Unvented · pressurised

Unvented pressurised cylinders in Dublin

Mains pressure hot water, no tank in the attic, and a shower that actually feels like one.

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What is an unvented cylinder?

An unvented cylinder — often called a pressurised cylinder — is fed directly from the mains rather than from a cold water tank in the attic. Because it runs at mains pressure, you get far better flow at the taps and the shower, and you free up the attic entirely.

It is the standard arrangement in most new builds now, and it is the single most common reason people ring us about hot water: the shower is weak, and they want to know what can be done about it.

Vented — the traditional way

Cold water tank in the attic feeds the cylinder by gravity. Simple and reliable, but the pressure you get depends on how far the water falls — which is why upstairs showers are often weak.

Unvented — pressurised

Fed from the mains — or from a boosted supply where the mains alone is not strong enough. No attic tank, strong flow upstairs and down, and the loft back. It needs safety controls, because the water is under pressure.

Why people switch

What an unvented cylinder gets you

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A proper shower

Mains pressure at every outlet, upstairs included. For most people this is the whole reason they do it.

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The attic back

No cold water tank, no overflow pipe, no header tank. That is real storage space returned, and one less thing to freeze in January.

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No shower pump

Where the mains is good, no noisy shower pump running every morning — the pressure is already there.

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Less to go wrong in frost

Attic tanks and their pipework are the classic burst-pipe culprit in a cold snap. An unvented system has neither.

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Two outlets at once

Where the mains supply is good, a shower and a tap together stop being a negotiation.

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Cleaner water

Water comes straight from the mains rather than sitting in an open tank in the roof.

What if your mains pressure is not up to it?

This is where a lot of people are told “sorry, your mains is too weak” and left at that. It is not the end of the road. An unvented cylinder does not have to run off the mains alone — the supply going into it can be boosted, and there are three ways of doing that.

Two rules that matter here

Boosting goes upstream. You boost the cold supply feeding the cylinder, never its hot outlet. Anyone suggesting otherwise should not be touching it.

In Ireland, boosting must be indirect. You are not permitted to simply fit a pump onto the incoming public main and pull harder — doing that takes pressure off your neighbours' supply. Uisce Éireann allow only indirect boosting, with a break tank required once you are drawing any real volume, and the details of such a system have to be submitted before connection.

That rules out the inline mains pump you will see advertised in the UK. What it leaves is two routes that are both perfectly workable.

1. A break tank and pump set

This is the normal answer here. A small cold water tank fills from the mains through a float valve — there is an air gap, so nothing is being sucked out of the main — and a pump then takes water from that tank and pressurises the system. Because the pump draws from the tank rather than the main, it is compliant, and it works even where the incoming supply is genuinely poor.

Works where mains pressure is bad — it does not depend on it
Consistent pressure regardless of what the mains is doing
The compliant way to get real pressure out of a weak supply
You are back to having a tank, though a much smaller one than the old attic tank
The pump makes noise, so where it goes matters

2. An accumulator

A pressure vessel that fills from the mains between uses and holds that water ready, so when you open a tap there is a reserve behind it. It is not pumping anything out of the main — it simply stores what the main gives it — so it sidesteps the problem entirely.

Silent — there is no pump at all
Nothing mechanical to wear out or service
Fixes flow — the shower dropping when someone runs a tap downstairs
Needs real space, as these vessels are large
Will not help much if the static pressure itself is very low

Our honest view

If your mains is good, a straight unvented cylinder is the cleanest answer — nothing running, nothing to service beyond the cylinder. If the issue is flow rather than pressure, an accumulator is usually the tidier fix because there is no pump. And where the mains is genuinely weak, a break tank and pump set will do it properly and legally. We will measure first and tell you which one your house actually needs, rather than selling you the biggest.

So the first thing we do is measure. We check the pressure and flow rate at your property before recommending anything — and if a better vented setup would serve you just as well for less money, we will say so.

What we check before recommending one

Incoming mains pressure and flow rate
Space for the cylinder and its expansion vessel
Where the safety discharge pipework can run
The electrical supply for the immersion
Whether your boiler can heat it efficiently

The safety side — and why it is regulated

A vented cylinder is open to the air through the attic tank, so pressure can never build. An unvented cylinder is sealed and pressurised, which is exactly what makes it perform — and exactly why it has to be installed properly.

Every unvented installation needs:

An expansion vessel to take up the expansion as water heats
A pressure relief valve
A temperature and pressure relief valve
Correctly sized discharge pipework, run to a safe place
A tundish, so any discharge is visible

None of that is optional, and none of it is covered by gas registration — RGI registration is about gas, and says nothing either way about pressurised hot water. It is separate work needing its own understanding, and manufacturers will generally expect evidence that whoever fitted it knew what they were doing before they will stand over the warranty.

So it is a fair question to ask anyone quoting you for one: have you done this before, and will the warranty stand?

They need servicing every year

Unlike a vented cylinder, an unvented one has safety components that must be checked annually — the expansion vessel charge in particular. Skip it and you risk both the safety devices and the manufacturer warranty. We can look after that at the same time as your annual boiler service.

What it costs

It depends on the cylinder size, the work needed to run the discharge pipework, and whether the existing plumbing needs altering. We price it per job after seeing the property, rather than quoting a figure that would be wrong for most houses. Ring us on 01 6991196.

Common questions

Unvented cylinder questions

What is the difference between vented and unvented?

A vented cylinder is fed from a cold water tank in the attic and relies on gravity, so the pressure depends on the height of the drop. An unvented cylinder is fed directly from the mains and runs at mains pressure, so flow is much stronger — especially upstairs.

Will an unvented cylinder fix my weak shower?

Usually yes. Where the mains supply is good it will do it on its own. Where the mains is poor, the supply feeding the cylinder can be boosted — with an accumulator, or a break tank and pump set — so a weak main is not the dead end people are often told it is. We measure the pressure and flow first and tell you which you need.

Can an unvented cylinder be pumped?

Yes, but it has to be done the right way. The boost goes on the cold supply feeding the cylinder, never on its hot outlet — and in Ireland it must be indirect, meaning a break tank and pump set rather than a pump fitted straight onto the incoming main. An accumulator is the other option where the problem is flow rather than pressure.

Do I still need the tank in the attic?

No. That is one of the main attractions — an unvented system takes its water straight from the mains, so the cold tank, the overflow and the header tank all come out.

Does an unvented cylinder need servicing?

Yes, every year. It has safety components — an expansion vessel and relief valves — that need checking. Annual servicing is also usually a condition of the manufacturer warranty.

Does it matter who installs an unvented cylinder?

Very much. It is a sealed, pressurised vessel with safety components and discharge pipework that all have to be right. It is separate work from gas, so gas registration on its own tells you nothing about it — and manufacturers will generally want evidence of competent installation before standing over the warranty. Ask whoever is quoting you.

Is stainless steel better than copper for unvented?

For an unvented cylinder, stainless steel is the practical choice — it handles the pressure better and lasts longer. See our hot water cylinder guide for the full comparison.

Thinking about an unvented cylinder?

We will measure your mains first and tell you exactly what your house needs.

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