Is It Better To Undersize Or Oversize A Boiler?

by | Last updated on January 24, 2024

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Contractors often add more capacity to the boiler because they want to make sure it isn’

t undersized

. However, an oversized boiler doesn’t do the customer any favors. Oversized boilers waste fuel and ultimately shorten the life of the system.

How many radiators can a 30kW boiler run?

How Many Radiators Can a 30kW Boiler Run? For combi boilers, sizes from 28 to 32kW can keep between 10 and 15 radiators nice and toasty. On the other hand, 30kW heat only and system boilers can heat

15 to 20 radiators

, making them the ideal choice for running larger properties.

What happens if my boiler is too small?

What happens if your boiler is too small? Having an undersized boiler may mean:

It won’t have the power to heat your home properly

, which is far from ideal in the winter. You may not have enough hot water when more than one person in the home is trying to have a shower or run the taps.

Do I need a bigger boiler?

As a general rule,

the bigger your house and the more radiators and hot taps and showers you have

, the more powerful the boiler you need. A single-occupant bungalow will require a much less powerful boiler than a family of six living in a five-bedroom house with three showers.

Is a smaller boiler more efficient?


The lower a boiler can go

, the more efficiently it can operate all year round. The minimum output tends to increase in bigger models and some exceed the maximum heat requirement for the average UK home, i.e. 6-8kW. Boilers with high minimum outputs will never be as efficient as a boiler that can go down to 2kW.

What happens if your boiler is underpowered?

An underpowered (undersized) central heating boiler

will struggle to meet the heating demands of your home

, resulting in a shortfall of heating to your radiators, hot water to your taps/shower or worse – both.

Should I turn radiators off in unused rooms?

And it’s not a good idea to turn a radiator off permanently in an unused room, as this can lead to damp and mould.

Leave them on low instead

, and close the doors.

Does a higher kW boiler cost more to run?

Will a higher kW boiler ever cost more to run? The short answers is

no

. … Modern boilers will fire up on a high flame, and as radiators or water reaches temperature, they’ll reduce the size of the flame (modulating), then stay simmering so they aren’t wasting gas.

What happens if a boiler is oversized?

Put simply, excessively

large boilers generate too much heat that the system cannot dissipate

. For example a 30kW boiler needs a flow rate of 1400 litres per hour through it.

How many radiators will a 35kW boiler run?

How many radiators can a 35kW boiler run? A 35kW boiler will run

20-25 radiators

with no problems at all.

What boiler size do I need?

As a rough estimate most flats, apartments and smaller houses, with up to 10, average size radiators and 1 bathroom* will require a

24 to 27kw

Combi Boiler. For a medium to large 3-4 bedroom house with up to 15 average size radiators, 1 bathroom and an ensuite* bathroom, a 28-34kw Combi Boiler would be installed.

What size boiler do I need for a 2 bedroom flat?

A two-bedroom house or flat is likely to have one bathroom, and at most four radiators. Based on this, the most suitable boiler size should be

between 24-27kW

. A combi boiler with an output of 24 kW can easily run up to 10 radiators. It can also comfortably supply hot water to one bathroom at mains pressure.

Is a 24kW boiler big enough?

Most flats, apartments and small terraced houses with just one bathroom and the number of radiators ranging from one to 8 will find that choosing a 24kW boiler is

usually good enough

, however it will depend on circumstances.

Is it OK to oversize radiators?

Rule-of-thumb methods often

oversize the radiator

so as to make sure there are no complaints that the space stays cold, but what no-one ever tells you is that a grossly oversized radiator can cause other problems within the system.

How many radiators can a 50kw boiler run?

The boiler can heat 300 litres of hot water between 1 and 2 high volume hot water tanks, and is best suited to large three storey Victorian or Georgian-type home or office. It is possible to heat

18 to 20 radiators

.

What size boiler do I need for a 4 bedroom house?

As your home has four bedrooms, you’ll need a boiler with an

output between 27kW-40kW and a cylinder with a 250-300 litre capacity

. If you have 2-3 bathrooms, you’ll likely just need a 27kW system boiler, whereas a home with 3+ bathrooms will likely need a 40kW boiler.

David Martineau
Author
David Martineau
David is an interior designer and home improvement expert. With a degree in architecture, David has worked on various renovation projects and has written for several home and garden publications. David's expertise in decorating, renovation, and repair will help you create your dream home.