What happens on the day your new boiler is fitted?

A new boiler is usually fitted in a single day, and most installation days follow the same pattern. Your home is protected, the old system is drained, the old boiler comes out, the new one goes in, the system is cleaned, and then everything is tested and you're shown how it works. Bigger jobs, such as moving the boiler or converting an old tank-fed system to a combi, often run into a second day.

This post walks through the day step by step, so you know what to expect, what to get ready and what paperwork you should have at the end. Any work on the gas supply, the flue or the boiler itself must be done by a Gas Safe registered engineer, so your part is limited to the simple preparations below.

What's decided before the day

By installation day, the big decisions should already be made. A survey visit beforehand settles which boiler suits your home, where it will go, how the flue will run and whether any pipework, radiators or controls need changing. If you're still weighing up the type of boiler, our guide to combi vs system boilers explains the difference.

If the plan includes moving the boiler or taking old tanks out of the loft or airing cupboard, that extra work is built in from the start. It's the main reason one boiler replacement takes a day and the next takes two.

How to get your home ready

A little preparation the day before keeps the job moving:

• Clear the area around the old boiler, and a path to it from the front door.
• Empty the airing cupboard if there's a hot water cylinder or tank being removed.
• Clear the space under the loft hatch if there are tanks up there.
• Leave space to park near your door, as there will be plenty of trips to the van.
• Keep pets in a separate room, because doors will be open a lot.
• Fill a kettle and a few bottles of water, and plan showers for the night before.

Your heating and hot water will be off for most of the day. You don't need to watch the work, but someone needs to be home to let us in and be on hand for questions.

Protecting your home and draining down

The first job is covering floors and carpets along the route the engineers will use, with dust sheets or protective film.

Next comes draining down, which means emptying the water out of the heating system through a drain valve, usually at the lowest radiator or pipe. The gas and electricity supplies to the boiler are isolated, meaning switched off and made safe, before any part of it is disconnected.

Taking out the old boiler

With the system empty, the old boiler comes off the wall and the old flue, the pipe that carries waste gases outside, is removed. On a conversion to a combi, the hot water cylinder and the cold water tanks in the loft come out too, and the pipework is re-routed.

We take the old boiler and the rest of the waste away with us.

Fitting the new boiler, flue and controls

The new boiler is hung on a wall bracket and connected to the gas, water and heating pipes. A new flue is fitted through the wall or roof. The condensate pipe, which carries away the slightly acidic water a modern condensing boiler produces, is run to a drain.

Many installations also include a magnetic filter. This is a small canister on the pipework under the boiler that catches magnetite, the black sludge made of rust particles from inside radiators and pipes, before it can reach the boiler. Then the new heating controls go in.

In England, fitting a new or replacement gas boiler also means fitting a carbon monoxide alarm in the same room. Carbon monoxide is a poisonous gas you can't see or smell, so an alarm is worth having near any gas appliance.

Cleaning the system and filling it back up

Older heating systems collect sludge over the years. A new boiler connected to dirty pipes and radiators can block up and lose efficiency quickly, so boiler manufacturers ask for the system to be cleaned as part of the installation. Depending on how dirty it is, that's either a chemical flush, where a cleaning solution is run round the system and drained out, or a power flush, which pumps water through at high flow to shift heavier sludge.

The system is then refilled and an inhibitor is added. This is a liquid that protects the inside of your pipes and radiators from corrosion. Finally, the air is bled out of each radiator.

Testing and showing you how it works

Commissioning is the set of checks that proves the boiler is installed correctly and running safely. The engineer tests the gas pipework for leaks, checks the gas pressure, and uses a flue gas analyser, a handheld meter that reads the exhaust gases, to confirm the boiler is burning cleanly. The results go into the Benchmark checklist, the boiler's commissioning record, which is usually in the back of the manual.

Then the heating is run and each radiator is checked. Some radiators warm up faster than others, so they may need balancing, which means adjusting the valves so every room heats evenly. Before we leave, we show you how to use the boiler and its controls, how to check the pressure and how to set your heating times. If you've chosen a smart thermostat, we can set up the app on your phone with you.

The paperwork that follows

You should be left with the boiler's manual and a completed Benchmark checklist. The manufacturer's warranty also needs registering, usually within a set time after installation.

Replacing a boiler is notifiable work under the Building Regulations. A Gas Safe registered installer tells your local authority about it through Gas Safe Register, and a Building Regulations compliance certificate is then posted to you. Keep it safe, as you may need it when you sell your home.

From then on, a yearly boiler service keeps the boiler running safely and is usually a condition of the manufacturer's warranty.

RKL Plumbing & Heating fits new boilers across Liverpool as Worcester Bosch accredited installers, with Gas Safe registered engineers. If your boiler is due for replacement, book a survey through our boiler installation service, and we'll talk you through exactly what your installation day will involve before it's booked in.

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