Boilers
How long does it take to fit a new boiler?
3 min read · Last updated · Reviewed for accuracy before publishing
A straightforward like-for-like combi swap is almost always a one-day job — out with the old, in with the new, flushed, tested, controls set and handed over, often with your heating back on by evening. It stretches to two or three days when the job is more than a swap: converting from a system boiler to a combi (or vice versa), moving the boiler to a new location, upgrading pipework, or fitting a cylinder. A proper installer tells you the realistic timeline at the quote, not on the morning.
The one-day swap
Replacing a combi with a combi in the same spot is the bread-and-butter one-day install: isolate and remove the old unit, fit the new one, run a full system flush (not a quick rinse), fit a filter, set up the controls, test everything and walk you through it. You lose heating for the working day, not longer.
The two-to-three-day jobs
Conversions mean pipework changes and often a cylinder going in or coming out. Relocating the boiler adds new gas, water, flue and condensate runs. Older homes can need pipe upgrades. None of it is unusual — it just isn't a one-day job, and any installer promising it in a day is either cutting a corner or hasn't looked properly. EnergyPages says labour is typically 1–3 days at £200–£400 per engineer-day, which is why extra days show on the quote.
What a good install day includes
A tidy, dust-sheeted work area, the old boiler and packaging taken away, a genuine flush, the system re-pressurised and balanced, the boiler registered for its warranty, and time spent showing you the controls — not a five-minute exit. Tidy-as-standard isn't a favour; it's the job.
“Any installer promising a conversion in a day is either cutting a corner or has not looked. A swap is a day; real pipework changes take two or three, and we say which at the quote.”
Sources
- Worcester Bosch — Step by step guide to boiler installation (usually 1–3 days; straightforward like-for-like swaps in about a day; accessed 19 Oct 2026)
- EnergyPages — Boiler costs (magnetic filter £100–£200; power flush £300–£600 on older systems with sludge; vertical flue +£200–£500 plus possible scaffolding; upsizing an older gas supply pipe £200–£600 depending on the run; 24–30 kW suits most 3-bed homes and 30–35 kW a 4-bed with two bathrooms; oversizing burns more gas per cycle and shortens boiler life; accessed 19 Oct 2026)
- GOV.UK — Approved Document L, volume 1: dwellings (before a new heating appliance is installed the central heating and primary hot water circuits should be thoroughly cleaned and flushed, a chemical inhibitor added, the feed water to combination boilers treated in hard water areas, and the system commissioned to BS 7593 — paragraph 8.8 of the 2021 edition incorporating 2023 amendments, 7.8 of the 2026 edition; a replacement gas boiler in an existing dwelling should meet 92% ErP efficiency, or in exceptional circumstances 78% SEDBUK; accessed 19 Oct 2026)
David founded Datum after years on the tools across heating and plumbing. He writes the boiler and cost guides from what actually happens on real installs.