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What Size Boiler Do I Need

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  • 09-07-2021
What Size Boiler Do I Need

Many homeowners want to know: How do I know what size boiler I need? Find out more about choosing the right size boiler for your home. We look at sizing combi boilers, heat-only boilers and system boilers.

Docklands Plumbing and Heating Ltd understands that finding a suitable boiler for your home can be a confusing search. Our professional heating engineers are skilled and qualified, with Gas Safe certificates to install, inspect and offer clients advice and answers to all "What size boiler do I need? How do I work out what size I need?" What type of boiler would be best?" kinds of questions that are so frequently asked.

We can provide and supply numerous installations and renovations for your gas and heating system boilers or cylinders. Our team of bespoke, experienced engineers whose training has been authorised and regulated by the financial conduct authority are on board to assist when you need a boiler or require maintenance. 

Our high-quality team is registered in England to perform all necessary tasks for our local customers and business clients across the UK. Our services offer a warranty on all the boiler installations and boiler or cylinder moves in your home.

Should I Replace My 20 Year Old Boiler?

How do I know what size boiler I need?

Sizing a combi boiler

Assess the size of your property type to find the best boiler sizes for your home. To find the best boiler for your property, we must measure the size against the property age, insulation levels, the number of bedrooms and bathrooms and the usage. Boiler heat and hot water levels are measured in Kilowatts. 

Choosing the right size boiler is essential for functionality and ensures no heat energy or hot water waste that could risk you losing money. It also allows your home to produce enough hot water to complete daily tasks like bathing, showering, cooking and cleaning.

How Do I Know What Size Boiler I Need? Newham and East London

Multiple households in the UK commonly have a heat requirement of 6-8kW or anything under 10kW on a cold day, which is straightforward to find and accommodate for small heat-only and system or regular boilers. 

However, when it comes to the size of combi boilers, the heating output is often found around 18kW, and the heating system utilised for hot water is 24kW and upwards. Plenty of modern boilers can be range rated down, meaning your professional installers can adjust your boilers overall output to around 6-8kW to suit your maximum heat requirement and ensure you are not wasting unnecessary productions of heat. Ensure you get your installer to adjust it for you safely. 

Property Insulation

We can assume the property's age from the insulation measures, such as loft insulation and double glazing retrofitting or when newer properties receive these insulation installations during construction.  Newer properties are more thoroughly insulated, whereas some older properties have been well protected.

Some homes have too high a level of insulation for the age of their property; the solution in this situation is to move a column or invest in a smaller boiler more suitable for your house. You may also need to factor your hot water demand rates into your calculations, especially before kitchen or bathroom renovations. 

Moving your boiler

When considering moving a column or boiler to accommodate the level of insulation, you must generously think of the costs. It is often less expensive to have a brand new boiler installed than repositioning your current boiler into a different location.

Moving your boiler to a different location in the home or property can cost more than a new boiler for numerous reasons, and these are; it requires much more time and hard labour and often requires more building parts, such as copper pipework.  However, it is far cheaper to shift your boiler elsewhere in the same room than to move from one room to another.

Typically, you will find that you will be expected to pay around £250 to potentially £500 for a central heating system boiler move. If further or substantial pipework or flue work is required, expect to pay an even higher fixed price. Docklands offers a warranty for all those investing in new boilers to guarantee that you will not lose out in the event of an emergency or dire maintenance repairs. 

Do you think you need a larger boiler?

According to Viessmann, a boiler manufacturer, few properties in the UK exceed 10kW. You can purchase a larger boiler for your household; however, your installer must adjust it by reducing the output on any installation.   

The hot water flow rate is also a great way of sizing up your combi boiler. Commonly expressed as litres per minute (LPM), this measurement shows how quickly a combi boiler heats your water.

Hot water rates vary widely from around 9.8 LPM to approximately 25 LPM (litres per minute). Suppose your household has several bathrooms, meaning the hot water flow in your home will be halved, especially in the case of two showers running at the same time. In this case, it is crucial to look closely at the overall flow rates.

It's best to consider you may be paying for a potentially more significant boiler or at least one with a higher flow rate as there is a higher demand for hot water. For properties with a large number of bathrooms, you may want to use a hot and cold water tank or system boiler.   

The best way to choose a boiler of a perfect property size is to look at how many bathrooms or showers you have. You may use an online boiler size calculator or use this guide:

For a household or small property with one bathroom or shower

The minimum boiler size would be 26kW

Our recommended boiler for this size is a Viessmann Votodens 050-W 29kW.

For a home with two bathrooms or showers in perhaps a 4 bedroom house

We recommend a boiler size of 30kW.

The best model to choose from is a Viessmann Vitodens 100-W 30kW.

A household or property with three bathrooms or showers

You should have a 35kW minimum sized boiler.

Our recommended boiler model is the Viessmann Vitodens 050-W 35kW.

For extensive properties with more than three bathrooms or showers that you may use simultaneously,

We recommend a minimum size of 35kW.

The model we would choose is the Viessmann Vitodens 111-W 35kW model.

You don't want to push the boundaries of the combi boilers; however, if you have access to a hot water tank, a conventional boiler system would likely be much more suitable. 

Whilst we display gas boilers at Docklands Plumbing and Heating Ltd in numerous different sizes above the 6-8kW rate, it is essential to be aware that these boiler options are somewhat oversized, especially for domestic homes and properties.

Choosing a boiler this size will likely end up in an enormous energy waste of heating and hot water even though it can guarantee immense boiler efficiency. There are various exceptions to the rule, and your specific engineer professional should ensure they carry out heat loss surveys to size up and analyse the peak heat requirements of the boiler you choose or acquire.

Our Gas Safe registered engineers and plumbers at Docklands Plumbing and Heating Ltd are thoroughly qualified to prepare all necessary surveys and inspections to assess your current or new boiler or any storage tanks and any potential issues you have reported. 

Sizing a heat-only boiler

It is entirely possible to receive a rough idea from looking solely at the age of your property, the size of it and by looking at the insulation levels. Property ages assumed all standard insulation measures suitable for a UK property, such as double glazing and loft insulation to warm the home. 

Similarly to combi boilers, if your insulation levels are relatively too high for your heat-only boiler alongside its property age, you can opt to move a column to the right. 

Another avenue to explore when sizing your heat-only boiler is the output necessary for your hot water cylinders, and these often lie above 8kW. Therefore you must consider all your gas boilers hot water needs and requirements. 

For those homes with multiple bathrooms in the home or on the property site that are frequently in use, potentially at the same time, you may require a larger boiler on your more conventional set-up. We can offer the best advice to recommend a boiler that can reach low kilowatts on the heating side to achieve the best efficiencies. 

 Minor cylinders often require 12kW of heat to function, whereas much larger cylinders can withstand 25kW and more amounts of heat. 

 Heat-only system boilers or conventional boilers deal more with radiators as opposed to bathrooms.

Calculating your boiler size

The best way to calculate your boiler size is to be aware that every radiator available in your home is 1.5kW, and you add that onto the 3kW used in your hot water cylinder. 

An example of the calculation done is, first, take the number of radiators in your home. Let's say your household or property has six radiators. The equation would be (6x 1.5kW + 3kW). The minimum size of the boiler your home requires will be 12kW, and therefore, the recommended boiler you need is the Ideal Logic 12kW model.

Here are further example calculations and their recommended boiler models:

For 12 Radiators:

12 x 1.5kW + 3kW

Minimum size boiler:

Recommended boiler model:
Ideal Logic 24kW

For 15 Radiators:

15 x 1.5kW + 3kW 

Minimum size boiler:

Recommended boiler model:
Viessmann Vitodens 100-W 26kW

For 20 Radiators:

20 x 1.5kW + 3kW 

Minimum size boiler:

Recommended boiler model:
Viessmann Vitodens 100-W 35kW

Sizing a system boiler

Suppose your home has seen improvements in terms of the standard levels of its insulation, including double glazing and loft insulation. In that case, the best option to opt for is a boiler with an applicable age bracket that suits your household. 

However, suppose your properties insulation levels are incredibly high compared to the overall age of the building. In that situation, an ideal decision, in this case, is to move a column to the right.  Similarly to heat-only boilers, installing a boiler with a heat capacity greater than around 18kW when dealing with a system boiler is not necessary. However, it is vital to factor in all the hot water requirements to suit your home cylinder accurately.

When dealing with system boilers, your property may only require approximately 6-8kW of heating, although a more considerable hot water output may be necessary.  Engineers inspecting your system boiler cylinders must still conduct heat loss surveys and inspections to determine the specific figures, the water pressure and kilowatts suitable for your homes property age.

When deciphering the best sizing for your new hot water cylinder, there are many factors you should be aware of. 

A 30-40kW heat-only or system boiler could successfully heat a large mansion house. In contrast, large houses with several bathrooms in regular collective use will require a much larger boiler with substantially more output to fuel and heat the house entirely.   

Sizing a system boiler - Newham and East London

An ideal set-up for your heating and hot water systems would be a boiler that meets all your demands perfectly and can drop quite low on the heating aspect so that you aren't frequently wasting immense amounts of heat. Wasting your heating, hot water supply directly from the mains and burning extra gas is not a wise idea for you economically as your energy bills could skyrocket, nor is it good environmentally.

An undersized but overpowered boiler will not guarantee any additional benefits to you or your home. Modern condensing boilers can often modulate down to meet your lower requirements. However, it will mean you have spent a considerable amount on a boiler model that is not offering you more than the right size boiler for a more reasonable price. 

There a various gas boiler brands that Docklands Plumbing and Heating Ltd offers here in the UK to our local Docklands clients and customers. 

Many of them are leading brands in the market, for example, Viessmann, Ideal Logic, Worcester Bosch Greenstar and even newcomers to the market like Alpha Innovations. 

The choice you make has an overall impact on the final fixed price, so it's crucial to pick carefully and select the types of boilers most suited to your home and budget.

Do you need boiler installation services in East London? 

Contact us for boiler installation in Docklands, Waltham Forest, Tower Hamlets, Lewisham, Newham, Hackney, Southwark or Greenwich.