How Big A Boiler Do I Need?

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When it is time to look for a replacement boiler, or a new one, you should do some research to get the best size for your home. You will need to consider the type of boiler, but importantly, how big it is.

Each home is unique so think about your heating needs and the amount of heat loss when working out the size of your boiler. 

In this guide, we will look at how the size of a boiler is calculated, why it is important to get the correct boiler size, and how big a boiler do you need?

How The Size Of A Boiler Is Calculated

Size typically means the physical dimensions of an object yet for a boiler, the calculation of size is based on the power of a boiler. The output rating is the depiction of a boiler’s size and that is measured in kilowatts (kW).

This should make it easier to find the right boiler as you simply match up the required heating output with the number of kilowatts.  

Why It Is Important To Get The Correct Boiler Size

Once you have the correct boiler size to match your home, you no longer need to think about it. The boiler should work throughout the year pretty effortlessly.

Should the boiler be the wrong size, that could mean that the radiators struggle to heat up sufficiently, the hot water barely gets hot enough, and it seems that the boiler is simply running on insufficient power.

Then again, too powerful a boiler and your heating bills will go up and you would be wasting money. 

It is important to remember that a higher output does not necessarily mean the best boiler. A low output boiler could mean higher bills if you work it harder so make sure the size is just right.

Finally, do not think that you need to match the size of your existing boiler, even if you imagine it has been performing well as you could save money.

Take the time to make the calculations based on the current heating requirements of your home and then get the correct boiler size.

How Big A Boiler Do You Need?

Several considerations need to be made to work out how big a boiler you need. That magical number of kilowatts will be key to ensuring you have enough hot water for the entire household.

Of course, the size of your home, the existing heating system, and the type of boiler you have will all matter. 

Making sure that heat loss is not an issue, considering an underfloor heating system such as Wundatherm’s Premium+ Overfloor Boards, and even renewable energy options are all factors too.

Ideally, the plumber will size up your boiler based on the hot water and heating requirements in your home, though an additional load from the underfloor heating system would also be available from a quote.

Replacing radiators with an underfloor heating system should not necessitate a change in boiler size as the load is typically reduced due to reduced demand for hot water.

The Number Of Bathrooms, Showers, And Bedrooms

One simple way to work out the size of your combi boiler would be to look at the number of bathrooms/showers in your home as this will dictate the hot water demand.

How Big A Boiler Do I Need?

The fewer bathrooms/showers you have, the fewer kilowatts you will need. For instance, a single bathroom would need at least a 26kW boiler, though two may require 30kW and three or more bathrooms/showers could necessitate a 35kW boiler size.

Also, the amount of bedrooms needs to be considered as just two bedrooms may only require a 26kW boiler while three or more bedrooms should mean a 35kW-sized boiler.

The Number Of Radiators

A simple rule of thumb does exist to calculate the boiler size based on the number of radiators and the water cylinder. A single water cylinder should command 3kW so keep that in mind with the radiators which should each command 1.5kW.

However, bigger radiators would command a bigger load so you may want to use 2kW as the number to calculate with. 

Let’s say you have six radiators, 1.5 multiplied by six equals nine, with the water cylinder that means 12kW. Double the amount of radiators should mean a 21kW-sized boiler and then 20 radiators would mean a 33kW-sized boiler.

This is only a general rule of thumb so you may want to go higher just to make sure your heating requirements are covered, but not too high.  

Frequently Asked Questions

Is It Possible To Buy A Boiler That Is Too Powerful?

Ideally, the boiler should have sufficient power to heat every room and supply just enough hot water, even in the depths of winter. While you would notice if the boiler was not powerful enough, it is possible to buy a boiler that is too powerful for your home.

Though it would heat up your home, you would suffer from expensive heating bills and the energy efficiency would not be perfect.

How Long Should A Boiler Last For?

You should be able to rely on a boiler for several years, around ten to fifteen years. Therefore, if you do get a new boiler to replace your old one then you should not have to worry about it for at least a decade.

You should still look to service and maintain the boiler yet replacing it should not be necessary for a few years.

Final Thoughts

Other factors will also need to be taken into account when working out how big the boiler is, such as the type of fuel. Most UK homes typically use gas boilers for efficiency yet an oil-fired or liquefied petroleum gas system would need some professional advice.

Poor water pressure, the type of boiler, and the overall size of the home will also have an impact on the size of the boiler. However, the number of radiators, bathrooms, showers, and bedrooms are usually factored into working out the size of the boiler in kilowatts.

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