A like-for-like combi boiler replacement usually takes one day and costs £2,000 to £4,500 supplied and fitted. The day covers removing the old boiler, a power flush, fitting and commissioning the new boiler, and registering the warranty. As a Vaillant-approved installer, we can offer extended Vaillant warranties of up to 10 years on qualifying boilers.
Most people only replace a boiler two or three times in their lives, so it is no surprise the process feels like a bit of a mystery. The good news is that a like-for-like combi swap is one of the most predictable jobs in heating. If the survey is done properly, the engineer turns up in the morning and you have heating and hot water again by the evening. Here is how that day actually unfolds, and what you should expect from whoever fits your new boiler.
Before the Day: The Survey
Every good install starts with a proper look at the existing setup. Anthony visits, checks where the current boiler sits, how the flue leaves the property, the size of the gas supply and the condition of the pipework and radiators.
- Boiler size: A combi needs to be matched to your hot water demand, not just the number of radiators. Too small and the shower runs lukewarm when someone turns on a tap.
- Flue and location: Most swaps go back in the same place, but if the flue position no longer meets current regulations it may need moving.
- Gas supply: Modern combis can need a larger gas pipe than the old boiler. Better to find that at the survey than on the day.
- System condition: Cold spots on radiators or dirty water when bleeding them are signs of sludge, which affects how long the flush will take.
You then get a clear written quote, so you know exactly what is included before anything is booked.
The Morning: Out With the Old
On the day, floors and the area around the boiler are protected first. The gas and water are isolated, the system is drained down and the old boiler is disconnected and removed.
With the old unit off the wall, any pipework that needs adjusting for the new boiler's connections is altered, and the new mounting bracket and flue are fitted.
Thinking About a New Combi Boiler?
Call Anthony on 07896 000354 for a free survey and a clear written quote. We cover Dartford, SE London and Kent.
Call for a Free QuoteWhy the Power Flush Matters
A new boiler is only as good as the water running through it. Older systems collect sludge, rust and debris over the years, and if that is left in place it heads straight for the new boiler's heat exchanger. That is one of the most common reasons a brand-new boiler starts having problems early.
That is why a power flush is included as standard on our combi swaps. The system is flushed through at high flow to lift out the sludge, so the new boiler starts life on clean water and the radiators heat more evenly.
Occasionally a system is so heavily sludged that it needs a second day of cleaning. If that happens, we explain why and agree the extra cost with you first. Nothing is added without your say-so.
Fitting, Commissioning and Handover
Once the system is clean, the new boiler is hung, connected to gas, water and heating pipework, and the controls are wired in. Then comes the part that matters most: commissioning.
- Gas checks: The gas supply is tightness tested and the boiler's combustion is checked to make sure it is burning safely and efficiently.
- System checks: The system is filled, pressurised and bled, and every radiator is checked for heat.
- Hot water: Taps and showers are run to confirm the hot water flow and temperature.
- Walk-through: Before we leave, you are shown how to use the new controls, how to set the heating and what the pressure gauge should read.
All gas work is carried out by a Gas Safe registered engineer, and you are left with the commissioning paperwork and warranty details for your records and any future sale of the house.
Cost and Warranty
Most boiler installs we carry out fall between £2,000 and £4,500 supplied and fitted. Where yours sits depends on the boiler you choose, where it is going and how much pipework needs changing. A straight combi swap in the same position is towards the lower end; moving the boiler or changing system type costs more.
As a Vaillant-approved installer, we can offer an extended Vaillant warranty of up to 10 years on qualifying boilers, registered for you on completion. Other brands carry their own manufacturer's warranty, and all of our labour comes with a 12-month workmanship guarantee. You can read more about how we fit new boilers on our boiler installation page.
Frequently Asked Questions
On a like-for-like combi swap, the heating and hot water are off for the working day only. The old boiler is drained down in the morning and the new one is normally running and tested before we leave that evening.
No. Most people replace a combi with a combi, but if your hot water needs have changed we can talk through a system boiler and cylinder instead. That is a bigger job than a straight swap, so it is quoted separately after the survey.
A power flush is included as standard. If a system is so heavily sludged that it needs a second day, we explain why and agree the extra cost with you before any further work is done.
No. As a Vaillant-approved installer, we register the warranty for you when the boiler is commissioned, and Vaillant boilers can qualify for an extended warranty of up to 10 years. Our own labour carries a 12-month workmanship guarantee.
