Changing to a combi boiler is one of the most popular home heating upgrades in the UK right now. Thousands of homeowners and landlords are switching from old conventional (regular) or system boilers to modern combination boilers every year because they save space, reduce energy bills, and provide instant hot water without a cylinder.
In this guide, I’ll explain everything you need to know – from costs and benefits to suitability, installation process, and real-world considerations.
Why Are So Many UK Homeowners and Landlords Changing to a Combi Boiler?
Changing to a combi boiler remains the number one boiler upgrade choice across the UK because of these key advantages:
- Space saving – removes the need for a hot-water cylinder and loft tanks
- Lower energy bills – typically 10–25% cheaper to run than old G-rated boilers
- Endless hot water – no waiting for a cylinder to reheat
- Easier and usually cheaper installation when converting
- Higher property value and better EPC rating (important for landlords)
- Neater appearance – everything in one wall-mounted unit
Is Your Home Actually Suitable for Changing to a Combi Boiler?
Not every property is ideal for a combi swap. Here are the main factors Gas Safe engineers check:
Mains Water Pressure and Flow Rate
Combi boilers need good incoming mains pressure (ideally ≥1.5 bar) and flow rate (≥12 litres per minute). Older houses with lead pipes or properties at the top of hills often have low pressure – this can make a combi perform poorly.
Number of Bathrooms and Simultaneous Demand
If you regularly run two showers + kitchen tap at once, a powerful 35–40 kW combi (e.g. Worcester 4000, Vaillant ecoTEC Plus, or Viessmann Vitodens 100-W) can cope. Larger homes with 3+ bathrooms are usually better with a system boiler + unvented cylinder.
Existing Pipework Layout
Properties that already have a hot-water cylinder can almost always be converted, but very old gravity-fed systems may need significant pipework upgrades.
How Much Does Changing to a Combi Boiler Cost (UK Average Prices)?
| Item | Average Cost (inc. VAT) |
| New combi boiler supply only | £2,300 – £3,800 |
| Full installation & conversion | £3,800 – £6,500 |
| Powerflush | £450 – £750 |
| System upgrades (filters, valves) | £300 – £800 |
| Removing old tanks & cylinder | £350 – £650 |
| Total typical cost | £4,500 – £7,500 |
Landlords can currently claim 100% capital allowances, and homeowners may qualify for the Boiler Upgrade Scheme (£7,500 grant) if switching to a heat pump, but not for gas combi upgrades.
Step-by-Step: What Happens When You Change to a Combi Boiler
- Free home survey by a Gas Safe engineer
- Old boiler, cylinder, and header tanks removed
- New high-efficiency combi mounted (usually in the same place or kitchen)
- System powerflushed
- New pipework alterations (bypass, filling loop, condensate drain)
- Magnetic filter and scale reducer fitted (mandatory on most warranties)
- New wireless thermostat/controls installed
- Full commissioning, Gas Safe certificate & Building Regulations sign-off
The whole job normally takes 2–4 days.
Pros and Cons of Switching to a Combi Boiler
| Pros | Cons |
| Huge space saving | Can struggle with very high demand |
| Cheaper running costs | Dependent on mains pressure |
| Instant hot water | Not ideal for homes with old/small pipes |
| Cleaner installation | Slightly higher upfront cost than like-for-like |
| Better for flats and smaller houses |
Best Combi Boilers for Conversion in 2026 (UK)
- Worcester Bosch Greenstar 4000 (Best all-rounder)
- Vaillant ecoTEC Plus (Quietest)
- Ideal Logic Max (Best value)
- Viessmann Vitodens 050-W (Best for tight budgets)
- Baxi 800 series (Excellent warranty)
All the above come with 10–12 year warranties when fitted by an accredited installer.
How to Prevent Problems After Changing to a Combi Boiler
- Have the system powerflushed before installation
- Fit a magnetic system filter (e.g. Adey MagnaClean or Fernox TF1)
- Install a scale reducer or water softener if you live in a hard-water area
- Annual servicing (essential to keep 10+ year warranty valid)
- Bleed radiators and check pressure regularly
FAQs
Can I keep my old radiators when changing to a combi boiler?
Yes – in 95% of cases your existing radiators can stay. Most engineers will recommend oversize radiators or add a couple if the house feels cold after the swap.
Will changing to a combi affect my shower pressure?
Usually improves it! Because combis deliver mains-pressure hot water, most people notice stronger showers – provided your incoming mains is decent.
Do I need planning permission to remove loft tanks for a combi conversion?
No. It’s considered permitted development and only requires Building Regulations approval (which your installer handles).
How long do modern combi boilers last?
With annual servicing, 12–15 years is normal. Many Worcester and Vaillant regularly see boilers still running perfectly at 18–20 years.
Can landlords legally install a combi boiler in a rental property?
Yes – in fact it often helps achieve the minimum EPC rating of E (soon to be C by 2030). Just make sure the installation is by a Gas Safe registered engineer and you issue the certificate to the tenant.
Stay warm, The Boilers2Go Team
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.