When a boiler is on its way out, the first decision is not which brand to buy. It is whether you are replacing the boiler or replacing the system. Most homes in South East London need the first, not the second. This guide explains the difference, when a straight swap is the right answer, and when it is not.
What a like-for-like swap actually means
A like-for-like swap is a new boiler of the same type, in the same place, on the same fuel, using broadly the same flue route and the same pipework. Combi out, combi in. Heat-only boiler out, heat-only boiler in. The radiators, the pipework under the floor and the way the hot water is produced all stay as they are.
Because nothing else is being redesigned, it is the quickest job (usually one working day) and the cheapest way to get a modern, efficient boiler with a full manufacturer warranty. It is the work we do most, and it is the work we are set up to do fastest through the busy months.
"Like-for-like" does not mean "identical". The new boiler will be a current model, so it will be a condensing boiler with modern controls even if the old one was not. It will almost certainly be a different size and shape, so there is usually a little making good around the old position.
When a straight swap is the right answer
A like-for-like replacement is normally the correct choice if all of the following are true:
- The boiler is in a sensible position already (a kitchen, a utility room, an airing cupboard, a proper cupboard with access)
- The flue can exit through an outside wall close to the boiler, or the existing vertical flue route is sound
- The heating and hot water arrangement suits the house. For most one-bathroom terraces and flats that is a combi. For larger houses that already have a cylinder and are happy with it, that is a system or heat-only boiler
- The radiators and pipework are in reasonable condition and have not been leaking
- You are not planning to add a bathroom, a loft conversion or an extension in the next few years
If that describes your house, do not let anyone sell you a relocation or a system change. You are paying for disruption you do not need.
When it is not enough
There are real situations where a straight swap is either impossible or a false economy. The honest ones are:
The boiler is in the wrong place
Some older properties have the boiler in a bedroom cupboard, a bathroom, or an awkward corner with no drain for the condensate. If the existing position cannot take a modern condensing boiler safely and to regulations, it has to move. That is a bigger job.
It is a back boiler behind a gas fire
These were common in houses built or modernised in the 1960s and 1970s. Replacement means a wall-hung boiler somewhere else and removal of the old unit and fire. It is not a swap in any sense, and it needs a proper survey.
The gas supply is too small
Modern combis need a bigger gas supply than many older boilers did. If the run from the meter is undersized, it has to be upgraded before the new boiler can be commissioned. This is a common finding in older South East London houses and it is not optional.
The system is full of sludge
If the radiators are cold at the bottom, the water is black, and the old boiler died of a blocked heat exchanger, dropping a new boiler onto the same dirty system will shorten its life and can void its warranty. A power flush becomes part of the job.
The hot water does not keep up
A combi heats water on demand. If two showers run at once in a house with a combi, the second one suffers. If your household has outgrown a combi, or you have added a bathroom, the right answer may be a system boiler with a hot water cylinder rather than a bigger combi. Equally, if you have a cylinder and tanks in the loft in a one-bathroom flat, a combi conversion frees the airing cupboard and the loft. Either way, that is a system change, not a swap.
You are about to build
If a loft conversion or extension is planned, size the boiler and position it for the finished house, not the current one. Doing it twice costs more than doing it once properly.
The middle ground: a swap with the right upgrades
Most jobs are not a pure swap and not a full redesign. They are a like-for-like boiler with a handful of sensible upgrades that make the new boiler last. The ones we would normally recommend on a straight replacement are:
- A magnetic filter on the return pipe. Cheap, and it protects the heat exchanger for the life of the boiler
- A chemical flush with inhibitor as a minimum, or a power flush if the system is dirty
- Thermostatic radiator valves on any radiators that do not have them
- A programmable or smart room thermostat. This is required on a new combi in England anyway, and it pays for itself in gas
- A gas pipe upgrade if the survey shows the run is too small
None of these change the type of system. They just stop the new boiler inheriting the old boiler's problems.
How we decide which one you need
We survey every job before we price it. On the survey we look at where the boiler is, how the flue runs, what size the gas pipe is, what the water looks like when we bleed a radiator, how many bathrooms the house has and how many people use them, and whether anything is planned for the property. Then we tell you which route we would take if it were our house, and give you a fixed written price for it. If a straight swap is right, we say so. We do not upsell a relocation to make the job bigger.
Timescales and disruption
A like-for-like combi swap is normally done in a day. You will be without heating and hot water for most of that day and the engineer will need access to the boiler, the radiators and the gas meter. A system change or relocation is usually two days, sometimes more, and involves draining down, lifting floorboards where pipes run, and more making good afterwards. If you are replacing before winter, the swap is the route that gets you sorted quickest, which matters once the cold weather arrives and diaries fill up.
What it costs
A swap is the cheaper end of the market, a conversion or relocation the dearer end. We have set out honest guide ranges and what moves the price in our South East London new boiler cost guide. For your own house, the only reliable number is a fixed quote after a survey, which we do for free.
Get a straight answer for your home
Send us a photo of your boiler, tell us the postcode and how many bathrooms you have, and we will tell you whether we think it is a swap or something more before anyone comes out. We install across South East London from our base in Bromley, including Brockley, Lewisham, Lee, Catford, Sydenham, Chislehurst and Beckenham. Full details on our boiler installation and replacement page.
Thinking about a new boiler? We give you a fixed written price after a free survey, and nothing is added on the day.
See our boiler installation service or get in touch for a free quote.