How to write a system description for an O&M manual (with examples)

It's the Thursday before handover. The datasheets are in, the certificates are in, and the manual is nearly there. Then you hit the section headed "System description" and it's blank.

So someone copies the spec clause in, or pastes the first page of the boiler manual, and moves on. A fortnight later the comments come back: "System description doesn't describe the installed system."

Fair comment. Here's how to write one that does, in well under an hour a system.

What a system description is for

A system description tells the person running the building what you actually installed, how it works and how to keep it working. It's written for the facilities manager on a wet Monday morning, not for the design team.

The datasheets tell them about one product. The drawings tell them where things are. The system description is the bit that joins them up: this boiler feeds these circuits, this controller runs them, this is what "normal" looks like.

There's a legal angle too. Where the energy efficiency rules in Part L apply, regulation 40 of the Building Regulations 2010 says the person carrying out the work must, no later than five days after the work is completed, give the owner enough information about the building, the fixed building services and their maintenance requirements for the building to be run without using more fuel and power than is reasonable. A clear system description is a big part of that.

The five-part structure

Use the same five headings for every system. Your reader learns where to look, and you never stare at a blank page.

1. What it is. One or two sentences. The system, what it serves and the main kit.
2. How it works. The flow, in plain words. Where it starts, where it goes, what controls it.
3. Controls and settings. What the user can change, where, and what it was set to at handover.
4. Maintenance in brief. The main tasks and how often, pointing to the full schedule and the manufacturers' literature.
5. Where to find things. Locations of the plant, isolators and key valves, and the drawing numbers.

Keep each part short. If a paragraph runs past five lines, it probably belongs in the maintenance schedule or an appendix.

Example 1: heating

This is an illustrative example, not from a real job.

What it is. Low temperature hot water heating serving the ground and first floor offices. Two wall-hung gas boilers in the second floor plant room feed a low loss header and two variable speed pump sets.

How it works. The boilers heat water to the header. Pump set P1 serves the radiator circuit on the ground floor. Pump set P2 serves the first floor fan coil units. The radiator circuit has a weather-compensated mixing valve, so its flow temperature drops as it gets warmer outside. The fan coil circuit runs at a constant flow temperature.

Controls and settings. The BMS outstation in the plant room runs the time schedules. At handover the occupied period was set to 07:00 to 19:00, Monday to Friday, with frost protection at all other times. Room setpoints are on the BMS front end, not on the radiators.

Maintenance in brief. Annual boiler service and combustion check by a Gas Safe registered engineer. Check system pressure and inhibitor levels yearly. Full tasks are in section 6, maintenance schedule.

Where to find things. Plant room 2.04. Gas isolation valve at the plant room door. Schematic on drawing M-501, plant room layout on M-210.

Notice what's not in it. No sales blurb from the boiler brochure, no copy of the spec, no "the contractor shall". Just what's there and how to live with it.

Example 2: ventilation

What it is. Supply and extract ventilation with heat recovery for the open plan office, from one roof-mounted air handling unit.

How it works. Fresh air comes in through the AHU, passes the heat recovery wheel and a filter, and is ducted to ceiling diffusers. Stale air is drawn back through grilles in the corridor ceiling and leaves through the same unit. A CO2 sensor in the office speeds the fans up when the room gets busy.

Controls and settings. On the same BMS time schedule as the heating. Fans set to run at minimum speed when occupied and quiet, rising on CO2. The fire alarm shuts the AHU down.

Maintenance in brief. Filter checks every three months, changes as the manufacturer recommends. Belt and bearing checks, and cleaning of the heat recovery wheel, as set out in section 6.

Where to find things. AHU-01 on the roof, access by the stair core hatch. Local isolator on the unit. Ductwork layout on drawing M-301.

Example 3: lighting and emergency lighting

What it is. LED lighting throughout, with presence detection in the offices and toilets, and self-contained emergency luminaires on escape routes.

How it works. Each office zone has a sensor that switches the lights on when someone comes in and dims them in daylight. Emergency luminaires have their own batteries and light up if the normal supply fails.

Controls and settings. Presence sensors set to switch off after 15 minutes with nobody there. Scene switches by each office door. Emergency lighting test key switches in the riser cupboards on each floor.

Maintenance in brief. Emergency lighting needs regular testing, a monthly functional test and an annual full rated duration test, as BS 5266-1 recommends, with results logged. The test regime and log sheets are in section 7. Clean sensors and diffusers during routine cleaning.

Where to find things. Distribution boards in the riser cupboards on each floor. Lighting layouts on drawings E-401 and E-402. Emergency lighting test certificate in section 8.

Five mistakes that get system descriptions sent back

• Copying the spec. The spec says what should be installed. The manual says what was. If a pump changed during the job, the description has to follow.
• Pasting the manufacturer's brochure. The FM doesn't need to know the boiler is "efficient and stylish". They need to know which circuits it feeds.
• No settings. "Controlled by the BMS" isn't enough. Write down what it was set to on the day you handed over. That's the first question when someone gets cold.
• No locations. If nobody can find the isolator, the description failed. Room numbers and drawing numbers, every time.
• Writing it last. The person who commissioned the system knows how it works. Get a few lines from them on the day, while it's fresh.

A quick way to get it done

You don't need to write these at the end. Try this on your next job:

• When a system is commissioned, ask the engineer for five lines against the five headings. A voice note works.
• Drop the commissioning settings straight into part 3 while the sheet is in front of you.
• Add the drawing numbers when the as-builts are issued, not before.
• Read it back as if you'd never seen the building. If you'd have a question, answer it in the text.

Do that and the blank section on the Thursday before handover is already filled in.

Where Handover Works fits

If you'd like a tidy home for all this, Handover Works builds the O&M manual from the job's documents, flags what's missing, and nothing is published until you approve it. Your client opens it in one scan. You can start free and pay when you publish.

Related reading: What goes in an O&M manual?, O&M manual template: how to write one and Why O&M manuals get rejected.

Handover Works team