What the job involves
A combi boiler replacement means taking out your existing boiler and fitting a combination boiler that heats the radiators and produces hot water on demand. There is no hot water cylinder and no loft tank. Most jobs fall into one of two types.
Like-for-like swap. You already have a combi, and the new one goes on the same wall in the same place, using the existing gas, water, flue and heating connections. This is the simplest job.
System conversion. You have a regular or system boiler with a cylinder and possibly tanks in the loft, and you switch to a combi. This is more involved. The cylinder and tanks are usually removed, pipework is altered, the heating circuit is changed to a sealed system, and the boiler may move to a different spot. Sometimes the gas supply or flue route needs work too.
In both cases the work includes isolating and draining the system, removing the old boiler, fitting and connecting the new one, installing a flue, filling and venting the system, and commissioning. Commissioning means testing combustion, checking gas pressure, checking safety devices and setting the boiler up properly. Gas work must be done by a Gas Safe registered engineer, and a boiler replacement should be notified to building control, which a registered installer normally does for you. You should receive paperwork afterwards.
What affects the price
- Type of job. A straight swap needs the least labour. A conversion from a system with a cylinder costs more because of the extra strip-out, pipework changes and making good.
- Boiler make and model. Output, build quality, brand and the length of the manufacturer's warranty all vary. A higher output or a premium range costs more than a basic model.
- Right size for the home. A combi suits smaller properties with modest hot water demand. A house with several bathrooms may need a different solution, and an installer who recommends one should be able to explain why.
- Boiler position. Moving the boiler means new gas, water, condensate and flue runs. Keeping it where it is costs less.
- Flue route. A short, direct flue through an external wall is simple. A longer run, a vertical flue through a roof, or extra bends and extensions add parts and labour.
- Condensate disposal. Modern boilers need a condensate drain. If there is no convenient internal waste, an external run or a pump may be needed.
- Gas pipe size. If the existing gas supply is too small for the new boiler, it will need upgrading, and this can involve digging or lifting floors.
- Condition of the existing system. Old pipework, sludgy radiators and tired valves may need attention. Cleaning the system, whether by a powerflush or a lighter chemical flush, adds time and cost.
- Extras. A magnetic filter, scale protection in hard water areas, new thermostatic radiator valves, a smart or programmable thermostat and weather compensation all add to the total.
- Access and making good. Lofts, tight cupboards, upstairs boiler locations and the need to patch plaster or boxing-in affect labour.
- Your location and the installer. Local labour rates, how busy the installer is, and whether it is an emergency breakdown job all play a part.
Ways to keep the cost down
- Keep the boiler where it is. Moving it is one of the biggest avoidable extras.
- Get several itemised quotes. Comparing like with like shows where the differences are. Use the prices on this page as a guide to what homeowners nearby have actually paid.
- Choose a sensible boiler, not the most expensive. A reputable mid-range model with a decent warranty is often better value than the top of the range.
- Plan ahead if you can. Replacing before the old boiler fails gives you time to compare, and avoids rushed decisions in the cold.
- Ask what the system really needs. If it is in good condition, a proper clean may be enough. If it is not, paying for the work now is cheaper than a failed boiler later.
- Prepare the space. Clearing the cupboard, loft access and floor around the boiler saves the installer time.
- Do not cut corners on safety items. Filters, controls and commissioning protect the boiler and your warranty.
What should be included in a quote
A proper quote is written, itemised and specific. Check that it covers:
- [ ] Make, model and output of the boiler, and warranty terms
- [ ] Whether it is a like-for-like swap or a conversion, and what that involves
- [ ] Removal and legal disposal of the old boiler, and any cylinder or tanks
- [ ] New flue and any extensions, bends or terminals
- [ ] Condensate pipework and where it will drain
- [ ] Any gas, water or heating pipework alterations
- [ ] System cleaning, and which method is proposed
- [ ] Magnetic filter and any scale protection
- [ ] Heating controls, thermostat and radiator valves
- [ ] Commissioning, the commissioning record and building regulations notification
- [ ] Making good, and what is and is not covered
- [ ] Gas Safe registration number of the engineer or company
- [ ] Payment terms, start date, expected duration and VAT treatment
- [ ] What happens if extra work turns up, and how it will be priced
Questions to ask a heating engineer
- Can I see your Gas Safe card, and which gas types is it valid for?
- Why have you chosen this boiler size and model for my home?
- Will my water pressure and flow rate cope with a combi?
- Where will the flue and condensate pipe go, and will it be visible?
- Does the existing gas pipe need upgrading?
- Is my system clean enough, or do you recommend cleaning it?
- What controls will I get, and are they included?
- What does the warranty cover, and does it depend on servicing or on a filter being fitted?
- Who notifies building control, and when will I get the paperwork?
- Is there a fixed price, and what could change it?
- How will you protect floors and furnishings?
How long it takes and what to expect
A like-for-like swap is usually done within a day. A conversion from a system with a cylinder usually takes longer, often a couple of days or more, especially if the boiler moves or the pipework changes a great deal. A system clean adds time.
Expect the heating and hot water to be off for much of the working day. The engineer will drain the system, remove the old boiler, fit the new one and connect everything, then fill, vent and test. There will be some noise, dust and disruption, particularly when a flue goes through a wall or when cylinders and tanks come out.
At the end, the engineer should show you how to use the boiler and controls, explain the pressure gauge, and hand over the commissioning record and warranty details. Keep these safe. A boiler that is serviced regularly is more likely to keep its warranty valid and keep running well.
Signs a quote is too cheap or too expensive
Too cheap
- No itemisation, just a single vague figure
- No mention of a flue, filter, controls or system cleaning
- The installer cannot show Gas Safe registration, or avoids the subject
- No commissioning record or building regulations notification
- An unknown or unusually basic boiler with a short warranty
- Pressure to pay a large sum upfront or decide on the spot
Too expensive
- A premium boiler recommended without a clear reason
- Charges for extras your home does not need, such as a flue extension or moving the boiler when neither is required
- Large allowances for unspecified work
- A figure far above other quotes for the same scope, with no explanation
If one quote is much higher or lower than the others, ask the installer to explain the difference line by line. A good engineer will be happy to do so. Compare the quote against the prices on this page for similar jobs in your area before you decide.