What Size Combi Boiler Do I Need?

We take a look at the factors to consider when choosing a suitable

Combi Boiler size

for your home
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A Combi Boiler is an essential investment for your home; after all it will be responsible for keeping your home heated, while also ensuring you have a constant supply of hot water all day, every day.

With that said choosing the right sized Combi Boiler to support your home’s heating needs can be a confusing task, get it right and your home will benefit from optimum heating, get it wrong and you’re likley to waste energy, making your energy bills more expensive.

So, how do you choose the right sized Combi Boiler for your home? Here’s what you need to think about.

* Please note that before making a final decision on a Combi Boiler we’d always recommend you seek advice from a qualified installer who will more accurately be able to assess your home’s heating needs.

How is Combi Boiler size measured?

Combi Boiler size has nothing to do with the physical appearance or size of the unit – instead, it refers to power output. A Combi Boiler’s size is measured in kilowatts (kW) – the more kilowatts it has, the bigger and more powerful it is.

In essence, the more heat and hot water you need, the bigger (in kW) your Combi Boiler must be. However, this is a generalised view; you must consider other factors before making your final decision, i.e. insulation levels and hot water requirements.

Should I choose an oversized Combi Boiler for my home?

An oversized Combi Boiler comes with greater output (kW) than required to heat your home. The theory behind the higher output is that it compensates for heat loss. This was a typical approach taken by installers in the past, advising homeowners to oversize their Combi Boiler should their home (upon inspection) be susceptible to heat loss.

Thankfully, this is no longer the case as modern Combi Boilers are super-efficient (at least 92% as of 2018) and better than ever at compensating for heat loss.

So what’s the difference? Well it’s simple, when you oversize you run the risk of wasting energy – which leads to wasted money. If you choose the right sized Combi Boiler then your home heating runs more efficiently, helping you to save on energy bills in the long-run.

Hot water demand

Combi Boilers are popular because they supply both central heating and hot water on demand. The unique thing about Combi Boilers is that they don’t have a hot-water tank (as seen in system and regular boiler setups) – instead water is heated directly via the mains, all within a single compact unit.

When it comes to hot water output then choosing a Combi Boiler with a sufficient hot-water flow rate is essential, this ensures it can meet the hot water demands of your home.

A Combi Boiler’s hot water capabilities are expressed by the amount of water it can heat per minute, to a specific temperature rise. As an example, 15 litres per minute at 40C rise.

Therefore, if you have a high demand for hot water, then you need a Combi Boiler that offers a higher hot-water flow rate.

Number of radiators

The number of radiators in your home will play a huge role in determining the size of your Combi Boiler. The more radiators you have, the more stress there is on your central heating system. Think about it; you don’t need as much energy to heat up ten radiators as you do to heat up fifteen or twenty.

Use the table below to identify your home’s size (by bedroom and radiator totals); this will help you to estimate a suitable sized Combi Boiler for your home’s central heating needs.

Home Size Bedrooms Radiators Size/Output
Home Size: Small Bedrooms: 1-2 Radiators: 10 Size/Output: 24kW to 27kW
Home Size: Medium Bedrooms: 3-4 Radiators: 11-15 Size/Output: 28kW to 34kW
Home Size: Large Bedrooms: 4+ Radiators: 15+ Size/Output: 35kW to 42kW

Heat loss

The physical dimensions of your home are still a factor when choosing a suitable sized Combi Boiler and it’s advised you seek advice from a qualified installer to calculate your home’s heat loss.

With the help of a heating expert, you’ll be able to determine where and what causes heat to escape your property; window types, insulation quality, the age of the home, etc… The heating engineer will then be able to advise you on an optimum Combi Boiler size suitable for your home.

Fuel type

The majority of UK households are connected to the UK’s gas grid, meaning most choose gas to heat their homes; however for those who aren’t then alternative fuels can be used namely LPG (liquefied petroleum gas) and oil and the less popular but rising option, biomass.

The type of fuel at your property will affect the sizing of your Combi Boiler, as different fuels mean different efficiencies; hence we recommend you seek advice from a qualified installer, who can advise on the best option for your home.

Mains water pressure

Whilst regular boilers take their water supply from a large hot water cylinder a Combi Boiler is fed directly from the mains; meaning it’s essential that your Combi Boiler is big enough to handle the amount of water that passes through it to be heated.

The size of your Combi Boiler will be influenced by the flow rate (pressure) of the mains at your property – the higher the flow rate then the bigger the Combi Boiler needs to be.

Should you be unsure of your mains pressure then we’d recommend you seek advice from a qualified installer who will be able to measure the flow rate and temperature of your mains water for you.

Need more help?

There are numerous factors that need to be considered when choosing an ideal Combi Boiler size suitable for your home. It’s important to ensure that you invest in a system that’s big enough to provide all the heating and hot water your home needs while keeping energy waste to a minimum.

If you’re still unsure and would like more advice on your new Combi Boiler then complete our Quick Quote Form, we’ll then arrange for someone to contact you to discuss your requirements and budget further.

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Combi Boiler Size Calculator

Use our Combi Boiler Calculator below to work out what size Combi Boiler you need. Please note that our calculator provides a rough estimate, a heating engineer will need to accurately size your Combi Boiler during a home visit.

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