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Combi vs system boiler

Updated: 17.10.2026

Combi vs system boiler is a decision about stored hot water and how many taps run together. The amazon.co.uk listings in this set do not sit in that decision. They are eleven instantaneous gas water heaters, from £150.44 to £379.99, and none describes a radiator circuit or a cylinder. The Gasland GS318NG title uses the word sealed. A system boiler is still a central-heating appliance with a cylinder. Peter F., DIY & tools editor, wrote this from Which?, British Gas, and Baxi on 15 October 2026, beside our [gas boiler comparison](/hub/hot-water/gas-boiler). We did not survey a house. If you buy through our links, we may earn a commission at no extra cost to you.

Combi vs system boiler is a decision about stored hot water and how many taps run together. The amazon.co.uk listings in this set do not sit in that decision.

Our top picks

Camplux Camplux BW158BC 6 L with rain cap

Best overall

Camplux Camplux BW158BC 6 L with rain cap
8.9/10about £219.99
View offer
Gasland Gasland GS318NG 12 L/min natural gas

Alternative

Gasland Gasland GS318NG 12 L/min natural gas
7.7/10about £151.99
View offer
Camplux Camplux AY132B 5 L portable

Preis-Tipp

Camplux Camplux AY132B 5 L portable
8.4/10about £150.44
View offer
On this page
  1. 1.These Amazon listings are not in the choice
  2. 2.When a combi is the right boiler
  3. 3.When a system boiler is the right boiler
  4. 4.When a regular boiler still fits
  5. 5.What does not decide it

These Amazon listings are not in the choice

A combi, a system boiler, and a regular boiler are fixed central-heating appliances. The kept Amazon listings are tankless water heaters: three indoor flued Gasland units and eight portable or outdoor LPG showers. The indoor flows, 10 to 12 L/min, are tap figures. They are not a heat-loss result, and they are not evidence that the unit can stand in for a combi. Energy Saving Trust’s £3,500 is a typical Great Britain cost for a G-rated to A-rated gas replacement, including thermostatic radiator valves. It is not a price for converting a combi to a system boiler. None of the sources we used prints that conversion cost. Who each listing is for is on the best gas boiler page.

When a combi is the right boiler

Choose a combi when one bathroom runs at a time and you have no room for a cylinder. British Gas describes one wall unit on the mains, with no separate cylinder. Baxi says hot water is available at one outlet at a time, so a combi is a poor fit when more than one bathroom is used together. A higher badge does not store a cylinder of water. If two people shower at once, the second outlet feels the flow drop. Which? treats poor incoming mains pressure as a reason to look at a system boiler instead.Ask the engineer to measure mains pressure before the quote assumes a combi.
Camplux Camplux BW158BC 6 L with rain cap

Testsieger

Camplux Camplux BW158BC 6 L with rain cap
8.9/10about £219.99
View offer

When a system boiler is the right boiler

Choose a system boiler when two outlets must run together and you can house a cylinder. Baxi says it stores water so several taps can run, and that you need space for the cylinder. British Gas says it does not need a cold loft tank, because more of the pumps and valves sit in the case. Which? places it where demand is high and a cold tank is a poor fit, including poor incoming pressure. A system boiler is not automatically the family choice. If the only fault is a tired combi and you still shower one at a time, replacing like for like can be the smaller job.

When a regular boiler still fits

Keep a regular, or heat-only, boiler when the cylinder and the cold tank are staying and demand is high. Which? points a larger home with tanks and high simultaneous demand that way. British Gas calls this the conventional layout: the boiler, a hot-water cylinder, and a cold tank in the loft. If those tanks are sound and the house already runs two bathrooms from that store, removing them so a brochure can say system boiler is an extra project. Leave the regular layout when the cold tank keeps failing, or a loft conversion puts the tank in the way. A tank that has been quiet for years, with a cylinder that recovers between showers, is a reason to keep both and replace only the boiler.

What does not decide it

An Amazon flow rate does not decide the type. Neither does the GS318NG efficiency cell that says “Wysoki”, nor a kilowatt badge copied from a neighbour. The buying guide sets out why published kW tables disagree. A 30 kW combi badge will not turn a combi into a cylinder. Two showers at once still need stored water, or one of them waits. Ask the engineer to write down how many outlets must run together, whether a cylinder or a loft tank is worth keeping, and the heat-loss figure for the radiators. The limits on who connects the gas are on the gas boiler installation page.

Common questions

Should I choose a combi or a system boiler?
Choose a combi for one bathroom at a time and no room for a cylinder. Choose a system boiler when two outlets must run together and you can house a cylinder, especially if the loft tank or the mains pressure is a poor fit.
When is a regular boiler still the right swap?
A regular boiler still fits when a sound cylinder and a sound cold tank are staying, and more than one outlet needs hot water. Leave that layout when the loft tank fails, or when the tank blocks a loft you are about to use.
Can two showers run together on a combi?
Baxi says a combi supplies one outlet at a time. A higher kilowatt badge does not add a store of hot water, so the second shower feels the drop. If both showers are a normal morning, you need a cylinder and somewhere to put it.
I have a loft tank. Do I have to remove it?
You do not have to remove a loft tank that still works. A regular boiler is built around that tank and a cylinder. Which? points to a system boiler when demand is high and the cold tank is a poor fit, including frost or a loft room in the way.
Is the GASLAND GS318NG a system boiler because the title says sealed?
The GS318NG at £151.99 is an indoor water heater for natural gas at 20 mbar and 12 L/min. “Sealed” on the title is seller wording. A system boiler stores hot water and heats radiators. This listing describes neither, and “Wysoki” is not an ErP letter.
Does a higher kW combi fix two bathrooms?
A higher kilowatt figure can raise the hot-water claim for one outlet. It does not store water for a second shower. Baxi’s limit stays one outlet at a time. Two bathrooms together are a cylinder question, not a jump from 24 kW to 34 kW.

Related pages

Hot water cylinder comparison

Gas boiler

Best gas boiler: who each listing in this set is for

What size gas boiler do I need? Book a heat-loss survey

Gas boiler installation: what only Gas Safe may do

LPG or natural gas boiler: match the supply you have

Sources

Public UK sources and the eleven amazon.co.uk listings, last checked on 17 October 2026.

  1. Energy Saving Trust: Boilers (retrieved 17 October 2026)
  2. Which?: How to buy the best boiler (retrieved 17 October 2026)
  3. BEIS: Boiler Plus policy clarification (retrieved 17 October 2026)
  4. HSE: Who can do landlord gas safety checks (retrieved 17 October 2026)
  5. Regulation 36, Gas Safety (Installation and Use) Regulations 1998 (retrieved 17 October 2026)
  6. Gas Safe Register: Overview (retrieved 17 October 2026)
  7. GOV.UK: Smoke and carbon monoxide alarms, England Q&A for landlords (retrieved 17 October 2026)
  8. Liquid Gas UK: Consumer Guidance Sheet 39 (retrieved 17 October 2026)