How do I know whether to repair or replace?

Ask which part has failed and whether it is still available. A heat exchanger, or a third major component inside a year, or a model that no longer has parts support, points to replacement. A fan, a sensor or a pump on an otherwise healthy appliance is a repair, and we will say so.

Can I have a combi if I currently have a hot water cylinder?

Sometimes, and it depends entirely on your incoming main and how many outlets you run together. We measure the flow at the kitchen tap first. If the main is weak, or you have two bathrooms in regular use, keeping the cylinder and fitting a system boiler gives you a better shower than any combi will.

How long does a boiler replacement take?

A like for like swap in the same position is normally a single day. Changing the type of boiler, relocating it, or running new gas pipe through the property usually takes two. You are told which it is before the work is booked.

Do my radiators and pipework have to be replaced too?

Usually not. Existing radiators normally stay, provided they are sound and the system is cleaned properly before the new boiler is commissioned. What does sometimes need changing is the gas supply pipe and any pipework in poor condition close to the boiler.

The labour rate is fixed before we attend and the total depends on how long the job takes, plus the appliance and materials. We survey first, so you know the scope, including anything like a gas pipe upgrade, before committing to the work.

My boiler has been turned off as unsafe. Does that mean I need a new one?

Not automatically. An appliance is turned off and labelled because of what was found on the day, and some of those findings are correctable: a blocked flue, a missing ventilation opening, a leaking joint on the gas side. What forces a replacement is a fault that cannot be corrected on that appliance, such as a flue route that can no longer be made compliant or a cracked heat exchanger. You are told which of the two you have, and what it would take to make it safe.

Can I keep my hot water cylinder when the boiler is replaced?

Yes, and in a two bathroom home you usually should. The cylinder stays in place, the flow and return to its coil are adapted to the new appliance, and the cylinder thermostat and zone valve are wired into the new controls. Swapping a cylinder for a combi to free up a cupboard often costs you shower performance, particularly where the incoming main is already working hard, and we will say so rather than sell the simpler installation.

Can you replace a boiler in a flat?

Yes, and the flue is the part that decides the detail. It has to terminate with the right clearances from windows, doors and the boundary, and in a flat it may run through a void that needs inspection access. Where a lease or managing agent has conditions about flue positions, that is checked at survey rather than discovered on the day.

Boiler replacement

A boiler is worth replacing when the failed part is the expensive one or when parts for it are no longer made. A cracked heat exchanger, a third major component inside a year, or an appliance out of parts support is a replacement. Almost everything else is a repair, and we will tell you which of the two you are actually looking at before anything comes off the wall.

When replacing beats repairing

Age on its own decides nothing. A twelve year old boiler that has been serviced, holds pressure and lights cleanly is not scrap. What decides it is which component has failed, and whether that component can still be bought. That is what a boiler diagnosis visit is for, and it is where this conversation should start: the fault gets proved with a manometer and an analyser before anybody talks about a new appliance.

The primary heat exchanger is the one that ends an appliance. When it corrodes through or splits, the boiler loses water into the case or into the combustion chamber, and the symptoms are pressure that drops with no leak anywhere in the house, staining inside the casing, or a plume off the flue that looks heavier than normal. On most domestic boilers the exchanger and the labour to fit it come to a large share of what a new appliance costs, which is why an engineer who is being straight with you will raise replacement at that point rather than quote the part.

The other honest trigger is a run of expensive parts. A fan, then a gas valve, then a control board, each defensible on its own, is a pattern once they arrive inside eighteen months. The components are all roughly the same age and they are all failing for the same reason, so paying for the next one rarely buys much time.

Parts availability is the quiet one. Manufacturers support a model for a set period after it goes out of production. Once a specific board, a diverter cartridge or a sealed fan assembly cannot be sourced new, the next fault is terminal whatever condition the rest of the boiler is in. We check that before recommending a repair, because there is no point fixing an appliance this month that cannot be fixed at all next year. Serviceability is also the only sensible test behind any list of boiler brands to avoid, since what matters is whether boards, cartridges and fan assemblies are still supplied years after the model is discontinued, not which badge is on the case.

Primary heat exchanger leaking, corroded or split
Several major components failed in quick succession
Parts for the model no longer available from the manufacturer
Appliance turned off as unsafe for a flue or ventilation fault that cannot be corrected
Heating and hot water demand the boiler was never sized for

Combi, system or heat only

A combi heats water on demand straight off the incoming main, so there is no cylinder and no stored hot water. Worth separating from this decision: a combi that runs the radiators happily but gives no hot water is usually a stuck diverter valve or a scaled plate heat exchanger, and that is a repair rather than a reason to replace the appliance. The real limit on a combi is not the boiler, it is the mains. If the cold tap at the kitchen sink gives a weak flow, the shower will be weak too, and no amount of boiler output fixes a poor supply. Two outlets running together will share whatever the main can deliver.

A system boiler is sealed, has its pump built in and heats a cylinder. Stored hot water is what lets two showers and a bath run properly at once, because the flow comes out of the cylinder rather than being made as it passes through a plate. How quickly the cylinder reheats afterwards depends on the coil inside it and the output of the boiler feeding it.

A heat only boiler still uses a cistern in the loft and a vented cylinder. It remains the right answer in some older London properties where the layout is settled and the mains cannot support anything more demanding.

The short version: one bathroom and a strong main usually points to a combi, while two or more bathrooms, or a household where showers run at the same time, points to keeping a cylinder and fitting a system boiler. Put another way, the combi or system boiler question is settled by the incoming main and the number of outlets that run together, and it is worth working through properly before anyone specifies an appliance.

What a survey checks before anyone says what to fit

The survey matters more than the brand. Which boiler is best in the abstract matters far less than which boiler suits the gas run, the flue route and the mains you actually have, and the same appliance can be an excellent choice in one property and the wrong one next door. Fitting the wrong type in a property that cannot support it produces a boiler that works perfectly and a customer who is disappointed every morning.

A common one in older London housing is the gas supply itself. Plenty of installations run 15mm pipe from the meter, which was adequate for the old appliance and will not carry a modern combi at full rate. Upgrading that run is part of the job and we want to know about it before we quote, not halfway through the day. Because all of it is notifiable gas work, it is also worth knowing how to check a plumber on the Gas Safe Register before anyone takes a panel off a wall, since the registration and the categories an engineer is actually signed off for are a matter of public record and take a moment to confirm.

Flow and static pressure measured at the kitchen tap, not estimated
The gas run from the meter, its size, its length and what else is on it
Where the flue can terminate and its clearances from windows, doors and the boundary
Where the condensate can discharge, and whether any part of that run is exposed to frost
Condition of the existing radiator circuit and pipework
Condition of the cylinder if you are keeping one
Electrical supply, wiring centre and what controls you want

Gas work is notifiable and must only be carried out by an engineer registered for it. Our engineers are qualified for the work described here.

What happens on the day

The system is drained, the old appliance and its flue come out, the new boiler is hung and the pipework adapted to it. The flue is cut and sealed at the wall or roof, and the condensate is taken to an internal waste wherever possible, because an external condensate run is the first thing to freeze in a cold snap and lock the boiler out.

Flushing is not an upsell. A new heat exchanger has narrow waterways, and pushing years of magnetite out of old radiators through it blocks it. The system is cleaned, a magnetic filter is fitted on the return and the water is dosed with inhibitor so the new appliance is not fed the debris that helped kill the old one.

Commissioning is where an installation is made safe rather than just made to run. The gas rate is checked at the meter, combustion is analysed at the flue with a calibrated analyser, the system and any relevant pressures are set, the controls are set up and shown to you, and the installation record is completed and left with you.

A like for like swap in the same position is usually a day. Changing the type of boiler, moving it to another wall, or running new gas pipe through the property takes two.

Keeping your cylinder when the boiler changes

A lot of London homes run an unvented cylinder off a system boiler, and that is a good arrangement. Replacing the boiler does not mean losing it. The cylinder stays where it is, the flow and return to the coil are adapted, and the cylinder thermostat and zone valve are wired into the new appliance and its controls.

Swapping that setup for a combi to free up a cupboard is often a downgrade in the shower, particularly in a flat with a shared riser or anywhere the main is already working hard. We will say so rather than sell you the simpler install.

If the cylinder is near the end of its life as well, doing both at once avoids draining the system twice and paying for the same labour twice. Whether that is a straight hot water cylinder replacement in the same cupboard or a fresh unvented cylinder installation depends on what is there now and what the mains will carry, and the survey answers both at the same visit as the boiler.

Work on an unvented cylinder is notifiable and must be carried out by an engineer holding the unvented qualification. Ours hold it, so the boiler and the cylinder do not need two separate trades and two separate visits.

After the new boiler is in

Keep the paperwork together: the installation record, the appliance instructions and the model and serial number off the data plate. Register the appliance with the manufacturer, and book it in for an annual service. A service is also what keeps the combustion figures on record, which is what tells an engineer two years from now whether something has drifted.

If the new boiler shows a fault code in the first weeks, write it down before resetting it. A code that repeats in a pattern, for example only on hot water or only on the coldest mornings, tells us far more than the code alone. Most early faults are commissioning items or something in the wider system rather than the appliance, and they are sorted quickly when the history is clear.

Questions we get asked

How do I know whether to repair or replace?: Ask which part has failed and whether it is still available. A heat exchanger, or a third major component inside a year, or a model that no longer has parts support, points to replacement. A fan, a sensor or a pump on an otherwise healthy appliance is a repair, and we will say so.
Can I have a combi if I currently have a hot water cylinder?: Sometimes, and it depends entirely on your incoming main and how many outlets you run together. We measure the flow at the kitchen tap first. If the main is weak, or you have two bathrooms in regular use, keeping the cylinder and fitting a system boiler gives you a better shower than any combi will.
How long does a boiler replacement take?: A like for like swap in the same position is normally a single day. Changing the type of boiler, relocating it, or running new gas pipe through the property usually takes two. You are told which it is before the work is booked.
Do my radiators and pipework have to be replaced too?: Usually not. Existing radiators normally stay, provided they are sound and the system is cleaned properly before the new boiler is commissioned. What does sometimes need changing is the gas supply pipe and any pipework in poor condition close to the boiler.
What will it cost?: The labour rate is fixed before we attend and the total depends on how long the job takes, plus the appliance and materials. We survey first, so you know the scope, including anything like a gas pipe upgrade, before committing to the work.
My boiler has been turned off as unsafe. Does that mean I need a new one?: Not automatically. An appliance is turned off and labelled because of what was found on the day, and some of those findings are correctable: a blocked flue, a missing ventilation opening, a leaking joint on the gas side. What forces a replacement is a fault that cannot be corrected on that appliance, such as a flue route that can no longer be made compliant or a cracked heat exchanger. You are told which of the two you have, and what it would take to make it safe.
Can I keep my hot water cylinder when the boiler is replaced?: Yes, and in a two bathroom home you usually should. The cylinder stays in place, the flow and return to its coil are adapted to the new appliance, and the cylinder thermostat and zone valve are wired into the new controls. Swapping a cylinder for a combi to free up a cupboard often costs you shower performance, particularly where the incoming main is already working hard, and we will say so rather than sell the simpler installation.
Can you replace a boiler in a flat?: Yes, and the flue is the part that decides the detail. It has to terminate with the right clearances from windows, doors and the boundary, and in a flat it may run through a void that needs inspection access. Where a lease or managing agent has conditions about flue positions, that is checked at survey rather than discovered on the day.