You paid for a service. Did you get one?

A filter, a visual look, and a cursory check that the thing runs. That is an inspection, and there is nothing wrong with selling one — but it is not what the word “service” is generally understood to mean, and it is not what most annual agreements are priced as.

Combustion has to stabilise before it can be measured, which takes several minutes of running on its own. Getting a panel off, taking readings, putting a gauge set on and taking it off again, and cleaning anything properly all take real time. An hour is a reasonable floor for a single appliance, and more where there are two.

Ask, before booking, what will be measured and what will be written down. That one question changes the visit, whoever you book it with.

A service is a set of measurements. Without instruments there are no measurements, and without measurements there is no comparison with last year — which is the entire point of doing it annually rather than waiting for a breakdown.

Looking at equipment tells you what has already failed. Measuring it tells you what is going to.

At minimum an analyser or a manometer, depending on what you have. See the panel below for your equipment. None of it is exotic; all of it is ordinary kit that any properly equipped van carries.

A technician attending a breakdown rather than a service may reasonably diagnose a specific fault without a full instrument set. This applies to an annual service, which is a different visit sold at a different price.

A dated sticker records that somebody attended. It records nothing about what they found, which means next year’s visit starts from zero again and no trend is ever visible.

The value of an annual service is almost entirely in the comparison. A combustion figure on its own means little; the same figure drifting over three years is a heat exchanger telling you something well before it becomes a shutdown.

• The readings, with units, for the tests listed below.
• What was cleaned or changed, specifically.
• Anything found that needs doing, separated into now and coming, with prices against the first.
• The model and serial number, so the next person does not have to look for them.

Ask for the sheet. If it does not exist, ask why not. It is a reasonable request and a reasonable contractor will already have one.

Changing the filter is genuinely important and it is also the single easiest thing to do on the whole appliance. On its own it is a five-minute job that most householders could do themselves, and it is worth roughly what a filter costs.

Everything that needs an instrument, everything that needs a panel off, and everything that produces a number. On a fuel-burning appliance that includes the combustion test and the heat exchanger inspection, which are the two items that exist for safety rather than efficiency.

If you are on an annual agreement, ask what it covers in writing. If the answer is a filter and a visual, that is a decision you can make with open eyes — but it should be a decision rather than an assumption.

Sometimes it is exactly right. A cracked heat exchanger ends a visit — the appliance is shut off and tagged and no repair is attempted, and a replacement quote is the honest next step. The same is true of a failed compressor on an old system where the parts cost approaches the equipment cost.

In those cases there is always a finding behind it, and a finding can be shown to you.

• What was measured, and what the number was. Not “it’s at the end of its life” — the reading.
• The age from the serial number, not from an estimate. It is on the plate and it is frequently a decade from what people assume.
• Whether the appliance has been condemned or is merely old. Those are entirely different statements and only one of them means you cannot keep using it.
• What the repair would cost, even if it is not recommended, so there is something to compare against.

On a replacement-sized decision it is entirely reasonable to get one, and no contractor worth using will be offended. We will check somebody else’s numbers and tell you if they were right — which they often are.

Evidence. Next year’s visit can be compared with this one, which is the whole reason for doing it annually, and if anything is ever disputed you have figures rather than impressions.

• Keep it. Three years of sheets is worth more than any single visit.
• Read the “coming” items, not just the urgent ones. That list is what lets you budget instead of reacting.
• Check the model and serial are on it, so you are never again guessing the age of your own equipment.
• If you sell the house, it is a genuinely useful thing to hand over.

Whether the readings are actual measurements or a column of ticks with numbers that are suspiciously round and identical to last year. It happens, and the way to spot it is that real readings move.