The Annual Service

The recommendation itself

CPSC puts it plainly: "Have the heating system professionally inspected and serviced annually to ensure proper operation." That is a safety recommendation about combustion and venting, not an efficiency suggestion.

Source: US Consumer Product Safety Commission, Carbon Monoxide Fact Sheet.

Why oil systems get it and gas systems often do not

An oil burner has consumable parts — a nozzle, a filter, an electrode — and it will stop working if they are neglected. The annual visit is therefore forced by the equipment, and most oil households have a service arrangement whether or not they think of it as safety work.

A gas boiler has no equivalent consumable. Nothing stops it running to make the appointment happen, so the appointment does not happen — and the combustion and venting check that CPSC is actually asking for is the thing that gets missed.

What a service is looking at

Combustion and its products, the venting path, the safety controls, and whether the appliance is doing what its controls are asking. On a sealed or condensing unit the condensate path matters too, because a blocked one shuts the boiler down at the coldest moment of the year.

The one thing to ask for afterwards

What the combustion readings were, and whether they changed from last year. A single set of numbers is a snapshot; two sets are a trend, and a trend is what tells you something is drifting before it fails.

And the CO alarm is not a substitute

An alarm tells you combustion has already gone wrong. The service is what stops it going wrong. Both, not one — and CPSC recommends alarms on every level of the home and outside sleeping areas, but not in kitchens or above the appliance itself.