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What size electric boiler do I need? It is a question we are often asked. It may be in relation to a new property or replacing existing gas boilers or oil boilers. The following guide will take you through the process and things you need to consider when selecting the correct size boiler. We supply a full range of electric boilers, which are electric system boilers for central heating, be it for underfloor heating and hot water demands, or for radiators and hot water demands (domestic hot water via an indirect cylinder), or just for electric central heating only. So, if you're looking for a 'what size electric boiler do I need calculator', keep reading.
Sizing an electric boiler for your home
The first part of selecting the correct electric heating boiler for a wet heating system is the heating load, which is expressed as your kW requirement. It doesn't matter whether you are using radiators or an underfloor heating system ' it's the same for both, as a heating load is a heating load, as they say!
Here's a quick chart ' for properties built after that have a ceiling height of 2.7 metres, which is the standard, and an insulation value, or U value, of 0.3 ' again, this should be the building standard in the 's and onwards.
You can do this in a couple of different ways '
What size electric boiler chart ' quick reference
Power Output of Boiler Heating floor area
4.5 kW ~ 55 m2
7.5 kW ~ 85 m2
9 kW ~ 110 m2
13.5 kW ~ 165 m2
15 kW ~ 180 m2
22.5 kW ~ 278 m2
24 kW ~ 300 m2
As a general rule, for properties built after the 's, you should allow 1.5 kW for each radiator in your home. For example, if you have eight radiators, you should choose a 12kW electric boiler. This rule may change depending on the height of your ceilings, the type of glazing you have (single or double), and the effectiveness of your property's insulation.
These rules of thumb are a good basis or guideline, but it's a good idea to confirm this with a heating engineer before you make a potentially costly error, but this guideline will give you a good understanding of the size of electric boiler you require.
Or should you have an older property / home , than prior to the 's such as an older Victorian type home or a property with different construction, such as stone, or all wood etc, and require a new boiler installation. Then the following is recommended
- The cost nothing, but your time method '
This is our recommended sizing method, to be absolutely certain, even for homes built after .
You can calculate the heat load yourself using the following website ' this will tell you the heating load required to heat your home or flat by inputting all your rooms into the calculator and adding the total kW required together to give you the total heat load.
Heat Loss calculator is here
2. Consult a plumber or a professional heating engineer 'they will be able to do a heat loss calculation for your property and do the boiler installation.
When you have your required output ' please see our dedicated webpages on the appropriate boiler you require
Central heating electric boilers
or
Electric combi boilers
Combi boilers do both heating and hot water in one unit normally without the need for a hot water cylinder, but there are conditions with regards to the Amps you have available ' see the above webpage for more information.
Electric boiler sizing
The electric boiler sizing process is exactly the same as for gas and oil boilers. It all comes down to how well insulated your property is. The higher the insulation level, the lower the electric central heating system output you require and visa versa. It's all down to the heat loss of your property , as electric boilers work the same as gas boilers or an oil boiler in that they produce a kW output, just that they are powered by electricity.
Frequently asked questions on electric boilers
How much does an electric boiler cost to run?
The easiest way to work out how much an electric boiler will cost to heat your property is to check your annual heat use via your existing energy bill. This will be shown in kilowatt hours for a gas boiler. Multiply the number of kWh by your electricity tariff / , to find your annual electric heating running costs. You may find the amount of kWh used by your electric boiler is lower than that of your gas or oil boiler because it is more efficient, so that figure will be reduced.
1. For Gas boilers
You get last year's gas bills ' and get the kWh's that the boiler used ' then deduct at least 10% off as boilers are at maximum around 90% Gross efficient (electric boilers are 100% efficient) see a full explanation of Gross V Nett efficiency of boilers here ' The total quantity of heat that can be produced by burning the fuel either gas or oil and how much of that heat the boiler can deliver inside your home is referred to as gross efficiency, so ultimate gross is what you get out of the boiler as usable heat.
So at the very best, a condensing boiler in a gas boiler or oil boilers are 90% efficient gross ' but we've seen readings of analysers that had the efficiency as low as 63 % for boilers that were only 15 years old ' so there can be some large differences in the real world figures
Get the existing installation checked with a flue gas analyser. It will give a true reading.
So if you have used 1,500 kWh of gas, multiply this by your efficiency figure ' let's just say in this example it is a highly efficient boiler at 90% net.
1,500 x 90% = 1,350 kWh
If the boiler is only 63% efficient, then the revised calculation would be
1,500 x 63% = 945 kWh
For more information, please visit Partedon Group.
When you have this figure multiply it by the price you pay for electric per kWh , as electric boilers are 100% energy efficient' then that's what it would have cost you for the last years usage. Thus, you are comparing like for like electricity running costs based on the same usage data for gas and oil boiler.
2. For Oil boilers
Same applies as above with regards to efficiency levels etc , but normally kerosene is sold by the litres ' Thus, get the litres bought / consumed for the home for the year .
In 1 litre of kerosene, there is 10.35kWh of thermal heat available.
So if some used 500 litres, then they used
500 x 10.35 kWh = 5,175 kWh of energy
Again, multiply by the efficiency ' say 90% at the very best ( if it's a condensing oil boiler) which can be as low as 60% etc as per the above point for a gas boiler.
5,175 x 90% = 4,657.50 kWh
Then multiply this figure by the price you pay for electricity per kWh, and that's what it would have cost you for the last year's usage as compared to an oil boiler. Thus, you are comparing electricity running costs like for like based on the same usage data for your home.
What's is the largest electric boiler that can be run on 240 volt single phase ?
The largest electric central heating boiler or electric combi boiler you can run of a single phase , 240 volt electric supply is 15kW. Electric boilers tend to be limited by the maximum amps a property has available ' normally 100 Amps for a UK property.
What size electric boiler do I need for underfloor heating ?
Exactly the same size as you would require for a radiator system , a 'heat loss' is as they say 'a heat loss'. Please see the above 'what size electric boiler do I need calculator' above on this webpage.
Is my house too big for an electric boiler?
No, electric boilers are available all the way up to 500kW of output, so we can cover any boiler size you would ever require. Your limiting factor will be the amps required to power the electric boiler. After 15kW output electric boiler size , you will require a 3-Phase electrical supply.
Electric boiler size need
Hopefully the above advice on which size electric boiler do I need / what size boiler for heating ' which are referred to as an electric system boiler (be it underfloor or radiators heating). These electric boilers can also be coupled up to cylinders for heating domestic hot water for your home hot water services/system.
Remember installation costs are lower for electric boilers as the boiler installer does not have to be a gas safe registered engineer or OFTEC registered (for oil) which are required for a gas or oil boiler install.
So, get in touch with us for your electric boiler costs, whether you require an electric system boiler, which works in conjunction with a hot water cylinder for your domestic hot water demand.
And generally, an electric boiler cost less than an equivalent sized gas and oil boilers.
Electric boilers have always been most popular in off grid areas with no mains gas, but with the increases in oil prices and the desire to reduce our reliance on fossil fuels, electric heating systems are on the rise and are becoming a much more realistic option.
The best time to design and fit any heating system is when you're building the property. But swapping out a gas boiler for an electric one later down the line is quite a simple process. This is largely due to the comparatively simple design of the system and vastly increased safety of an electric boiler.
Remove the old system
Most of the hassle will be related to removing your old system. A gas boiler needs to be removed by a gas safe heating engineer so that the gas line can be capped off and made safe. With an oil system the oil feed to the house and the old oil tank will all need to be removed and disposed of safely.
When you've removed a gas or oil boiler, you will also remove the flue. With a gas or oil boiler there has to be a flue to vent combustion gases to the outside. Depending on where your boiler was installed, this will leave holes in external walls and possibly ceilings which need to be filled and made good.
If your boiler was manufactured and installed after then it is likely that you have a condensing boiler. This means there will also be a condensation pipe running through the wall to the outside. This is the pipe that transfers wastewater from your boiler to the sewer, and if your boiler is going, it needs to as well!
Your old boiler and tanks need to be disposed of safely, so you should contact your local council or visit their website to make sure you or your contractor is adhering to the relevant local regulations.
Installing a new electric boiler
When you've got your old system out of the way, installing the new one is relatively easy but there are some considerations to bear in mind.
The main consideration for an electric boiler is the power supply. ThermoSphere electric boilers up to 12kW can be connected to a standard 230V single phase supply. Larger output boilers from 18 to 24kW boilers must be connected to a 400V three phase supply, which is usually only available in larger domestic and commercial properties in the UK.
Your boiler will need to be connected to a dedicated and suitably sized breaker back at your distribution board using the correct size and type of electrical cable. An isolation switch should be provided next to the boiler to allow easy and safe isolation from the electrical supply before any checks or maintenance is carried out.
An electric boiler power supply should be installed by a competent electrician as they will be able to specify and install the correct size breaker in your distribution board and use the correct size and type of cable.
Plumbing connections
The plumbing connections on an electric boiler are very similar to a gas or oil boiler. The main difference is that there are fewer pipes to connect on an electric boiler because there is no fuel being pumped in. So that means there are no gas or oil pipes to worry about, no flue to install and no condensate pipes. With an electric flow or system boiler there is simply a cold input (return) and a hot output (flow) to connect to your plumbing system. If you're having an electric combi boiler installed there will also be flow and return connections for the domestic hot water plumbing circuit. These water connections are exactly the same as you would find on a gas or oil boiler, but your plumber might need to modify the pipe work near the boiler to make them all fit the connections on the boiler.
Unlike a heat pump, there is no need to upgrade all of your pipes and radiators when you choose an electric boiler. You can keep everything other that the boiler the same so there are no extra hidden costs. So long as the heating circuit has not changed, and you're not installing a higher output boiler, any expansion vessels that are already installed can be kept, so long as they are working as they should.
Heating controls
ThermoSphere electric boilers have wiring inputs for central heating thermostats. These are a pair of volt free contacts that are compatible with most central heating controls and wiring centres on the UK market. So, changing your boiler doesn't mean that you need to change your thermostats and controls!
What size electric boiler do I need?
The size or output of your boiler will dictate the size of the electrical power supply that it will need to operate. As with any heat source, in order to know which size boiler to buy, you first need to know the energy demand for your heating system. You could simply check your old boiler's output and put in a like for like electric model. For example, if you have a 12kW gas boiler you could install a 12kW electric boiler to meet the same demand.
However, it is worth checking your required heat output before you commit to a new boiler. This is because boilers are often very over-spec'd which means that you'll sometimes find that the boiler installed is actually too big for the property, not operating efficiently and could actually be replaced with a smaller, less powerful boiler that will use less energy and be cheaper to run while giving you the same levels of comfort.
For a heating only boiler, the general rule of thumb used by heating system designers is to allow 1.5kW of boiler output per radiator. So, using this rule, if you have 8 radiators you should install a 12kW boiler. However, some radiators are going to be much larger than others and also used at different times so the smart thing to do is consider whether you actually need 1.5kW per radiator in your home. For example, a small landing radiator or bathroom towel rail might only require 500W or less, so that is 1kW off your required boiler output straight away, and 1kW of electricity that you don't have to spend money on. As you can see, it is well worth walking around your house counting radiators and considering where you may not need to allow that 1.5kW, just to see if you can choose a smaller boiler that is more cost effective to run and has less environmental impact.
If you are specifying a boiler to provide hot water for heating and showering, the popular school of thought is to add 3kW to the required output of the boiler. This is considered a 'failsafe' but is not really needed, even if you have an electric combi boiler.
The reason for this is that your boiler will only ever be heating up water either heating or hot water at any one time. So, if you calculate that you need 9kW for your heating, system, that same 9kW of heating power will be more than enough to warm the relatively small amount f water running through a coil in your hot water storage cylinder. If you need 9kW to heat your home and you're also using the boiler to heat a hot water cylinder, then a 9kW boiler would be suitable. There's no need to overcompensate with a 12kW boiler because this will only lead to unnecessarily high running costs and a more expensive electrical installation.
The ideal way to specify the right boiler is to have an energy assessor visit your home and provide you with heat loss calculations and a required heating output. This is a much more in-depth method specifying a boiler, and it does have a cost attached, but in the long run it's the best way to ensure you don't over spec your boiler and pay for energy you don't actually need to use.
So, there you have it! It is relatively simple to change an old gas or oil-fired boiler for a new zero emissions electric boiler and here's a checklist of the main considerations:
- Safe removal and disposal of your old boiler.
- Safe removal and disposal of any tanks or oil lines.
- Don't overcompensate! Reconsider the heating output that you actually need before simply ordering a like for like output boiler.
- Ask an electrician to specify and install the correct power supply.
- The plumbing connections are the same as a gas boiler, just fewer!
- No flue required!
- No annual safety checks or servicing.
- It will work with your existing heating system and controls.
Get a new electric boiler quote
Complete this short form to receive a new electric boiler quote along with the right specification for your property.
To discuss electric boilers for larger projects, please call one of the team on 019 .
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