Boilers & Heating
Combi vs system vs regular
TL;DR
Combi: one boiler, no tanks, hot water on demand — best for 1-2 bathroom homes. System: boiler + unvented cylinder — best for 3+ bathrooms or simultaneous showers. Regular: boiler + cylinder + loft tanks — best when replacing like-for-like in an older property. The right answer is set by hot-water demand, not house size.
The single biggest mistake in UK boiler replacement is fitting a combi to a house that really needs a system boiler. Two showers running at 7am, a combi rated for 15 L/min hot water, and you end up with two lukewarm showers instead of one hot one. Sizing this correctly costs nothing at quote stage and everything if you get it wrong.
This page compares the three types honestly — including the situations where regular boilers still make sense in 2026 — so you can walk into a quote knowing what you actually need.
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What is a combi boiler and when is it right?
A combi (combination) boiler heats water on demand — no cylinder, no loft tanks. Open a hot tap and cold mains water flows through the boiler's plate heat exchanger and out the tap hot. It's compact, cheap to install, and about 90% of new UK installs are combis.
A combi is right when: you have one bathroom (or two used at different times), your incoming cold-water main delivers at least 1.5 bar pressure and 12 L/min flow, and you want more storage space (no cylinder, no loft tanks). It's wrong when two showers might run simultaneously — a combi splits its flow rate between them and you get half the power at each.
What is a system boiler and when is it right?
A system boiler heats radiators and heats an unvented (mains-pressure) hot-water cylinder that stores hot water ready for use. There are no loft tanks — the cylinder is fed directly from the mains, so you get full mains pressure at every tap and every shower, and multiple showers can run at once without any drop.
A system boiler is right when: you have 2+ bathrooms in regular use, you want strong pressure at every outlet, or you're planning to add solar thermal or a heat pump later. It costs more to install than a combi (mostly the cylinder) but it's the right long-term answer for family homes with high peak demand.
What is a regular (heat-only) boiler and when is it right?
A regular boiler heats radiators and heats a hot-water cylinder that's fed from a cold-water tank in the loft — the traditional 1970s-90s UK setup. Water pressure at hot taps is gravity-fed, which is why the shower in a house with a regular boiler is often weaker than the cold tap next to it.
In 2026, regular boilers are almost never the right answer for a new-build or major refurbishment. But when your existing house has a working cylinder, sound loft tanks, and a lot of pipework already set up for gravity feed, a like-for-like regular swap is often the cheapest and least disruptive option. If you'd otherwise be re-routing every hot pipe in the house, sometimes the honest answer is: stick with the same setup.
How do I choose based on my house?
Answer these four questions and you have your answer:
- How many bathrooms are used simultaneously at peak times? 1 → combi. 2+ → system.
- What's my incoming mains flow rate? Under 12 L/min → cylinder-based system, not combi.
- Do I have loft tanks and a cylinder already, and are they sound? Yes → regular swap is cheapest.
- Am I planning a heat pump in the next 10 years? Yes → system with a cylinder makes the transition easier.
What about running costs?
All three types can now hit 92-94% ErP efficiency, so day-to-day running cost is similar per unit of heat delivered. The real cost difference is behavioural: combis waste less heat because they don't store hot water, but system and regular boilers with cylinders let you time hot-water production for cheap tariff periods (e.g. Octopus Agile) or use solar thermal top-up.
For most homes, the annual gas bill difference between the three types is under £60 either way. Choose based on hot-water performance and layout, not on running cost.
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 install (like-for-like) | 2,200 – 3,200 | One day, one boiler on the wall |
| Combi install (upgrade with flush, filter, smart stat) | 2,600 – 3,800 | Boiler Plus compliant |
| System boiler + new unvented cylinder | 3,800 – 5,800 | Two-day install typically |
| System boiler swap (keep cylinder) | 2,800 – 3,800 | If cylinder is under 8 yrs old |
| Regular boiler like-for-like swap | 2,400 – 3,600 | Cheapest option with sound tanks |
| Regular → Combi conversion | 3,400 – 4,800 | Removes tanks and cylinder |
| Regular → System conversion | 4,200 – 6,200 | Removes F&E tank, keeps cylinder |
Which brand should you fit?
All the major brands (Worcester, Vaillant, Ideal, Baxi, Viessmann, Alpha) make all three types. Combi ranges get more R&D attention because they're the biggest sellers, but system and regular ranges from Worcester (Greenstar Ri, CDi Compact) and Vaillant (ecoTEC plus) are excellent choices. 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".
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