What to leave behind

The job is not finished when the system runs. It is finished when somebody who has never met you could pick up the paperwork in five years and understand what was installed, why it was sized that way, and what it was set to do.

That somebody might be another engineer, a surveyor, a buyer's solicitor, or you, having forgotten the job entirely.

The certificate

Register the installation and get the certificate to the customer. It is the document that unlocks their grant, gets them paid for exported electricity, and answers the question a conveyancer will ask when they sell.

Do it promptly. A certificate chased for six weeks undoes a great deal of goodwill earned on site, and it is the commonest complaint made about installations that were otherwise fine.

The design as installed

Not the one on the quotation, if they differ.

For a heat pump: the room-by-room heat loss calculation, with the areas, the U-values you assumed and the design temperatures. The schedule of emitters, showing what is in each room and what it delivers at the design flow temperature, and the design flow temperature itself.

For solar: the five inputs to the generation estimate — capacity, postcode, pitch, orientation, shading. The shading assessment if one was needed. The string layout showing which modules are on which input.

This is the document that makes a later problem diagnosable. Without it, whoever comes next is guessing, and the first thing they will guess is that you got it wrong.

What you set it to

The commissioning record is a baseline, and its whole value is that it lets somebody compare against day one.

Heat pump: the flow temperature and the weather compensation curve as left. The system flow rate and the temperature difference across it. The hot water setting, how the pasteurisation cycle is arranged, and any refrigerant work.

Solar: the string test results — open circuit voltages, short circuit currents, insulation resistance, earth continuity — and the electrical certificate for the AC side.

Three years on, when somebody says the output looks low, those string results are how the fault gets found instead of argued about.

How to run it, written for them

Not the manufacturer's service manual. Something a page or two long, written for the person who lives there.

What the controls do, which of them they should touch and which they should not. What normal looks like: lukewarm radiators, running most of the time, steam on a cold morning. What the schedule is set to. What to check before calling anybody — usually pressure, and whether something obvious has tripped.

The section that saves you the most calls is the one headed "this is normal". A heat pump owner who does not know a defrost cycle is normal will report it as a fault, every time.

What needs doing, and when

Honestly, including where the honest answer is "very little". Solar systems need almost nothing, and padding the list with invented servicing is transparent.

Heat pumps: keeping the outdoor coil clear, system pressure, the strainer, antifreeze concentration where there is any. If the manufacturer's warranty depends on an annual service, say so here in plain words, so it is not discovered when a claim is refused.

Unvented cylinders need annual attention from somebody qualified for them, which is a separate thing from the heat pump service and is routinely forgotten.

The three warranties, separately

They are different things and customers conflate them constantly.

The manufacturer's warranty on the equipment, with its length and whether labour is included. The workmanship warranty from you. And the insurance-backed guarantee that stands behind your workmanship if you stop trading — which is a separate policy with its own number, and the one people most often assume they have without ever having seen it.

Hand over the actual policy document, not a mention of it.

The notifications you made on their behalf

Building control compliance. The network operator notification, whether that was a connect-and-notify or an approval obtained beforehand. Copies of both, so that when somebody asks in eight years there is an answer in the folder.

Put it in one place

A single folder or a single email with everything in it, handed over as one thing and not trickled out over a fortnight.

Two practical reasons. It is the difference between the customer feeling the job is finished and feeling it is still in progress. And when they sell the house, that folder is what gets handed to the buyer, which is when your work gets seen by somebody who might be your next customer.

The test

Before you drive away: if this house sold in five years, and a surveyor and an engineer went through the folder, would they be able to tell what was installed, why it was sized that way, and what it was set to do?

If yes, the job is finished.