What size boiler do I need?

For a gas combi, size on hot water: 24–28kW for one bathroom, 30–35kW for two. For system and heat-only boilers, size on your home's heat loss plus a little for the cylinder. For electric boilers, size closely to heat loss, as most single-phase models stop at 14.4kW. The kit builder estimates this from floor area and insulation; confirm it with an installer's heat loss survey.

Can an electric combi run a shower?

One modest shower, yes. An electric combi delivers about 0.41 litres a minute per kW, so a 14.4kW single-phase model gives roughly 5.9 L/min. For two outlets at once, a system boiler with a cylinder, or a three-phase combi, is the better choice.

Do I need a cylinder with underfloor heating?

Not necessarily: a combi can run underfloor heating. But UFH runs for long periods at low temperature, which leaves time to reheat a cylinder, and larger homes with UFH usually have more than one bathroom. With two or more bathrooms, choose a system boiler and cylinder.

Can I replace an open-vented system with a combi?

Yes, but it means converting to a sealed system: removing the loft tanks and cylinder, and checking that old pipework and radiators can take mains pressure. If you'd rather keep the existing pipework, a heat-only boiler is the like-for-like swap.

Is a magnetic filter required for the boiler warranty?

For most manufacturers, yes. A filter, plus cleaning and inhibiting the system to BS 7593, is a common condition of extended warranties. Check the terms for your boiler, and keep the commissioning checklist (Benchmark) your installer completes.

Can I use any flue with my gas boiler?

No. Flues are tested and certified with a specific brand's boilers, so you need the same manufacturer's flue kit, extensions and plume kit. Many boilers we sell include a horizontal flue; the kit builder only adds a flue when it isn't already in the box.

Do I need a carbon monoxide alarm?

For a gas or LPG boiler, yes. Building Regulations require a CO alarm in the room when a fixed combustion appliance is installed or replaced. Electric boilers don't burn fuel, so a CO alarm isn't required for them.

Can a natural gas boiler run on LPG?

Many can, using the manufacturer's LPG conversion kit, fitted and commissioned by a Gas Safe engineer. Some models ship with the kit included. Check the boiler's LPG approval before you order.

Why does the builder pick a Vaillant controller for Vaillant boilers?

Vaillant boilers communicate with Vaillant controls over their own eBUS connection, which unlocks load and weather compensation. That helps meet Part L and Boiler Plus and gets the most from the boiler's modulation. You can still swap it in the parts list.

Can I run an electric boiler from solar panels?

Yes. Many electric boilers can be range-rated or have multiple outputs to match available solar power, and a diverter can send surplus PV to a cylinder's immersion heater. Read our guide to running an electric boiler with solar panels.

Kit de préparation à l'achat d'une chaudière - Votre guide ultime

Boiler kit builder

Choose a boiler and everything it needs, in one basket

Build your boiler kit

What will the boiler run on?

This decides which boilers, flues and safety parts apply.

Your home and how you heat it

Rough answers are fine. They set the boiler size and whether you need a cylinder.

What will the boiler supply?

Heated floor area (m²)

Insulation

Bedrooms

Bathrooms and shower rooms

Your current system

Heating zones

Hard water area?

Hot water: on demand or stored?

The options depend on what the boiler supplies and how many bathrooms you have.

Choose your boiler

Smallest suitable size first, in stock before out of stock. Prices and stock are live.

Choose your cylinder

Sized from your bedrooms and bathrooms.

Flue route

Flues are made for each brand's boilers. The builder only picks the matching brand.

Where will the flue exit?

Extra 1m extensions

Plume management kit

Your parts list

Each line says why it's there. Untick what you don't need, change quantities or swap a product, then add the kit to your basket.