The boiler decision you'll live with for the next twelve winters.

A proper boiler quote starts with a heat-loss calculation, not a glance at the old boiler's badge. The badge tells you what was fitted in 2008, not what your home needs in 2026; after the cavity wall insulation, the loft top-up, and the new windows.

If your installer hasn't asked these four questions, they're sizing by guesswork:

• How many radiators do you have, and what size?; determines real heat demand, not boiler badge.
• How many bathrooms, and what's the mains pressure at the kitchen tap?; decides combi vs system.
• What's been done to the fabric since the last boiler?; every retrofit shrinks the kW you need.
• Where does the flue go, and is the gas pipe 22mm or 15mm?; the install detail that blows budgets.

Combi: heats water on demand from the mains. No cylinder, no loft tank, more cupboard space. Brilliant for flats and small houses. Falls over when two showers run at once, or when mains pressure is poor (most Victorian terraces).

System: a sealed boiler feeding a hot water cylinder. More space needed, but consistent hot water to multiple outlets, easy to pair with solar thermal or a future heat pump. The right answer for any home with two or more bathrooms.

Regular (heat-only): a system boiler with a separate loft tank. Specify only if you're keeping an existing gravity setup intact. Otherwise, system every time.

Modern UK boilers; Worcester, Vaillant, Viessmann, Ideal, Baxi; are mostly excellent at the premium end and broadly fine in the mid-range. The brand argument is over-played. The kW argument almost never happens, and it costs households thousands over the boiler's life.

An oversized boiler short-cycles: it heats the radiators to setpoint quickly, switches off, cools, switches back on. That cycling wears the boiler out faster and uses 10–15% more gas than a correctly sized unit. A 24kW combi running hard is cheaper to own than a 35kW combi loafing.

Rule of thumb for a UK home with normal insulation: 24kW for a 2-bed flat or small terrace, 28kW for a 3-bed semi, 32–35kW only for a large 4-bed with two bathrooms running off the combi. If you're being sold bigger, ask for the heat-loss calc.

Current government direction: no new gas boilers in new-build homes from 2025, no new gas boilers in existing homes from 2035 (subject to political reversal; this has shifted twice already). Replacement boilers for failing units are likely to remain legal beyond that date, but expect rising gas standing charges and Climate Change Levy from 2030 onwards.

Practical translation: a boiler fitted in 2026 will see out its warranty. A boiler fitted in 2032 may not. If your boiler is healthy now, plan your heat-pump-ready upgrades (insulation, larger radiators, hot-water cylinder) in the meantime so you have options at end-of-life.

A heat pump is the better answer when: your home is well insulated (EPC C or better), your gas bill exceeds £1,500/year, you can use the £7,500 Boiler Upgrade Scheme grant, and you have somewhere sensible to put a cylinder. In that scenario the maths usually beats a new combi within the warranty period.

It is not the better answer if your home is poorly insulated, you've no space for a cylinder, or your mains supply is single-phase 60A and already maxed out. Replacing the boiler buys you time to fix the prerequisites.