What size Gas Boiler does my house need?

What Size boiler do I need?

“What size boiler” this question is always a hot topic for discussion, and customers are almost always misinformed.

Anyone researching for info on replacing their gas boiler will inevitably search on google, and this is where the problem begins.

In this post I’ll try to explain, what on the face of it is a very basic thing, but because of all the bad info or misunderstanding, has left people simply confused.

Research (Google)

This is the first piece of info I was met with when searching this topic.

It explains that 10 average sized rads in an average house = 24-30 kw.

It comes from a UK based web site, and in the UK the norm is to have a COMBI gas boiler, which provides instant Hot Water.

So Straight away the Irish home owner is receiving the wrong info.

The info is accurate, but only applies if your in the market for a COMBI, as 24-30 kw is the output energy required on the Hot Water side, NOT the radiator output.

The reason this level of output is so high is because instant hot water requires a lot of energy .

This is why the smallest COMBI available is a 24 kw, simply because anything smaller won’t give you sufficient hot water to your taps.

The table above is from a Worcester Bosch 30kw I Combi Boiler, it has a max Hot Water output of 30kw and only 24kw for the Radiators.

On closer inspection (depending on the boiler make/model) you will see info showing the heating output for the rads is much lower.

How do We calculate the boiler size?

Lets take the typical Irish home. This is a 3 bed semi d with 10 Rads and a HOT press storage cylinder for hot water needs.

Now lets look at standard gas boilers that don’t provide instant hot water.

These boilers come in the following sizes-

  • 7 kw
  • 12 kw
  • 15 kw
  • 18 kw
  • 21 kw
  • 24 kw
  • 28 kw
  • 32 kw
  • 35 kw
  • 37 kw
  • 40 kw.

So which one for our standard Irish home?

The answer is a 15 kw or 18 kw.

Most people simply don’t believe me when I say this, but simple addition shows how it works.

Lets take the 10 rads and the output from each one.

  • 2467 w Kitchen
  • 2878 w Living room
  • 1176 w Hall
  • 2878 w Dining room
  • 1234 w Main Bathroom
  • 588 w En-suite
  • 1372 w *3 for Bedrooms

Add all these rads = 15, 337 watts or 15.3 kilowatts.

Add 2 kw for your Hot water and 1 kw for pipework and heat loss = 18 kw.

The sizes I’ve used here are generous and in most cases the rads would be even smaller, so a 15 kw will may suffice also.

So why are there so many different sizes?

The boilers are all identical, except for the circuit board which is pre-programmed to be whichever size is required. Everything else is the same.

All these boilers are condensing boilers, they basically condense the water that’s a by product of burning gas.

In traditional non condensing boilers this water was simply burnt away, but the efficiency of the whole process suffered as a result.

If this water doesn’t have to be burnt away then energy is saved and this is what makes modern condensing boilers so efficient.

The boiler will condense this water if the operating temperature of the boiler is kept low enough, and this is the reason We shouldn’t oversize the boiler.

If We fit a larger boiler in our 3 bed semi, then it will burn hotter as it’s expecting to heat rads that don’t exist.

The condensing is harder to produce and as a result We’re left with a less efficient system.

Another major issue is the existing gas supply

Condensing boilers, unlike traditional types, suck gas in via the fan and require a certain amount of gas to run.

The larger the boiler, the more gas it requires and anything above 18 kw will certainly require a new larger bore gas line to meet it’s requirements.

This will only cost more money, on top of working less efficiently.


So installing a boiler too large for your home is not only pointless, it will end up costing you more money.

This can all be easily avoided by simply adding up your rads total output,  and fitting the suitable size.

Also a simple trick is to see what size your existing boiler is.

If it heats the rads sufficiently then why would you need to fit a boiler with a larger output?

It will almost certainly be measured in BTU, and is a quick and easy way to see what you require.

50,000BTU=15 kw, 60,000BTU=18kw 80,000BTU=24kw

However, this could be misleading if the boiler is oversized to begin with.

Be careful of anyone selling you a boiler way oversized.

A common tactic is to advertise a low price for lower output boilers like a 15 kw. Then during a survey,advise the customer that a larger boiler is required at a much higher price.

Remember 9-10 rads will require maximum of 18 kw, so question anyone pushing 24-30 kw  promising faster heat up times and hotter rads.







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