Boilers & Heating
Boiler replacement
TL;DR
If your boiler is 12+ years old, has failed twice in two years, or would cost more than half the price of a new one to repair, replacement usually wins. Most replacements are a one-day job, £2,200-£3,800 for a like-for-like combi, and cut annual gas use by 20-35%.
Replacement is different from installation. On an install we're often designing a system from scratch. On a replacement we're swapping out an existing appliance, which means we have to make sensible decisions about what to keep (pipework, radiators, controls) and what has to go (asbestos flue, undersized gas run, ancient F&E tank). A good replacement quote itemises all of that in writing before you commit.
This page walks through when replacement actually makes sense over repair, what a proper like-for-like swap includes, and what to watch for in the quotes you get from national brands and cheap outfits — because both extremes tend to hide the same things.
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When does replacement beat repair?
There are three honest triggers. Any one of them is a reason to seriously price replacement; two of them and it's almost always the right answer.
- Age — over 12 years old and out of manufacturer warranty
- Repair cost — over 50% of the price of a straight replacement
- Repeat failure — two significant faults in the last 24 months
- Efficiency — non-condensing ("heavy" cast-iron style) boiler still in place
- Parts availability — manufacturer no longer stocks OEM parts for the model
What does a proper boiler replacement include?
The single biggest reason boilers fail early is that the system water was never properly cleaned when the new boiler was fitted. Old iron oxide sludge chews through modern aluminium heat exchangers in months. A KJ boiler replacement always includes:
- System flush — chemical or powerflush depending on condition, with a water sample
- Magnetic filter fitted on the return — sized for the boiler
- Inhibitor dosed to manufacturer's concentration with a treatment tag
- New expansion vessel or check of existing pre-charge to spec
- Condensate re-routed to internal soil stack if currently going outside
- Boiler Plus compliant control (weather comp, load comp, or smart stat)
- Gas supply verified for adequate working pressure at max fire
- Benchmark filled in, manufacturer registered, Building Control notified via Gas Safe
Can I keep the same radiators and pipework?
In 90% of cases, yes. Modern condensing boilers work well with existing 15mm and 22mm copper. The exception is when we're upgrading from a low-pressure gravity-fed regular boiler to a mains-pressure combi — then the flow and return may need increasing at the boiler and some horizontal runs may need re-piping to remove airlocks.
Radiators very rarely need replacing at the same time as the boiler unless they're already leaking or badly corroded. We check each one with a thermal camera during the commissioning walk-round.
Should I upgrade boiler type at the same time?
This is the one big architectural decision that comes up during replacement. Common upgrades and when they make sense:
Regular → Combi: right call when you have one bathroom, want more loft space, and your mains pressure is at least 1.5 bar with 12+ L/min flow.
Regular → System: right call when you have 2+ bathrooms and want to keep a cylinder for simultaneous showers.
Combi → System: makes sense when you've added an ensuite or family bathroom and simultaneous hot-water demand is stretching your combi.
System → Heat pump: possible but usually not on a like-for-like schedule — it's a separate design project with insulation upgrades and larger radiators.
How long does a replacement take?
Like-for-like combi swap: one day. Regular to combi conversion: two days. System boiler replacement with new cylinder: two days. We do our best to leave you with heating and hot water at the end of each day, and we plan the disruption around when you'll be home.
Typical prices
Real-world ranges from recent Northwest jobs. Your final price is confirmed in writing before any work starts.
| Item | Typical price | Notes |
|---|---|---|
| Like-for-like combi swap | 2,200 – 3,200 | Existing flue reused |
| Combi swap with flush + filter + smart stat | 2,600 – 3,800 | Boiler Plus compliant |
| System boiler + new unvented cylinder | 3,800 – 5,800 | G3 certificate included |
| Regular boiler swap (keep tanks) | 2,400 – 3,600 | Cheapest option if tanks are sound |
| Regular → Combi conversion | 3,400 – 4,800 | Removes cylinder + loft tanks |
| Combi → System with cylinder | 4,200 – 6,200 | Adds cylinder + controls |
| Extended warranty (up to 12 years) | 0 – 250 | Free with accredited installer offers |
| Full powerflush | 450 – 650 | If system water is dirty |
Which brand should you fit?
For most Northwest homes we recommend Worcester Bosch and Vaillant for their parts network — you'll never wait more than 24 hours for a spare. Ideal Logic Max is the best value at the budget end. Baxi Assure gets landlords a longer standard warranty. Viessmann has the best aluminium heat exchanger but parts are slower. We install and service Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann and Alpha — none of them pay us commission and we don't push a "boiler of the month".
Our promise on this job
Flat £50 diagnostic — deducted in full from any work booked. Fair, transparent quote before we start. If we can't fix the fault we diagnosed, we refund the diagnostic. All gas work is signed off by a Gas Safe registered engineer.
Talk to KJ about boiler replacement
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