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.