sufficient concentrations.
I explained the readings clearly to the customer. I could not sign off this appliance on a standard service. The only responsible course of action was to escalate to a full strip down — dismantle the combustion components, identify the cause, and physically clear it.
💡 Why Flue Gas Analysis Matters A visual boiler check will not tell you if your boiler is burning gas safely. Flue gas analysis is the only way to measure what is actually coming out of the flue — and it is the difference between signing off a dangerous appliance and catching a serious problem before it causes harm. We carry out a calibrated flue gas analysis on every Vaillant boiler service we complete in Ealing and across West London. It is non-negotiable — and it is exactly what uncovered the problem in this case. |
My Solution
With the customer’s full understanding and approval, I isolated the gas and electrical supplies and began the strip down. This is a labour-intensive procedure — it involves working through each combustion component systematically, in the correct order, and following Vaillant’s technical instructions throughout.
Burner and Fan Assembly
I removed the front casing and room-sealed chamber panel, then disconnected the gas valve and fan to expose the burner. There was a build-up of carbon deposits across the burner face — enough to disrupt the flame picture and reduce the even distribution of gas. I cleaned it thoroughly and confirmed the flame ports were clear.
Heat Exchanger
The stainless steel heat exchanger is where the problem was worst. The pathways between the coils — which allow exhaust gases to pass through and transfer heat to the system water — were heavily restricted with hardened scale and debris. Using the correct Vaillant cleaning tools and a specialist cleaning solution, I worked through each section methodically until the coils were completely clear. Restore the flow here and you restore the combustion.
Electrodes
While the combustion chamber was open, I inspected the ignition and flame sensing electrodes. Both showed wear and carbon build-up. I cleaned them with an abrasive cloth, checked the spark gap, and confirmed they were functional without needing replacement — a useful check that saves a potential lockout call in winter.
Condensate Trap
Modern condensing boilers extract latent heat from the flue gases, producing acidic condensate that must drain away cleanly. I removed the condensate trap and flushed out a significant accumulation of aluminium oxide sludge. A blocked trap backs condensate up into the heat exchanger — it adds to combustion restriction and accelerates internal corrosion. Cleared and refitted.
Burner Door Gasket
This step is not optional. Once the burner door is removed, the original graphite gasket cannot be reused — it has been compressed and will not re-seal. I fitted a new manufacturer-approved gasket before reassembling the burner, fan, and gas valve, torquing all fastenings to the correct specifications.
The Result
Once reassembled, I restored water pressure and gas supply, fired the boiler in chimney sweep mode at maximum load, and let it stabilise for ten minutes to ensure any residual cleaning solution had burnt off before taking a final flue gas analysis.
The numbers were exactly where they needed to be. CO levels had dropped dramatically from the readings at the start of the job, and the CO/CO₂ ratio was well within Vaillant’s specified safe and efficient parameters. The incomplete combustion issue was completely resolved.
I walked the customer through the final readings on the analyser — showing them the before and after figures directly. The boiler is now safe to operate, will run measurably more efficiently over the winter months, and has had a full record of the work retained for their documentation.
Frequently Asked Questions
Common questions about boiler strip down services, high CO readings, and flue gas analysis from Ealing and West London homeowners.
Frequently Asked Questions |
Q: What is a boiler strip down service? A: A strip down service goes well beyond a standard annual service. It involves dismantling the main combustion components — including the burner, fan, gas valve, and heat exchanger — to physically remove carbon deposits, scale, and debris that a basic visual check cannot reach. It’s typically required when flue gas analysis reveals unsafe combustion readings. |
Q: How do I know if my boiler needs a strip down service rather than a standard service? A: The clearest indicator is a high CO/CO₂ ratio during flue gas analysis. If a Gas Safe engineer carries out a proper flue gas test and the numbers fall outside the manufacturer’s acceptable parameters, a strip down is the appropriate next step. Symptoms that may point in this direction include increased gas bills, sooty marks around the boiler, intermittent lockouts, or a yellow/orange burner flame instead of a crisp blue one. |
Q: Is a high CO reading dangerous? A: Yes. A high CO (carbon monoxide) reading during flue gas analysis indicates incomplete combustion — the boiler is not burning gas efficiently. Left unaddressed, this can result in dangerous CO escaping into your home. Carbon monoxide is odourless and colourless, making it impossible to detect without a CO alarm. If a Gas Safe engineer flags high readings, the appliance should not simply be signed off — it needs investigation and rectification. |
Q: How long does a boiler strip down service take? A: For a standard condensing combi boiler such as a Vaillant ecoTEC plus, allow around 2.5 to 3.5 hours on-site. The exact time depends on the level of scaling and debris found, the boiler model, and the age of the appliance. We always walk customers through what we’ve found before starting, so there are no surprises. |
Q: Why does the burner door gasket need replacing during a strip down? A: The burner door seal (gasket) is a one-use component. Once the burner door is removed, the original graphite seal cannot be safely reused — it has been compressed and will no longer form an airtight seal. Refitting an old gasket risks highly toxic combustion gases escaping into the boiler casing. We always fit a new, manufacturer-approved gasket as part of every strip down service. |
Q: How much does a boiler strip down service cost in Ealing? A: Pricing depends on the boiler make, model, and the parts required. We operate a transparent pricing model — our £95 upfront diagnosis fee covers the initial assessment and flue gas analysis, and we confirm the full strip down cost with you before any additional work begins. There are no hidden charges. Contact us for a specific quote for your boiler. |
Got a Similar Problem in Ealing or West London?
We carry out annual boiler services across Ealing, Hanwell, Acton, Southall, Greenford, and the surrounding West London boroughs. Every service includes a calibrated flue gas analysis as standard — because without it, you simply cannot know whether your boiler is operating safely.
If you’ve received a warning about high CO readings, or if your boiler hasn’t had a proper flue gas test recently, don’t leave it. The risk is real, and the fix — when caught early — is straightforward.
Whether you need a routine annual check or suspect you might need a full boiler strip down service, we are here to help.
Call Ealing Boiler Services: 0208 538 1891 | ealingboilerservices.co.uk
