5 min read

Guide to Installing Electric Boilers

Guide to Installing Electric Boilers

Electric boilers are similar in size to gas boilers and, on average, slightly cheaper to install.  They have appeal to homeowners who live off the gas grid or are to looking to reduce their carbon footprint sooner rather than later. Where they are much more efficient than gas boilers, they are less efficient than heat pumps, but are still a great option for space and water heating.

In this heating blog we provide some of the top FAQs for installing ThermoSphere’s range of electric boilers focusing on the electrics. 

If you have a question that has not been covered in this article, please reach out to one of the team here at ThermoSphere HQ. 

Who should be wiring up an electric boiler?

First, electric boilers should only be installed by skilled electrical or heating engineers. This is because an electric boiler installation requires competence with electrics and pressurised hot water systems. Both aspects can be extremely dangerous if not installed and maintained safely by skilled people.

A skilled electrician must always install the dedicated electrical supply circuit and make the final electrical connections to the mains. Sometimes an electrician will also take care of the control wiring between the room thermostats, valves, and wiring centre, especially if the control circuit requires a 230V power supply.

A plumber or heating engineer will take care of the plumbing connections to the boiler, the design and installation of the heating and the hot water plumbing system.

Normally, if you’re using the services of a specialist heating engineer, they will often take care of both the electrical and plumbing aspects of the job or employ a trusted electrician for the electrical part of the installation.

Whoever you decide to use, the person or people installing or servicing your electric boiler must be skilled and experienced with electrics and pressurised hot water. If you suspect that they might not have the required experience, get a second opinion because safety is paramount.


Getting power to your electric boiler

Can I power an electric boiler from a ring main?

No. An electric boiler uses a significant amount of electricity and is likely to be essential for everyday use, so an electric boiler should always be connected to the mains via a dedicated circuit, protected by a circuit breaker. An electric boiler must never be supplied from a ring main because it can easily overload the circuit, cause nuisance tripping and is a dangerous fire hazard.

Do electric boilers need their own electrical supply circuit?

In short, yes. Electric boilers, whether micro, flow, system, or combi, all use a significant amount of electricity. Just like an electric oven, your electric boiler will need a dedicated electrical circuit to supply them with power safely. An electrician will be able to specify and install the correctly rated cable runs and switching equipment to provide a compliant and safe power supply. All supply circuits for electric boilers must have a suitably rated safety isolation switch, located near the boiler. This is to provide a safe means of electrical isolation for maintenance purposes.

What is a dedicated circuit?

A dedicated circuit is a circuit that provides power to only one appliance. It is a direct connection from the appliance, in this case the electric boiler and the consumer unit. A direct circuit also has its own circuit breaker or MCB (Miniature Circuit Breakers) installed in the consumer unit, which is sized to withstand the maximum current draw of the electric boiler. 

Does it matter what cable I use?

Yes. A competent, qualified electrician will be able to select and install a suitably sized cable by checking the maximum load of the boiler and the conditions in which the cable is going to be installed. Cable sizing is important because incorrectly sized cables that are not capable of handling the electrical load of the circuit can get extremely hot and present a fire hazard.

Troubleshooting Electric Flow Boiler_24

Single phase or three-phase

Are Electric Boilers single or three-phase?

ThermoSphere Electric Flow Boilers can be connected to either single-phase (230 Volt) or three-phase (400 Volt) electrical supplies. Boilers up to and including 15kW come with a jumper pin, making installation easy, as it joins all 3 of the live terminals together for connection to a single-phase electrical supply.

ThermoSphere Electric Flow Boiler V3V4 wiring terminals

ThermoSphere Electric Flow Boiler V3/V4 wiring terminals.

To connect to a three-phase supply, simply remove the jumper pin and connect one of the live phases to each of the 3 live supply terminals.

ThermoSphere Electric Flow Boiler V3V4 wiring terminals 2

ThermoSphere Electric Flow Boiler V3/V4 wiring terminals.

18kW ThermoSphere Electric Flow Boilers and above are designed for connection to 3 phase supply circuits only.


Heating systems: S-plan and Y-plan

Within most UK domestic homes, you’re likely to have one of two heating systems, S-plan or Y-plan.

What is an S-plan heating system?

An S-plan system can have multiple zones and multiple thermostats so is an efficient way to control your boiler. For example, you might have separate thermostats and heating circuits for upstairs and downstairs, operated by a zone valve on each zone. You may also have a third zone for the hot water.

S-Plan systems require quite a lot of wiring between the thermostats, valves, and power supply, so a wiring centre is used to manage all of the connections.

What is a Y-plan heating system?

A Y-plan system is a closed temperature control of the heating system and hot water generation featuring one three-port valve, using timed operation. For example, it controls central heating and hot water, but not on a zonal level.

The Y-plan system tends to be found in older properties and is commonly installed where space is limited.

Wiring the different ThermoSphere Electric Flow Boiler models

ThermoSphere V4 electric boilers 

These require a call for heat wire (or switched live) to be connected to the °C terminal. The call for heat wire can be supplied from either a room stat or the wiring centre in an S-plan or Y-plan control circuit.

V4 electric boilers also have a 3 Amp supply for the controls circuit consisting of a Live out, Neutral and Earth. These terminals are located next to the call for heat terminal. This supply circuit can be used to provide 230V to the control circuit without the need to install a separate 3A fused spur for the wiring centre.

ThermoSphere Electric Flow Boiler V4 and S-Plan heating system wiring diagram

ThermoSphere Electric Flow Boiler V4 and S-Plan heating system wiring diagram.

ThermoSphere V2 and V3 electric boilers 

These models require a pair of control wires to be connected to the control terminals. It is possible to connect your V3 ThermoSphere electric boiler to an S-Plan central heating and hot water system, just like a gas boiler. To provide a call for heat signal to the boiler, the installer must connect both the grey and orange cables from each zone valve back to the wiring centre and then to the boiler control terminals.

Remove the red jumper from the control terminals before you do this!

Connecting the valves to the boiler in this way ensures that the boiler will fire up if any of the valves are opened (heating required by the zone). There is no direct connection between the thermostats and the boiler in an S-Plan system. It all happens via the zone valves and the wiring centre.

Can I control my boiler with a thermostat?

Yes, this is possible. We recommend using a room thermostat with a control circuit to control your boiler and ensure your heating and hot water system runs as energy efficiently as possible. Each ThermoSphere Electric Flow Boiler comes with a red jumper cable or pin that bridges the control circuit connections. This means, unless a control circuit is connected, the boiler will always be able to fire up if the set temperature on the internal thermostat is not satisfied. 

The control terminals on a ThermoSphere Electric Flow Boiler are located at the top of the boiler, inside the casing. To connect a thermostat or control circuit you must first remove the red jumper pin/cable. When a control circuit is connected, the boiler will fire up if the temperature of the circulating water is lower than the internal thermostat and the room or hot water temperature is lower than the set point of the room thermostat.

Need to ask a question?

ThermoSphere’s technical team are available to answer any questions you have before, during or after your boiler installation. You can reach us on 0800 019 5899 or sales@thermosphere.com.

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