THE SHORT ANSWER
WHAT GETS CHECKED
If none of the left-hand column happens, you have had a visual check and a sticker, not a service.
With instruments, not by eye
Combustion
Flue gas analyser readings, recorded
Gas rate
Measured against the manufacturer figure
Gas tightness
The installation tested for soundness
Safety devices
Each one tested rather than assumed
Flue integrity
Checked for spillage and correct termination
Ventilation
Checked against what the appliance needs
The parts that fail quietly
Case seals
A failed seal is a safety issue, not cosmetic
Condensate route
The run and the fall that stop it freezing
Heat exchanger
Inspected and cleaned where needed
Burner and injectors
Cleaned so combustion stays correct
System pressure
Set correctly, expansion vessel checked
The written record
What a manufacturer asks to see on a claim
WHAT HAPPENS ON THE DAY
1
Not a glance and a sticker. The analyser tells us whether the boiler is burning correctly, and it is the single best early warning of a fault worth catching. If someone services your boiler without taking readings, you have not had a service.
2
Gas rate checked against the manufacturer figure, case seals checked for soundness, the condensate route checked for the fall that stops it freezing in January, and every safety device tested rather than assumed working.
3
You get told plainly how the boiler is doing. If something needs attention you get what, why and what it costs, with no invented urgency. That record is what a manufacturer asks to see if you ever make a warranty claim.
WHEN YOURS IS DUE
Boiler fitted in the last year
Boiler you inherited with the house
You let the property out
It has been a few years
COMMON QUESTIONS
How often should a boiler be serviced?
What happens if I skip a year?
How long does a boiler service take?
Does a new boiler still need servicing in year one?
Is a service the same as a landlord gas safety certificate?
When is the best time of year to book?
BEFORE YOU GO
More of the same, written by the engineers who do the work.