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Electric Combi Boilers – 4.5 kW to 21 kW Output

Electric Combi Boilers or Electric Combination Boilers as they should really be referred too, are electric boilers that will do both your heating and your Domestic Hot Water requirements (often referred to as DHW). They are a great solution for properties or households that have no access to natural gas or where oil or LPG is not practical or available. With advancements in technology, electric combi boilers have evolved into an advanced and competitive product, mainly due to they are typically easier to maintain and install than gas boilers, and are extremely quiet in operation. Some of the benefits of electric combi boilers are that they are small and compact, so can often fit in a cupboard in the kitchen for example, or under the stairs etc. The other huge benefit is that they require no flue pipe, unlike a gas or oil boiler as they don’t release any gases into the environment, so the risk of CO2 poisoning from a faulty flue or boiler are eliminated, this means that you can locate your electric combi boiler in more locations to suit your needs, rather than having to position it to accommodate the flue system and regulations that you have with gas or oil boilers.


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Wall Mounted Electric Combination Boilers With Storage From Flexiheat UK

Electric combi boilers come in a variety of outputs and the most suitable option for your property or home will depend not only on your heating requirement but mainly on your hot water usage and the number of bathrooms you have. None of our electric combi boilers emit gases or direct pollution when operating and do not operate using fuels such as gas or diesel and therefore no risk of leaks or explosions. All of our electric combination boilers are highly efficient at 99%, and come with a control panel, and are fitted with the latest ErP low energy rated heating pumps. With all of our electric combi boilers – domestic hot water always has priority, so should you have a domestic hot water (DHW) demand ,and the boiler is in the central heating mode at that time , it will temporarily shut down or reduce the central heating output to satisfy the domestic hot water requirements. As soon as the DHW demand ceases, the electric combi boiler will automatically restart or ramp up the central heating. This is called Domestic Hot Water priority. Our electric combi boiler efficiency is 99.5%, and this is one of the biggest selling points of our electric combi boilers.

Electric Combination Boiler Range

The HDCS/HDCSM is an electric heating boiler with domestic hot water production, more commonly known as an electric combi or combination boiler. We supply two versions: a wall-mounted boiler, which is designated the “HDCSM” or a floor-mounted boiler, which is designated the “HDCS”. Both of these boilers benefit from a high-quality construction and are fully ErP rated products, with such features as a modulating ErP rated heating pump for the central heating side of the boiler.

The wall mounted electric combi boiler is available with various power outputs, from 4.5 kW to 21 kW, and has a 50-litre stainless steel domestic hot water tank inbuilt within the boiler casing.

Whilst the floor mounted electric combi boiler is available with again, various power outputs, from 4.5 kW to 21 kW, but it has a larger 80-litre stainless steel domestic hot water tank inbuilt within the boiler casing.

Whilst the heating side of these boilers are a sealed heating system, the domestic hot water side is not G3 compliant i.e. mains unvented hot water supply – and will need to be installed to an open-vented domestic hot water system. See picture below. If you require a mains unvented domestic hot water system – then please look at our other electric boilers that you can use in conjunction with an indirect unvented cylinder on web page – Electric Heating Boilers

Whilst the heating side of these boilers are a sealed heating system, the domestic hot water side is not G3 compliant i.e. mains unvented hot water supply – and will need to be installed to an open-vented domestic hot water system. See picture below. If you require a mains unvented domestic hot water system – then please look at our other electric boilers that you can use in conjunction with an indirect unvented cylinder on web page – Electric Heating Boilers

Open Vented Domestic Hot Water Diagram

With a simple adjustment, the power output can be adjusted from 4.5 kW to 9 kW in the HDCS and HDCSM 45/90, and from 10.5 kW to 15 kW in the HDCS and HDCSM 10/15. So, these electric combi boilers can be set to a specific kW output as detailed below –

HDCSM / HDCS  Model “45/90” – Combination Electric Boiler

This can be set to give an output of either 4.5 kW, 6 kW, 7.5 kW, or 9 kW

Whilst the HDCSM / HDCS Model “10/15” – Combination Electric Boiler

Can have the kW output adjusted to either 10 kW, 12.5 kW or 15 kW

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Floor mounted electric combination boilers from Flexiheat UK

Electric Combi Boiler Sizing

As an electric combi boiler will do both the heating and the hot water – the first element of sizing the correct boiler – will be the heat loss of your home or property. Below is a quick reference heat loss / combi boiler calculator for the heating requirement of the boiler.

There are some assumptions here – this is based on your property have a brick and block cavity wall, which has been insulated – which if your property, has been built from the late 1990’s onwards, then it should comply to these regulations. For the more technical of you, this is based on a “U” value of 0.3 Watts per Metre squared.

If you home or flat is constructed prior to that date, or you have different materials used in its construction, please use the heat loss calculator linked below in the webpage.

Please bear in mind to include all floor areas, i.e. upstairs and down stairs, basements etc

Power Output of BoilerHeating area with Height of 2.7 Mtrs & U Value of 0.3
4.5 kW~ 55 Mtrs/2
7.5 kW~ 85 Mtrs/2
9 kW~ 110 Mtrs/2
13.5 kW~ 165 Mtrs/2
15 kW~ 180 Mtrs/2
22.5 kW~ 278 Mtrs/2
24 kW~ 300 Mtrs/2

Domestic Hot Water side of the Combination Boiler-  Electrical Powered

The domestic hot water tank is made of high-quality stainless steel, an extremely hard-wearing material, and also compliant with the highest water hygiene requirements. No magnesium anode is required, avoiding costly inspection and maintenance requirements, something that is required with inferior glass lined domestic hot water tanks, especially in hard water areas. This guarantees a long system lifespan due to stainless steel’s inherent characteristics also with simplified maintenance because no magnesium anode is required.

All of our boilers have a domestic hot water priority system, which ensures that the hot water is always available for instant use. All of the boilers have a 6 kW heating elements dedicated to heating the domestic water, which ensure a great recovery time of the stainless-steel water tank, should you use all the water in one go – such as filling a bath. The recovery time of 50 litre tank, (which is used in the wall hung electric boiler) from 10 Degrees C to 60 Degrees C is only 29 minutes – and only 15 minutes from 35 Degrees C to 60 Degrees C. With the floor mounted electric boilers which have an 80-litre tank, the recovery time from 10 Degrees C to 60 Degrees C is only 47 minutes, and only 24 minutes from 35 Degrees C to 60 Degrees C.

This domestic hot water performance ensures a plentifully supply of hot water when you require it – immediate and at the right temperature.

A few points of note –

The floor mounted electric combination boiler – which has the 80 Litre domestic hot water storage tank inbuilt ,will provide 134 litres of domestic hot water @ 40C – which is the normal bathing temperature and according to BS6700 a standard 1700 mm x 700 mm bath uses approximately 100 litres of water at 40 Degrees C

So if you have a bath in your property, and want to be able to fill it quickly – then you need to look at the floor mounted version.

Or alternatively

The wall hung electric combination boiler version – which has a 50 litre domestic hot water storage tank inbuilt ,will provide 83 litres of domestic hot water @ 40C  – again the normal bathing temperature – According to all the data , a standard 5 minute shower uses approximately 45 litres of water at 40 Degrees C

So if you only have a shower in your flat or property, then usually the wall mounted unit will normally be suitable.

Electric Combi Boilers – Domestic Hot Water Heating Performance

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Domestic Hot Water Performance for our Electric Combination Boiler

Download the Electric Combi Boilers Brochure from Flexiheat UK

or download the full technical manual  –

Download the Flexiheat Manual for HDCS & HDCSM Electric Combination Boilers

Our Electric combi boilers also feature –

  • Adjustable output in 2 steps – i.e. either half power power or full power output
  • Can be used in conjunction with solar heating systems
  • A Wilo ErP rated energy efficient heating pump
  • A Frost protection system – If the temperature of the water inside the boiler is under 5 Degrees C, pump and heating elements will operate to bringing the water temperature up to the security level of 35 Degrees C
  • Instant and constant hot water temperature via a stainless steel hot water storage tank built into the boiler
  • Overheat protection system
  • Can be used with a wet underfloor heating system
  • Can be used on a traditional central heating system with radiators
  • Two levels of output settings i.e. These electric combi boilers can be set to half , or full of their maximum kW output – giving you maximum control of your electric combi boiler.
  • Anti – Block Function -The pump inside the electric combi boiler operates once every 18 hrs to ensure that there will be no blockage within the boiler
  • Can utilise either Single Phase ( 240 volt ) or Three Phase ( 400 volt ) as their power supply – depending on the electric combi boilers output


Are electric combination boilers the right choice for you and your home or flat ?

This is one of the most important questions to ask yourself , because there are limitations for electric combi boilers, and here is why –

  1. Unlike a gas fired combi boiler, whereby you can get a relative high amount of energy down a small diameter pipe, with electricity, the higher the power required – the large the diameter cable you require , and more importantly after 15 kW in Output you in reality have to jump from Single Phase 240 Volt ( which is the standard household domestic supply ) to Three Phase 415 Volt – which is normally the industrial electrical supply, as it can handle more Amp’s.
  2. Why does this change in Voltage matter ? Well it matters because most domestic properties in the UK have only 240 Volt  Single phase electrical supply to them , that have a 100 Amp Fuse Board / Consumer Unit ( there are some very old properties which may have only a 60 Amp supply, but this is very rare now – but it’s worth checking ). Which means that all the home electrical appliances , whilst being used simultaneously should not draw over 100 Amps , otherwise the fuse can blow, and out your electric.
  3. Following are some typical electrical loads for common household electrical appliances –
Domestic Electric ApplianceAmps Used
Kettle/Radiator (oil filled)13
Washing machine/Dishwasher/Hair dryer10
Toaster/ Vacuum cleaner9
Tumble dryer11

4. Ok, now for the electrical consumption of our Single Phase 240 Volt Electric Combi Boilers –

240 volt – Single Phase Electrical Combi Boilers – Electrical Consumption – For Heating and hot Water

ModelOutput in kWHeating side draw Current- in Amps  for 1 Phase 240 VoltDomestic hot water draw in amps for 1 Phase 240 Volt
HDCS 4.5/9.04.5 to 920 to 40.8 26
HDCS 10/1510 to 1543.4 to 65.2 26
HDCS 18- 60up to 1881.8 26

Thus at maximum draw , the boiler could pull –

For the 15kW Boiler on 1 phase / 240V–

15kW Heating – 65.2 Amps
Domestic Hot Water – 26 Amps

Giving a total of 91.2 Amps for the 15kW Option on full fire

For the 12.5kW Boiler on 1 phase / 240V–

12.5 kW Heating – 54.3 Amps

Domestic Hot Water – 26 Amps

Giving a total of 80.3 Amps for the 12.5 kW Option on full fire

The “diversity factor” – which is defined  as the (unofficial) term diversity in relation to electrics, refers to the percent of time available that a machine, piece of equipment, or facility has its maximum or nominal load or demand (a 70% diversity means that the device in question operates at its nominal or maximum load level 70% of the time that it is connected and turned on).So as you can see from the above table, if you are running a 12.5 kW electric combi boiler , which can draw 80.3 Amps of electrical power on a single phase supply when operating at full power, and you have a hair dryer on at 10 amps, plus a dishwasher at 10 amps , your going to end up with a demand of 102.8 Amps , which will probably mean you trip your fuse or getting very close to it. Now to be fair to this statement, there are a few factors to consider –

  • Is this potential situation applicable to your circumstances – If you live in a flat , or smaller house , with either one or two occupants , are you like to see this scenario ? e.g. if there’s only one occupant –  If you’re in the shower , then you can’t be putting a toaster on at the same time  ? or using a hair dryer at the same time.
  • On the other side of the coin – if you have 4 people living in a house, with say 2 adults and 2 teenagers – then yes, I would say this is definitely possible.

A Solution to this problem  ?

Split your heating and domestic how water requirements into 2 separate units , either an electric boiler for the central heating needs and a indirect cylinder or calorifier for your domestic hot water requirements, or even a direct ( with electric immersion heaters ) cylinder / calorifier, more information on this system can be found on our web page here 

3 Phase – 400 Volt – Electric Combination Boilers

The only solution in reality to getting the higher performance outputs in both your electric central heating requirements and domestic hot water performance is to use three phase as the electrical power supply. A three phase electrical supply isn’t fitted as standard to a normal house property in the UK, but can often be requested as the standard supply voltage especially when doing a new build, and can also be retrofitted to most existing properties. Many blocks of flats have a three phase supply to them, as they use this to power the lifts etc, so if you require an electric combi boiler for a flat, then please check with your property management company , to see if this is possible to have a three phase supply to your flat.

As you can see from the data below , the range of power outputs is much larger when the electric combination boilers are powered by three phase, going all the way up to 21 kW in output.

400 volt – Three Phase Electric Combi Boiler – Electrical Consumption and Cable requirements

ModelOutput in kWDraw Current – AmpsCable Section in mm2
HDCS/M – 4.5/9.04.5 to 93 x 6.8 to 13.6  –  When wired in 3 Phase2.5
HDCS / M- 10/1510 to 153 x 15.9 to 22.8   –  When wired in 3 Phase6
HDCS / M 18- 60up to 183 x 27.2 –  When wired in 3 Phase6
HDCS /M 21-60up to 213 x 31.9  –  When wired in 3 Phase10


The first part of selecting the correct electric combination boiler is the central heating load or kW requirement. You can do this in a couple of different ways –

What size Electric Combi Boiler do I require ?

1. Consult a plumber or a heating services engineer –

A good qualified plumber will be able to do a heat loss calculation , but probably won’t have the design insurance indemnities ( which means that they have an insurance policy to claim off – if the system they design / recommend doesn’t work as stated.

2. The cost nothing , but your time method –

Or you could calculate the load yourself using the following link – which is a heat loss calculation app, this will tell you the heat load for each room, input the room data for your property , and them all up – and you end up with your total heating load

Heat loss calculator 

Quick note – the result will be in BTU’s or Watts -( to convert Watts to kW – divide the Watts  by 1,000 to get the kW figure )

This heat loss calculator is very good an accurate if the information inputted is correct , I’m pretty sure that this ( or very similar) is what plumbers and heating consultants use on a daily basis to do heat loss calculations these days

3.Last but not least the ” Rule of Thumb ” , we don’t recommend using this at all , as they make a lot of assumptions and can be very dangerous ! but here we go – and use at your own risk

For underfloor wet heating systems – use 100 watts per m2 for concrete floors , and for suspended timber floors use 70 watts per m2

For a radiator heating systems – use an average of 1.5 kW of output for every radiator. Thus if the central heating system has 10 rads – 10 x 1.5 kW = 15 kW of heating output is required.

One kilowatt of heat will be required for heating every 10 square metres of your property. Thus, for heating a 50 square meters property or flat, you would need a 5 kW electric combi boiler.

We really wouldn’t recommend using these rules of thumb methods, use point 2 at the very least in our opinion, these heat loss calculators are very accurate these days, as long as the information entered is correct.


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Electric Combi Boilers / Electric Combination Boilers from Flexiheat UK


Domestic Hot Water Performance of Electric Combination Boilers

The most important feature or aspect of electric combi boilers is the domestic hot water performance i.e. your bathing water , or hot water tap on the sink or wash basins. The amount of hot water an electric combi boiler can produce, is somewhat limited in the amount of hot water it can produce at any one time, especially when using single phase as the power supply, and serious consideration should be applied to splitting the domestic hot water demand , via a storage cylinder or calorifier ( either direct or indirect type ) should you have a larger property, or those with a higher level of domestic hot water usage. We do a range of electric boilers that can be coupled up to large domestic hot water tanks – please see more on this web page 

The most important thing to consider with an electric combi boiler is water temperature and flow rates. Some of the electric combi boilers on the market are true Combi boilers in that they produce domestic hot water on demand. The problem with these boilers, which are often produced in the far east, is that not only is the flow rate incredibly low but the actual water temperature they can produce is often too low as well, as they are designed for use in areas that have different incoming water temperatures to the UK and northern Europe. Many of these types of boilers can only produce 6 L of hot water with a temperature rise of 25° C, when powered by the most popular 240 V single phase electrical supply that is used in a normal property in the UK. This poor temperature rise or uplift does not allow you to achieve your desired temperatures for showering / bathing or for dish-washing applications.

Flow rates for some of the single phase electric combi boilers on the UK market –

10kw Electric combi boiler – 5.8 Litres / minute with a Δt = 25°C – This means a temperature rise of 25 Degrees C
12kw Electric combi boiler – 6.9 Litres / minute with a Δt = 25°C – This means a temperature rise of 25 Degrees C

Here are some domestic hot water temperatures that we require in the UK –
For dish washing / kitchen applications = 50 Degrees C – this temperature is required to remove grease from cooking utensils
For bathing (bath) water = 43 Degrees C (maximum)
For showering = 41 Degrees C (maximum)

mains cold water temperature UK;

Please see the Blue line for the average mains cold water temperature for the UK over a 12 month period

As you can see from the blue line, which shows the average mains cold water temperature for the UK, for pretty much 7 months of the year, this temperature is only 10 Degrees C, or below. So why does this matter? well it matters because if you have a electric combi boiler, that can only add 25 degrees C to your incoming water temperature, then your final outlet temperature at your taps / shower head is only going to be 35 Degrees C, way to low for domestic hot water requirements- 35 Degrees C won’t remove grease for plates etc, and a 35 Degrees C shower or bath for most people will be regarded as a cold.

Although some of these “true” electric combi boiler can produce domestic hot water up to 50 Degree C , the flow rate drops dramatically, and becomes unusable in the real world. You would be lucky to get 2 litres a minute form a 10kW Electric combi boiler , and 2.5 litres from a 12 kW Electric combi boiler. With a standard UK sink holding 15 to 18 litres of water – it would take up to 9 minutes to fill for the 10 kw boiler, and over 7 minutes for a 12kW boiler .

These reasons are why having an electric combi boiler with storage is the correct solution for you heating and hot water requirements. The hot water storage element , removes these poor flow rate problems, and makes living with an electric combination boiler hassle free.

Electric Combination Boilers – System Requirements

Our Electric Combi boilers require a minimum pressure of 0.5 of a Bar to operate on the central heating system, and will work up to a pressure of 7 Bar on the cold water inlet for the domestic hot water. The temperature of this domestic hot water can be controlled between 35 to 65 degrees Centigrade.

Operating Parameters of our Electric Combi Boilers

 Technical SpecificationsHeatingDomestic Hot Water
Minimum Operation PressureBar10.5
Maximum Operation PressureBar37
Minimum Setting Temperature°C4035
Maximum Setting Temperature°C8065

Electric Combination Boilers For Flats
Electric combi boilers can be the ideal solution to your heating and how water requirements when it comes to flats or bed sits. They are often very easy to install when compared to a gas or oil boiler as there are no flue requirements to thing about , which means you can basically place the boiler where are you want to.With a control panel these electric combi boiler for flats are easy to use and control, they are also aesthetically pleasing , and extremely quiet in operation. So if your require an electric combi boiler for your flat or bedsit, then give us a call, as we have the best range of electric combination boilers for sale in the UK.



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Electric Combi Boilers for Flats from Flexiheat UK

Frequently asked Electric Combi Boilers questions and answers –

  • Electric combi boiler installation – are easier than a gas or oil boiler for a few reason – the biggest being there is no flue pipe to install , thus the cost of this and the associated builders cost, such as making holes in brickwork or the roof for the flue are eliminated. This will often mean that fitting an electric combi boiler, is the cheapest of all combi boilers to fit when compared to oil or gas.
  •  Electric combi boiler cost – as a general rule they are slightly higher than their counter parts in gas at first impressions – but when you look at the extra’s such as the flue pipe etc, then they are around about the same price when you look at them as a package. When compared to oil combi boilers, then are cheaper on a like for like kW basis, even before you add the flue pipes and accessories.
  • Electric combi boiler prices – Electric combination boiler prices are at the mature stage of pricing cycle, they are a well established product and have been for many years. The most important thing to look for is not only the price, but also the associated quality. There are a few company importing far east manufactured products, that are using inferior components, and you also have to question the back up when it comes to spare and technical help. Our Electric combi boilers are manufactured in the EU , with quality components, that by law have to have spare parts available for at least ten years if a model or component gets updated, ask the one you found on ebay from the far east can guarantee this, then ask him to put it in writing. So in a nutshell, our electric combi boiler prices are competitive and more than fair, for the quality of product your purchasing, with the full technical back up we give and spares we carry – should you ever need any.
  • Electric combi boiler running costs or annual consumption costs  – These are easy to calculate. Get your current price you pay for a kilowatt hour or (kWh) of electricity and multiply it by the kW output of the boiler i.e. 17 Pence per kWh x 12 kW ( Output of your Electric Combi Boiler) = 17p x 12 = £2.04 per hour if your boiler is running constantly.  Electric combi boilers should be sized that on a minus 3 degrees C outside temperature , you can hold a constant temperature of 20 Degrees C internally within your home or flat. But in the real world , it very rarely works like this , as the day goes on , often the external temperature rises, your property benefits from solar gains from the sun etc. Here is a link to a energy consumption calculator , that may help you predict your running costs of your electric combination boiler.

Electric Combi Boilers – Electric Combination Boilers for sale UK & Ireland

Whether you want an electric combination or electric combi boiler for your property, be it a house,flat or bedsit ,or if you require one for a commercial and industrial application, then we have the best range here at Flexiheat UK to satisfy you demand with a range from 4.5kW to 21kW Output, and options of single or three phase. Should you require anymore assistance or information on our Electric Combination Boilers / Electric Combi Boilers for sale in the UK and Ireland, then please don’t hesitate to contact our sales team on 01202 822221 or click here for our email contact form