Benchmark Commissioning: A CENWAT Guide

Fitting a boiler isn't finished when the gas is connected — it's finished when it's been commissioned: set up, proven safe and efficient, and recorded. The industry's framework for that is the Benchmark scheme. This guide walks through commissioning to it. It's study material; only a Gas Safe registered engineer may carry out the work.

What Benchmark is

Benchmark is an industry code of practice and a commissioning/service checklist supplied in the back of every domestic boiler manual. Completing it at installation records that the boiler was set up correctly — and it's usually tied to the manufacturer's warranty: an incomplete Benchmark can invalidate cover. Each service is logged on it too, building a history for the appliance.

Clean the system first — BS 7593

A new boiler must not be filled into a dirty system. BS 7593 sets out preparing the system water: cleanse/flush the system to remove installation debris and existing sludge, then dose a corrosion inhibitor to protect against rust and magnetite sludge. Fitting an in-line magnetic system filter helps keep the water clean and the heat exchanger efficient. Skipping this is a common cause of premature boiler failure.

The commissioning sequence

Working to the manufacturer's instructions, a typical sequence is:

1. Inspect & prepare — flue, ventilation, gas and water connections; flush/cleanse and add inhibitor.
2. Fill & vent — fill the system (via the filling loop on a sealed system), bleed air, set the cold fill pressure, check the expansion vessel.
3. Tightness test the gas installation and purge.
4. Check the gas rate and burner pressure against the data plate / MIs.
5. Combustion check — verify combustion with a flue gas analyser (the CO/CO₂ ratio, linking to CPA1).
6. Set the controls — prove boiler interlock and the time/temperature controls.
7. Check condensate disposal and any flue/spillage requirements.
8. Complete the Benchmark checklist, hand over the documents, and demonstrate the system to the customer.

Servicing later

At each service, you re-check combustion, gas rate, controls, condensate, ventilation and flue integrity, and log it on the Benchmark service section — keeping the appliance safe and the warranty valid over its life.

1. Benchmark = industry commissioning/service checklist in the boiler manual; tied to the warranty.
2. Clean first (BS 7593): flush/cleanse, dose inhibitor, consider a magnetic filter.
3. Fill & vent: filling loop, set cold pressure, check expansion vessel.
4. Gas: tightness test & purge; set gas rate and burner pressure to the data plate.
5. Combustion check with an analyser (CO/CO₂ ratio — links to CPA1).
6. Controls: prove interlock and time/temperature control; check condensate & flue.
7. Complete Benchmark, hand over, demonstrate; re-check and log at every service.