Boilers & Heating
Boiler installation
TL;DR
A good boiler install is 20% heat-loss sizing, 20% gas-supply and flue routing, and 60% flush, filter and controls. Get those right and the boiler you fit almost doesn't matter — get them wrong and even the best boiler will fail early.
Most quotes you'll get for a new boiler are worked out on a doorstep in ten minutes: bedrooms, bathrooms, done. That's how homes end up with a 35 kW combi in a two-bed terrace, cycling on and off every three minutes and eating gas. A proper install starts with an honest heat-loss calculation for the actual house, then confirms the gas supply can deliver the flow the appliance needs, then picks the boiler.
This page walks through how we quote and install boilers at KJ Plumbers, what a compliant job actually includes (not just "a new boiler on the wall"), and what you should expect to pay in the North West in 2026. If you want a written fixed price, KJ — our AI assistant — can put a rough estimate together in about three minutes, then we come and confirm it in person.
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How do I know what size boiler I actually need?
The rule of thumb of "1 kW per radiator + 3 kW per bathroom" is a starting point, not an answer. We do a room-by-room heat-loss based on wall construction, window U-values, exposure and target temperature. A well-insulated three-bed semi in Rochdale often needs a 24 kW combi, not the 32 kW model the merchant tries to upsell.
Oversizing a combi hurts efficiency: the burner short-cycles, condensing mode is barely reached, and the flow rate through your existing pipework becomes the real bottleneck, not the boiler's headline kW. The right size is the smallest boiler that comfortably meets your peak hot-water demand.
What does a compliant boiler installation actually include?
Building Regulations Part L and the boiler manufacturer's MI (manufacturer's instructions) are what dictate the job — not tradition. A code-compliant install in 2026 includes all of the following, and if any are missing your warranty is not valid:
- Powerflush or chemical flush of the existing system with a proven procedure and water sample
- Magnetic system filter fitted on the return, sized for the boiler output
- Inhibitor dosed to manufacturer's concentration, with a treatment tag
- Correctly sized gas supply pipework verified with a working pressure and standing pressure test
- Condensate run to an internal soil stack where possible, insulated externally if it must go outside
- Weather compensation or load compensation control (Boiler Plus requires one of these on combi installs)
- Benchmark commissioning book completed and boiler registered with the manufacturer within 30 days
- Notification to Gas Safe within 30 days and building notice lodged with your local authority
Combi, system or regular — how do we pick?
This is the biggest single decision and it's almost always driven by hot-water demand, not the size of the property. A four-bed with one shower and a bath used one at a time is a combi job. A three-bed with two showers used simultaneously in the morning is a system boiler and cylinder job.
Regular (heat-only) boilers are usually the right call in older houses that already have a good cylinder and loft tanks — a like-for-like swap is cheaper and less disruptive than converting the entire system. There's a full comparison on our combi vs system vs regular page.
How long does a boiler installation take?
A like-for-like combi swap is a one-day job. Converting a regular system to a combi is typically two days and involves removing the cylinder, cold tank and F&E tank, running new mains-pressure pipework, and often a powerflush over lunch. A first-time install (no existing gas or heating) can be three to five days.
We do our best to leave you with heating and hot water at the end of each day, and we sheet up work areas before we start. Landlord installs are prioritised so tenants aren't left without hot water.
What controls should be fitted?
Boiler Plus (in force since 2018) requires one of four extras on every combi install: weather compensation, load compensation, flue gas heat recovery, or a smart thermostat with automation and optimisation. We default to a smart thermostat (Nest, Hive, Tado or Honeywell Evohome) because it delivers the biggest real-world saving and gives you app control.
Where the boiler supports OpenTherm or eBUS, we wire it in so the boiler modulates properly instead of just switching on and off. That single change is often worth 8-12% on the annual gas bill.
Typical prices
Real-world ranges from recent Northwest jobs. Your final price is confirmed in writing before any work starts.
| Item | Typical price | Notes |
|---|---|---|
| Combi swap (like-for-like) | 2,200 – 3,200 | One day, existing flue reused |
| Combi install with system flush + filter + smart stat | 2,600 – 3,800 | Boiler Plus compliant |
| System boiler + unvented cylinder | 3,800 – 5,800 | G3 unvented certificate included |
| Regular (heat-only) swap | 2,400 – 3,600 | Keeps tanks and cylinder |
| Regular → Combi conversion | 3,400 – 4,800 | Removes cylinder and loft tanks |
| First-time gas boiler install | 4,500 – 7,000 | New gas run, flue, controls |
| Flue extension (per metre) | 90 – 140 | If flue route needs extending |
| Powerflush (up to 10 rads) | 450 – 650 | Water sample provided |
Which brand should you fit?
For most Northwest homes we recommend Worcester Bosch or Vaillant for their parts availability and dealer network, Ideal for good value on a tight budget, and Baxi where a landlord wants a longer standard warranty. Viessmann and Alpha are excellent but harder to get parts for on a Saturday. 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 installation
Real triage in seconds. £50 diagnostic visit — refunded if we don't fix it.