New Boiler Installation in London: What to Expect on the Day

Before Installation Day

A well-managed boiler installation begins before the engineer arrives. The installation company should have surveyed the property — either in person or via detailed photographs and measurements — to confirm the boiler location, flue route, condensate pipe route, gas supply capacity, and heating system configuration. The correct boiler model should have been specified for the property size, number of bathrooms, and hot water demand. A combi boiler that is undersized for a large property will struggle to deliver simultaneous heating and hot water; a boiler that is oversized for a small flat will cycle on and off too frequently, reducing efficiency and increasing wear. If a pre-installation survey has not been offered, request one — an engineer who quotes a fixed price without surveying the property is guessing at some of the costs.

Arrival and Initial Assessment

A qualified Gas Safe engineer arrives with the new boiler, flue components, and installation materials. The first task is a brief walk-through: confirming the boiler location, checking the existing flue exit point, identifying the gas meter and supply pipe, checking the system pressure if the existing boiler is still operational, and noting the heating controls to be replaced or retained. At this point the engineer should confirm the day plan with you — estimated start and finish time, whether there will be any periods without hot water or heating, and what access they need to various parts of the property.

Removing the Old Boiler

The old boiler is isolated electrically and at the gas valve before any work begins. The system is drained down or the affected circuit is isolated. The flue is disconnected and the boiler is unbolted from its fixing bracket or removed from its housing. The old boiler is heavy — most combi boilers weigh 25 to 40 kg — and removal typically requires two people for safety. Old flue sections, the old condensate trap, and the old filling loop are removed. In older properties, the removal process sometimes reveals issues with the existing pipework — corroded connections, inadequate pipe sizing, or incorrect configuration — that need to be addressed before the new boiler goes in.

Preparing the Installation

Before the new boiler is fitted, the engineer prepares the circuit: power flushing or chemically flushing the system if it has not been done recently (sludge and scale from an old system will damage the new boiler heat exchanger within months if not cleared), fitting a magnetic filter on the return pipe, and checking inhibitor concentration in the system water. In London, where water hardness accelerates scale formation in heat exchangers, a scale reducer or water softener on the cold mains feed to the boiler is advisable and should be discussed at survey stage. The gas supply pipework is inspected for condition and sizing; if the existing pipe is undersized for the new boiler gas rate, a section may need upgrading before the boiler is commissioned.

Fitting the New Boiler

The new boiler is fixed to its bracket or housing and the connections are made: flow and return to the heating circuit, cold mains inlet (for a combi), hot water outlet (for a combi), gas connection, condensate pipe, flue, and electrical connections. The flue route is one of the more variable parts of the installation — in a London flat where the external wall route for the flue has changed, or where a longer flue run is required due to building constraints, this part of the job takes longer. The condensate pipe from a condensing boiler must be routed to a drain; in cold weather, external condensate pipe runs can freeze and cause the boiler to lock out — the engineer should route the condensate internally where possible or insulate any external section.

Commissioning

Commissioning is the stage that separates a proper installation from a quick fit-and-go. The engineer fills the system to the correct pressure, bleeds all radiators, checks for leaks at every connection, ignites the boiler for the first time, and carries out a full gas tightness test on the completed installation. A flue gas analysis checks combustion quality — CO/CO2 ratio and flue gas temperature — and confirms that the boiler is operating within the manufacturer performance specification. The heating controls are programmed and tested. The engineer walks you through the boiler controls, the filling loop operation, and the system pressure gauge, and shows you where the gas isolation valve and the emergency shutdown are located. You should receive a commissioning report and, for a rental property, a gas safety certificate.

What to Check After Installation

In the days following installation, check the system pressure gauge daily — a new installation may need pressure top-ups as air works out of the system and the inhibitor circulates. Confirm that all radiators are heating evenly; if any are cold at the top, they need bleeding. Check that the condensate pipe is draining freely. If the boiler displays any fault codes or error messages, contact the installation company immediately rather than attempting a reset — a fault code on a newly installed boiler usually indicates a commissioning issue that the installer should return to rectify at no charge.