What size gas boiler do I need?
Updated: 17.10.2026
You may be holding a quote that leads with a kilowatt badge, or an amazon.co.uk result priced like a weekend away. What size gas boiler do I need is a heat-loss question for a combi, system, or regular boiler, and a Gas Safe engineer answers it after seeing the house. The eleven listings in this set are a different object. Peter F., DIY & tools editor, checked those amazon.co.uk pages on 15 October 2026 and did not lab-test them. Shelf prices run from £150.44 to £379.99, and each one is an instantaneous gas water heater. The ASIN table is the [gas boiler comparison](/hub/hot-water/gas-boiler). If you buy through our links, we may earn a commission at no extra cost to you.
You may be holding a quote that leads with a kilowatt badge, or an amazon.co.uk result priced like a weekend away.
You may be holding a quote that leads with a kilowatt badge, or an amazon.co.uk result priced like a weekend away. What size gas boiler do I need is a heat-loss question for a combi, system, or regular boiler, and a Gas Safe engineer answers it after seeing the house. The eleven listings in this set are a different object. Peter F., DIY & tools editor, checked those amazon.co.uk pages on 15 October 2026 and did not lab-test them. Shelf prices run from £150.44 to £379.99, and each one is an instantaneous gas water heater. The ASIN table is the gas boiler comparison. If you buy through our links, we may earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Role | Fuel | Stated output | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Camplux Camplux BW158BC 6 L with rain cap | Outdoor shower | Propane or butane | 12 kW, 6 L/min | 8.9 | about £219.99 | View offer |
| Alternative |  Gasland Gasland BG300NG 11 L/min natural gas | Indoor water heater | Natural gas, 20 mbar | 22 kW, 11 L/min | 8.1 | about £178.49 | View offer |
| Preis-Tipp |  Camplux Camplux AY132B 5 L portable | Portable shower | Propane | 10 kW, 5 L/min | 8.4 | about £150.44 | View offer |
On this page
A house boiler and an Amazon water heater are different buys
A fitted gas boiler in a UK home is a condensing appliance for radiators and hot water: a combi, a system boiler, or a regular heat-only boiler. Energy Saving Trust’s typical Great Britain figure for replacing a G-rated gas boiler with an A-rated boiler, including thermostatic radiator valves, is £3,500. That is a different job from this Amazon shelf, where prices run from £150.44 to £379.99.
Three Gasland units are wall-mounted indoor water heaters. The BG300NG is £178.49, listed at 11 L/min and 22 kW on natural gas at 20 mbar. That 22 kW is water-heater output. It is not the heat loss of a three-bedroom house, and the bullets do not say the unit feeds a heating circuit. An efficiency cell is not a Boiler Plus certificate. From April 2018, England asks for at least 92% ErP, with no opt-out. The GS318NG cell says “Wysoki”, which is not an A–G letter. How the three types differ in daily use is on the combi vs system boiler page.
Combi, system, or regular comes before kilowatts
Which? points a small flat with one bathroom towards a combi, and a Gas Safe registered engineer must do the installation. British Gas describes a combi as one wall unit on the mains, with no cylinder. A regular boiler needs a hot-water cylinder and a cold tank. A system boiler needs the cylinder and does not need the cold loft tank. Choose the type before anyone argues about a number.
The fork is how many outlets run at once, and what is already in the loft or the airing cupboard. One shower at a time, with no space for a cylinder, is the combi case. Two showers together need stored hot water. Poor incoming mains pressure is one reason Which? gives for a system boiler. An 11 L/min Amazon bullet does not settle that. If the type is settled and you are choosing fuel, the pressures are on the LPG or natural gas boiler page.
Why the kW tables do not match
Published tables disagree because they answer slightly different questions, and none of them is a regulation. British Gas sizes from radiator count and from baths or showers. On that page, size means output in kilowatts, not the width of the case. Vaillant publishes bands, including 24–27 kW for a small home and 28–34 kW for a larger combi, and tells you to ask an installer. Those bands are Vaillant’s.
The Heating Hub argues that space-heating load for many UK homes is under 10 kW, often 6–8 kW, while the large number on a combi badge is hot-water flow. A higher kW is not automatically safer. The badge can be the hot-water claim, while the radiators short-cycle if the space-heating load is small. We will not invent a single correct kilowatt for a UK house.
What to take to a Gas Safe survey
Book the survey before you pay for a model. Take the fuel you already have, mains gas or LPG, how many bathrooms run at the same time, and whether a cylinder or a cold loft tank is already there.
Ask for a heat-loss calculation rather than a badge copied from a neighbour. Your quote can sit either side of the published typical figure. Who may connect the gas is on the gas boiler installation page. If you clicked an Amazon listing, stop at the role. Indoor flued units still need a professional install, and the BG300NG bullets name a 110 mm flue that is not in the box. Portable units are outdoor showers.
Common questions
What size gas boiler do I need for a 3-bed house?
There is no single kilowatt for a three-bedroom house. British Gas sizes from radiators and from baths or showers. Vaillant’s bands and The Heating Hub’s 6–8 kW space-heating argument measure different things. Pay for a survey that separates them.
Is a higher kW always safer?
A higher badge can be the hot-water rating, not spare capacity on the radiators. Far above the heat loss, a boiler can short-cycle. Too little hot-water capacity shows up when two outlets run. Separate those loads in the survey.
Is a £180 gas boiler on Amazon the same purchase?
The BG300NG at £178.49 is 11 L/min of natural-gas hot water. The bullets do not describe radiator heat. Energy Saving Trust’s £3,500 is a typical fitted G-to-A replacement in Great Britain, including thermostatic radiator valves. That is the house quote.
Can I trust an A+ cell on a listing as the energy rating?
An Amazon efficiency cell is not Boiler Plus. From April 2018 a new gas boiler in England has to meet at least 92% ErP, with no opt-out. The GS318NG cell says “Wysoki”, which is not an A–G letter. A+ on a portable shower is seller text.
Who should calculate the heat loss?
A Gas Safe registered engineer quoting the boiler should calculate the heat loss, or work from a calculation you can read. Bring the fuel, the bathrooms you use together, and any cylinder or loft tank. Shared Amazon reviews are not that survey.
What does a fitted replacement cost, as a published typical figure?
Energy Saving Trust publishes £3,500 as a typical Great Britain cost to replace a G-rated gas boiler with an A-rated one, including thermostatic radiator valves. It is not your street’s quote, and it is not the price of these water heaters.
Related pages
Sources
Public UK sources and the eleven amazon.co.uk listings, last checked on 17 October 2026.
- Energy Saving Trust: Boilers (retrieved 17 October 2026)
- Which?: How to buy the best boiler (retrieved 17 October 2026)
- BEIS: Boiler Plus policy clarification (retrieved 17 October 2026)
- HSE: Who can do landlord gas safety checks (retrieved 17 October 2026)
- Regulation 36, Gas Safety (Installation and Use) Regulations 1998 (retrieved 17 October 2026)
- Gas Safe Register: Overview (retrieved 17 October 2026)
- GOV.UK: Smoke and carbon monoxide alarms, England Q&A for landlords (retrieved 17 October 2026)
- Liquid Gas UK: Consumer Guidance Sheet 39 (retrieved 17 October 2026)