What pressure should my boiler be?

Another hot topic for discussion is the boiler pressure. This is a very misunderstood area and comes down to a couple of factors.

•The boiler brand/ model

•How the system gets its water for the radiators.

A common gauge found on most boiler models, 1 bar is the normal setting, but this isn’t always the case

Boiler Brand/ Model

Most gas boiler brands require pressure to operate,  it’s as simple as that.

They either have a pressure switch or in most modern boilers the main circuit board is programmed to be pressure dependant.

However, some brands can operate on little or no pressure and these boilers are a personal preference of mine, for that very reason.

The filling point for my radiators.

Most boiler manuals or instruction videos will direct you to the boiler for the fill point.

However, in most cases the more likely position is going to be the hot press.

This is simply because all pipework is accessible there, where the boiler is usually in a cramped kitchen press or similar unit.

In the hot press you’ll likely find what’s called a filling loop. The filling loop is a special design, that only allows water to flow in one direction.

This is a safety measure to prevent water from your radiators mixing with domestic water you’ll use to wash with or drink.

An example of a filling loop with valves in the closed position.

Pressure or no pressure

If you have a pressure dependant boiler then this will require the filling loop to be directly connected to your incoming water mains.

In this case the filling valve is opened,  the system is charged to 1 bar and then the valve is CLOSED.

Closing the valve is vitally important, failure to do this can be detrimental to a heating system.

This is a direct result of a failed mains pressurised system, the brown stain is a result of the filling loop not been closed by the homeowner

What causes the damage?

By leaving the mains fill valve on, the pressure will go way past the recommended 1 bar.

If the pressure goes above 3 bar,  the safety valve will open and water from the system will disperse out side.

This is a catastrophic cycle as fresh water is constantly passing into the heating system, fresh water is rich in oxygen, this in turn causes rapid corrosion, hence the brown stains.

This is just the beginning,  the high pressure will eventually cause the expansion vessel to burst.

Expansion vessel

The expansion vessel is the main component of a system gas boiler, but can also be found on the heating system external to the boiler.

As water heats it expands, like steam would emit from a kettle.

To accommodate this in your radiators we use an expansion vessel.

As the water expands, a bladder inside the vessel expands and contracts as the water cools.

Over pressurising a system can cause the bladders to burst and when this happens the safety valve comes into play again.

The pressure from the expansion now finds its way through the safety valve, to the outside.

This is noticed by the Home owner who in turn opens the filling valve to let fresh water back in.  Eventually the home owner, fed up refilling, just leaves the valve on.

It’s only a matter of time before the boilers main heat exchange becomes damaged beyond repair, due to corrosion.

Why I prefer low pressure boilers

A low pressure boiler doesn’t need to be pressure dependant, which means it can get a low pressure supply from storage tanks.

Because of this the filling valve can be left open as the tank pressure will never get anywhere near the 3 bar required to lift the safety valve, and the high pressures that can burst the vessel.


Mains pressurising a system is absolutely sound practise and in some cases has to be done,  ie an attic conversion where the radiators are higher than the storage tanks.

A mains regulating valve can also be used, these are preset to not allow anything above 1.5 bar into a system.

However, they are expensive and can be prone to failure.

The main problem is the homeowner not closing after bleeding rads or topping up a system.

If you’re losing pressure check for a tell tail brown stain on the outside,  this is almost certainly a burst vessel and will need immediate attention.




combi boilers installation


Combination Boilers (combi boilers), are without question the most practical way of generating your hot water demands.

However, retrofitting one for your existing standard boiler is far from straightforward.

In this post i’ll explain the process, so you can decide if this route is an option for your home.

There are 4 main points to consider when installing combi boilers

A- The existing boilers location

Physically a combi boiler is identical in appearance and size, to a standard one.

The combi basically has a small tank called a plate heat exchanger inside it, which stores the hot water.

The main difference from an installation point of view is basically 2 extra pipes.

1- the cold water in, from the incoming mains.

And 2- the hot water out, that goes to the taps and showers, etc.

In all homes you are guaranteed to find these 2 pipes at the kitchen sink.

The below picture illustrates a perfect location, as We simply just run our 2 pipes from the sink to the boiler, behind the kitchen presses.

If the boiler is unfortunately in another location, then it’s trying to find the easiest most practical route to run these 2 pipes.

This picture is a perfect location for a new combi boiler, close to a kitchen sink. Most modern new built homes have the boiler in this location.

B- Your Hot Water requirements on combi boilers

In the domestic market the smallest combi boiler is a 24 kw, and the largest is around 40 kw.

Physically they are all identical, the heat exchanger is basically larger or smaller depending on the size.

A 24kw is perfect for someone just running a shower, say an apartment or a couple with no children.

If you have a bath then a 30 kw will deliver more hot water where a 24 might struggle.

If you’re planning on running multiple showers at the same time with baths and en-suites etc, then that’s when you will require the larger 40 kw types.

However these are incredibly expensive, upwards of €1000 or more when compared with 24 and 30 kws.

In this case it’s probably better to keep a hot water cylinder as it will guarantee hot water in this amount, and certainly will be far cheaper.

How much hot water do you require? Sinks, showers and Baths. The average 3 bed home will have 3 sinks, a shower and a bath with 10 rads. A 30- 35kw will be perfect in this case.

C- The Gas Supply on combi boilers

The Gas supply, from the installers point of view, is probably the main obstacle.

A combi boiler ,when supplying hot water, will “HIGH FIRE”, basically going into maximum output.

If there’s not enough gas it simply won’t work and if you have other gas appliances, this further complicates things.

A standard gas boiler rarely uses it’s maximum output when heating the rads and your hot press cylinder, that’s why the gas is not really an issue when installing them.

If it’s undersized a new larger gas supply may need to be installed.

Domestically a copper gas supply leaves the gas meter in the larger 3/4″ pipe . However it’s not uncommon for the existing gas boiler to be piped with the smaller 1/2″. Unfortunately this may not suffice for a combi boiler, as the smaller pipe simply doesn’t provide as much gas to the boiler.

D- Water pressure on combi boilers

Ideally a combi boiler should run directly from your incoming mains.

This will eliminate the need for any water storage in your home, and enables the removal of the attic tank and hot press tank.

In most modern homes and newly built estates I have found the pressure, although not fantastic, is greatly improved.

However older homes closer to the city, especially around the Dublin area, have incredibly poor pressures.

In this case We need to take a closer look at your hot water demands.

I’ve noticed most homes have electric showers, like TRITONS or MIRAS, these showers don’t have to be considered as they self heat the water.

Assuming there’s no bath, this will leave just sinks and even with a poor water supply a 24 kw should be fine in this case.

If you need to run a shower or bath then the only way to guarantee a decent water supply is to introduce a booster pump.

The booster will replace the mains, however everything (with the exception of wash machines and dishwashers) will now be pumped toilets, sinks, showers and baths.

The most likely location for the pump will be the hot press, it will guarantee fantastic showers but at a cost, they tend to be loud and are incredibly expensive.


Hopefully the above info helps you understand what’s involved.

A scenario where the boiler is close to the kitchen sink in a newer built home is perfect. In this case the gas line is usually fine also.

However, if you require allot of hot water with poor incoming water pressures and the gas lines size is questionable, then it becomes a costly venture.

In this case a new standard boiler with a good quality hot water cylinder is the most practical and without doubt cheaper option.

Click here for our latest combi boiler offers





Gas Boiler installer Accreditation

Is your Gas boiler installed by an accredited installer?

I recently installed a gas boiler, and a few days later I had an interesting conversation with the customers son.

On completion I normally register the boiler with the manufacturer, and I used the sons email as the lady was elderly and didn’t have one.

He rang me to thank me, and was delighted that he had received an email from BOSCH.

This indicated  that I had installed the boiler, and also when the warranty was due to expire.

He went on to explain that he never received a similar email with his own boiler, also a BOSCH model, installed 2 years previous by a different contractor.

Is your Gas Boiler even registered?

What happened here, sadly, is common practice with gas boiler replacements.

Because I’m an accredited installer for BOSCH I was able to offer extended cover on the new boiler, 10 years as opposed to 5 by a non-accredited.

This is the reason I have to register the installation details.

BOSCH need to know it was installed by me, and in turn, I log into a secure portal and input all the relevant details.

Unfortunately in the example here, the installer left the registration in the hands of the customer.

This installer clearly failed to explain the importance of completing this.

The end result is a customer with NO warranty.

This is an actual warranty registration from BOSCH. It keeps details of installation date , address and when the guarantee ends. One of these is emailed to every customer I install a boiler for.

How do I know my Boiler was registered?

All Gas installers must be RGII registered, this ensures that the installer is fully trained and insured.

This is all that’s required to install a gas boiler.

However, been accredited with a major player like BOSCH requires much more than this.

Reputation means everything to these large companies, and they are only interested in having their product installed by reliable professionals.

It took me years to get accreditation, which explains why so many boilers are installed by non-accredited gas installers.

It requires a lot of extra leg work and training to become accredited, most installers simply can’t be bothered.

Personally I think it’s essential, after all what customer doesn’t want extended warranty?


All accredited installers will have a listing on the manufacturers website. Click on logo for the General Gas BOSCH listing
Click here for our latest offers




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.