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What size boiler should I install?

Lets face it, choosing the right size boiler is absolutely imperative to the functioning of your home

If you get it wrong, you're going to have to live with poor heating and water performance for years to come.

However, finding the right size boiler isn't as easy as it seems, which is why we've provided some best practices below to help you size your new boiler so that you know what to do when the time comes to replace it! 

The basics of sizing your new boiler

How is boiler size measured?

Boiler size refers to the output of the boiler in kilowatts (kW). Generally speaking, the more heat and hot water you need, the higher the kilowatts will be. 

Should I oversize my boiler to be safe?

Old boilers, pre-condensing boilers, were extremely inefficient, so oversizing your boiler was a good way to compensate for heat loss. 

Nowadays with our energy-efficient boilers, oversizing can actually end up costing you money on your heating bills. 

So in other words, no, definitely avoid oversizing your boiler!

Sizing your boiler

Does your current boiler meet your needs?

Does your boiler meet your current needs? i.e. are you running out of hot water regularly? Does your home take too long to heat up? Is anybody moving out of your house in the near future?

And then ask yourself, would you be happy if you replaced your boiler with a direct replacement knowing what you know about the above? 

If no, then you should probably consider resizing your boiler to meet your current needs. 

Hot water demand

Most houses use combi boiler systems nowadays. 

They combine all of the old water tanks and holding tanks into one compact system that typically sits in the kitchen. They provide heat and hot water direct from the mains and they're practical and fairly low cost to run for smaller homes (with one bathroom). 

These boilers aren't however, efficient for larger homes with more than one bathroom. 

If you have a large family and/or two bathrooms, it's worth considering resizing your boiler to a larger system, likely to a conventional boiler system with hot water tanks so you always have enough hot water.

Number of radiators

How many radiators do you have in your home? 

  • An average 3-4 bedroom house will have around 10 radiators. And in most cases a 24-30kw combi boiler is suitable. 
  • Larger houses with more radiators or and en-suite bathroom generally need a boiler that powers between 30-35kw. 
  • If you have an even larger house with 3+ bathrooms and 15-20+ radiators, you'd be best off considering a 35-42 kw combi boiler, or changing over to a conventional system boiler. 

If your current boiler is below the recommended kw, then you should probably consider resizing your boiler. 

Other factors

There are other factors that you need to consider like heat loss, type of fuel (gas, oil, electric etc.) water pressure and a few other minor factors, all of which together can change your choice of boiler. 

When do I replace my boiler?

There are a number of indicators that it might be time to replace your boiler, including:


One of the primary indicators that it's time to replace your boiler is its age. The average lifespan of a boiler is around 10-15 years, depending on the make and model. If your boiler is approaching or exceeding this age range, it's worth considering a replacement. Older boilers tend to be less efficient and more prone to breakdowns, which can lead to higher energy bills and costly repairs.

Breakdowns on a regular basis

If your boiler is frequently breaking down and requiring repairs, it may be more cost-effective to replace it rather than continually paying for repairs. Frequent breakdowns can be a sign that your boiler is reaching the end of its lifespan, and it may be more cost-effective to invest in a new, more reliable model.

Increasing energy costs

Modern boilers are much more efficient than older models, and upgrading to a newer model can result in significant energy savings. If your energy bills are increasing, or you're noticing that your boiler is using more fuel than usual to heat your home, it may be time to replace your boiler.

Uneven heating

If certain rooms in your house are regularly colder than others, it may be a clue that your boiler needs to be changed since it is not operating well.

Safety concerns 

If you're experiencing issues with your boiler, such as strange noises, leaks, or carbon monoxide alarms going off, it's important to address these issues promptly. These can be signs of serious problems that could potentially be dangerous to you and your family. In some cases, it may be necessary to replace your boiler to ensure the safety of your home.

It is advised to have a qualified boiler expert inspect your system and make any required recommendations for repairs or replacements if you notice any of these symptoms or have concerns about the health of your boiler.

Boiler servicing is an essential part of maintaining the safety and efficiency of your heating system. Regular boiler servicing can help prevent breakdowns, improve energy efficiency, and extend the lifespan of your boiler.

Would you like help sizing your new boiler in Glasgow?

If you would like help sizing your new replacement boiler, please get in touch with our gas engineers in Glasgow. We cover Paisley, Clydebank and Cumbernauld areas.

Call us today on 0141 374 0274 or 07580 020343 or send us a message via our contact form

We'll look forward to hearing from you soon.