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Electric Central Heating: What size boiler do I need?

We are frequently asked what size electric boiler they need for their electric central heating. The boiler they need for this is called a ‘Flow Boiler’, so perhaps the best way to start would be to be clear as to exactly what a flow boiler actually is.

The easiest way to describe it would be an in-line water heater that heats and maintains the temperature of water pumped around a circuit such as a wet central heating system or maybe a wet underfloor heating system.

So the heater is actually part of the circuit, and it would typically be controlled by a thermostat and timed heating controls.

They can come in various forms but in the main they tend to be long thin items with elements or ‘pods’ doing the heating as the water passes through them. They require a pump and an expansion vessel, although some very neat looking models have the pump built in to the unit. Other variants, such as combination boilers would have a flow boiler either within the unit, or fixed to the outside of the cylinder.

Some brands have different models for underfloor heating or radiators, as the required temperature is generally not the same for different applications. Depending on the brand, flow boilers are available in a number of Kw sizes, ranging from 2kw up to 12kw.

To help determine to size needed for various applications, there are some simple rules of thumb that can be employed:

Wet underfloor systems: 70w per m2 for timber suspended floors, and 100w per m2 for concrete floors

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Central heating systems: Allow 1.5kw per radiator – this is an accepted average for most applications.

So, as an example, if you were looking to provide heat for a bungalow with a total of 6 radiators, then you should probably opt for a 9kw flow boiler.

Occasionally, there may be situations where you need to use a flow boiler to provide heat for the water to flow through the coil of an indirect hot water cylinder. In these cases, it would be best to check the Kw requirement or rating for the coil in the cylinder.

We hope this guide to size your electric central heating flow boiler has been of some help, and if you’d like to speak to a real live geek to discuss this further, then please give one of the super-friendly guys at Electrical Deals Direct a call on 01934 253600, or drop us an email at