What size of boiler do I need for my home?

Given that a new boiler is one of the most important investments that you will make, properly sizing a boiler for your house is crucial.

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What size of boiler do I need for my house?

A new boiler will typically last up to fifteen years and will be responsible for providing for your heating and hot water needs all day, every day. Choosing the right boiler size will ensure that your house is warm enough and that you have sufficient hot water whenever you need it without spending any more than you have to on your energy bills.

So how can you size a boiler for a house and what factors do you need to consider before choosing a boiler for your home? Here's what you need to think about.

How is boiler size measured?

Before we look at how to size a boiler for your home, it's important to ensure that we understand how boiler size is measured. Boiler size actually refers to the output in British thermal units (BTU’s) or kilowatts (kW), not the physical dimensions. BTU’s or kilowatts are units that are used to measure how much energy is provided by the boiler in the form of heat. Generally speaking, the more heat and hot water you require, the higher the output you will need, although other factors also play a part in choosing the right size.

Should I choose an oversized boiler?

Modern advances in boiler technology mean that they are now more efficient than ever. In the past, homeowners were often advised to choose an oversized boiler to compensate for heat loss, but this is no longer recommended. It will result in wasted energy and is likely to leave you paying more than you need to for your gas bill. Also, new boilers have the ability to modulate the amount of heat output required, so they can operate under partial loads.

Hot water demand

Many North American households use a combination of gas furnaces for heating the home and water heater systems for hot water. Today, modern boilers can replace both and make your home heating system more efficient and more comfortable.

When sizing a boiler for a house, you will need to consider how much hot water you will be using and whether a conventional boiler plus a hot water tank is the right choice.

Ultimately, the type of system you choose will have an impact on the size of boiler you need. A combi-boiler is sized to provide hot water on demand, and you will see its capability to do this expressed in terms of how much water it can heat per minute to a defined temperature rise, e.g. 3.5 gallons per minute (GPM) at a 70°F rise, or 12 liters per minute at a 35°C rise.

The higher the flow rate you require, for filling baths more quickly or to have a more powerful shower, the higher the output you will need from a combi-boiler. Depending on the hot water demand patterns for a household, a combi-boiler may not be the ideal choice and a system boiler plus indirect hot water tank might be the better option.

System boilers and conventional boilers preheat hot water to be stored in an insulated hot water tank. Therefore, hot water flow rate is not as critical. The heat load for an average home can vary from around 34,000 to 68,000 BTU (10 to 20 kW), which means that conventional boilers can be sized smaller than combi boilers. That means they will be more efficient when heating the home, however more space is required due to the addition of the hot water storage tank.

Number of radiators for my home

When sizing a boiler for your home, a key factor to consider is the number of radiators you have or wish to install. If you have just a few, for example in a small apartment, then choosing the smallest suitable boiler will offer you the greatest level of economy and ensure you avoid paying more than you need to for energy.

An average three- or four-bedroom house typically has around ten radiators and will usually require a small or medium combi boiler, or conventional boiler. A larger house with substantially more radiators or an en-suite bathroom will generally need a boiler that is larger.

If you have a large house with as many as twenty radiators, a large combi boiler may be needed to provide sufficient heat with the best economy. Another solution is to change to a system boiler with a hot water storage tank, which can be sized more appropriately.

Viessmann boilers and heating technology

Other factors to consider when sizing a boiler

Heat Loss

In the past, the physical size of your building was often used as a factor when sizing a boiler for your home. While this still plays a part, a much better way to choose the most effective and efficient boiler size is to have an expert calculate your home's heat loss.

Heat loss calculations take into account a range of factors including floor area, number and type of radiators, number and type of windows, insulation, and other aspects that will cause heat to be lost from your home.

A heating expert will then use this information to work out how much heat your house will lose on the coldest day of the year, allowing them to advise you on the optimum size of boiler.

Type of fuel

The type of fuel available at your property can also affect the size of boiler you choose, since boilers for different fuels can vary in terms of efficiency.

Most North American homes use natural gas which offers the greatest level of efficiency, while houses that are not connected to a main gas supply often choose to use an oil-fired or LPG (liquid propane gas) system instead. An alternative option that is less common but is growing in popularity is biomass. However, this is only an option in areas that allow such fuel to be burned.

The type of boiler system you use and the ideal size for your home may be affected by these factors and it is recommended that you seek advice from a professional in order to make the best possible investment.

Water pressure

Although this is not usually a concern for newer houses on a community water supply, some older homes may not have a suitable high pressure water supply and this can have an impact on the type and size of boiler you choose.

A combi-boiler must be big enough to cope with the amount of water passing through it to be heated. The higher the flow rate, the bigger the boiler will generally be.

While most manufacturers provide information on the maximum flow rate for their boilers, getting a professional to advise you may help you choose the most economical boiler for your home, helping you to save money in the long term.

Alternative energy sources

When asking how to size a boiler for their home, many people also want to know about alternative energy sources that may be more environmentally friendly. Some modern boilers are built ready for future energy sources. For example, it may be possible for your boiler to use a combination of natural gas blended with hydrogen gas where such blends may be available now or in the future. If you choose to install a boiler that supports renewable or future energy sources, this may also become an important factor in deciding the right size of boiler.

Think about service

Whatever size of boiler you ultimately choose, be aware that its effectiveness and efficiency will remain at their best if you have it serviced annually by a heating contractor trained or accredited by the manufacturer. Maintaining your system correctly is the best way to ensure that you always have the right amount of heat and hot water at the lowest possible cost.

Contact us for more details and to discuss your options.

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