Guides to the gas boilers we service, covering common faults and what they usually turn out to be, parts availability, service specifics, and the scaling problem that dominates combi performance in a hard water area.
What these pages cover
- Fault codes and what they usually turn out to be – not the manual’s description, but the actual cause we find most often.
- Common component failures on that model.
- Service specifics – condensate trap access, heat exchanger cleaning, seal replacement.
- Parts availability, honestly.
- Access quirks, because some appliances are far harder to work on than others.
- Known weak points, where they exist.
- Controls compatibility – whether the appliance supports OpenTherm or a manufacturer protocol, which determines whether a smart thermostat can actually modulate it.
Each model page below is populated from what we actually find on the job – common faults, part numbers, service quirks and access notes – rather than from manufacturer literature. That takes time to accumulate and it is worth more than a rewritten brochure. Pages appear as the records support them.
The fault code families
Whatever the make, codes fall into a small number of families and each has a characteristic set of real causes:
| Family | What the boiler noticed | What it usually is |
|---|---|---|
| Ignition lockout | Ran the sequence, no flame proved | Gas supply, blocked condensate trap, ignition or sensing electrode, gas valve |
| Flame loss during run | Lit, then lost it | Gas pressure under load, condensate blockage, flame sensing |
| Overheat | Temperature exceeded the limit | Poor circulation – pump, air, sludge, closed valve – far more often than a sensor |
| Low pressure | System below minimum | A leak, or a failed internal expansion vessel |
| Fan / air pressure | Fan not running, or flue obstructed | Fan, pressure switch, blocked terminal, or condensate backing up |
| Condensate | Cannot drain | Frozen or blocked condensate pipe, or a blocked trap |
| PCB / internal | Board detected an internal fault | Sometimes the board. Frequently something else it is misreading |
A partially blocked condensate trap causes several completely different-looking codes – ignition faults, flame loss, and fan or air pressure faults all appear. It is dirty and fiddly to clean, which is why it gets skipped at service, and it is behind a disproportionate share of intermittent faults. See understanding boiler fault codes.
The hard water problem with combis
This is the defining long-term fault on combi boilers in this county and it deserves stating plainly.
A combi heats domestic hot water instantaneously through a plate heat exchanger – a stack of thin plates with very narrow passages. Narrow passages transfer heat well, which is why they are used, and they scale readily.
Scale in a plate exchanger does two things at once: it insulates, so the water leaves cooler, and it restricts, so less water passes. The result is the pattern everybody here recognises – hot water that is not as hot as it was, at a flow that is not as good as it was, declining gradually over years until somebody finally notices.
It is the strongest practical argument for scale protection on a combi in a hard water area, and it is why we raise water treatment more often than a firm in a soft water region would. See what limescale actually does and scale inhibitors.
What determines how long a gas boiler lasts
- Annual servicing, properly done and recorded – see what a gas boiler service should include.
- A clean system. A modern heat exchanger has narrow waterways and is intolerant of magnetite. This is the most common declined warranty claim.
- Correct sizing. An oversized boiler short-cycles and spends its life in the least efficient part of its range.
- Adequate gas supply. An undersized pipe starves the appliance under load and the fault gets blamed on the boiler – see gas supply and pipe sizing.
- Condensate routing. Internal where possible.
- Water treatment in hard water, for the plate exchanger on a combi.
- Correct flow temperature. A boiler left at factory maximum is a condensing boiler that is not condensing.
Boiler showing a code? Write it down exactly before you reset – it is the most useful thing you can tell us.
Questions we get asked about this
What does my fault code mean?
It tells you which internal check failed, which narrows the cause to a handful of possibilities. It does not name the faulty part. The manufacturer’s manual lists the codes for your specific appliance.
Why has my combi’s hot water got worse over the years?
Almost certainly scale in the plate heat exchanger, which insulates and restricts at the same time. It is the defining hard water fault on combis and it develops so gradually nobody notices until it is substantial.
Can a scaled plate heat exchanger be cleaned?
Frequently, with a chemical descale. Whether it is worth doing depends on the appliance’s age and how quickly it will re-scale without treatment.
Why do fault codes keep coming back?
Intermittent faults are usually marginal rather than failed – a sensor that misreads when hot, gas pressure adequate standing but not under load, or a partly blocked condensate trap. They need patience rather than part substitution.
Will you work on my make?
We work on the common makes. Where a specific appliance needs a manufacturer-trained engineer or a proprietary tool, we will tell you rather than making a poor job of it.
Are the model pages available?
They are being built from actual job records rather than manufacturer literature, which takes time. Pages appear as the records support them.
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