Selecting a new hot water cylinder can feel like a daunting task.

The market is full of technical terms – single coil, twin coil, unvented, solar compatible – and unless you work in the trade, it isn’t always obvious what these mean in practical terms.

For most homeowners, the real question is straightforward: which cylinder will give me enough hot water, work efficiently with my heating system, and last for years without hassle?

At its most basic level, a hot water cylinder is simply a storage unit.

It keeps water hot so that it is available the moment you turn on a tap, step into the shower, or run a bath.

But not all households use hot water in the same way.

A couple in a two-bedroom home will have very different needs compared with a family of five in a busy house with multiple bathrooms.

The right cylinder for you depends on three main factors:

• Household size and demand – How many people live in your home, how many bathrooms you have, and whether you mainly use showers or baths all affect how much stored hot water you’ll require.
• Your heating source – Whether you heat your home with gas, oil, solar thermal panels, a heat pump, or a mix of these will influence whether you need a single, twin, or even a triple coil cylinder.
• Future-proofing your system – Many people are now considering renewable options, such as solar or even heat pumps. Choosing a cylinder that allows flexibility for future upgrades can save money in the long run.

By breaking things down into these simple considerations, choosing a hot water cylinder doesn’t have to be confusing.

Once you understand the types available and how they work, it becomes much easier to match the right model to your lifestyle and heating system.

In this guide, we take you through the key considerations for choosing the right hot water cylinder for your home:

Coil types explained
Explore single, twin, and triple coil options and how they work with different heat sources.

Capacity: 200 Litres vs 300 Litres
See which cylinder size suits your household and daily hot water needs.

Vented vs Unvented
Compare traditional loft-fed systems with mains pressure cylinders for performance and space.

Compare Joule Hot Water Cylinders at a glance
A quick reference table to check capacities, coil types, and system compatibility.

Which one is right for me?
Find out how to match your household, water use, and pressure needs to the right model.

Energy sources and compatibility
Learn which cylinders work with oil, gas, solar, heat pumps, and other renewable setups.

Making the right choice
Tips to combine capacity, coil type, and system type for an efficient and reliable hot water system.

Let’s dive in:

Coil types explained

When comparing hot water cylinders, one of the first terms you will come across is coil. But what exactly does this mean?

Inside the cylinder, coils act as heat exchangers. Hot water from your boiler or renewable system flows through these coiled pipes, transferring heat into the stored water without the two water supplies ever mixing.

The number of coils – single, twin, or triple – determines how many different heat sources your cylinder can work with.

Single coil cylinders

A single coil cylinder is the simplest design and is usually the best option if your home only relies on one heat source, such as a gas or oil boiler.

It provides reliable, straightforward performance without the added complexity of managing multiple inputs.

For example, the Stainless Steel Cylinder 200 Litre Single Coil from Joule is ideal for smaller households using a conventional boiler-based system.

It delivers ample hot water without unnecessary features that you won’t use.

Twin coil cylinders

A twin coil cylinder is designed to work with two heat sources.

In most homes, this means a boiler paired with solar thermal panels.

The lower coil connects to the solar system, harvesting free energy whenever the sun is shining, while the upper coil is linked to your boiler, which automatically takes over when solar energy isn’t sufficient.

The Stainless Steel Cylinder 300l Twin Coil is a great example, ensuring your household always has hot water while also cutting energy bills by making the most of renewable energy.

This naturally raises a common question: Do I really need a twin coil cylinder if I don’t yet have solar panels?

The answer is that if you’re planning to install solar in the future, it can be worthwhile choosing a twin coil now.

That way, your system is ready for the upgrade without needing a new cylinder later.

Triple coil cylinders

For larger or more complex systems, a triple coil cylinder may be the answer.

These are designed to integrate three different heat sources, giving you maximum flexibility.

A popular set-up could include solar thermal, a traditional boiler, and a stove with a back boiler.

The Stainless Steel Cylinder 300 Litre Triple Coil from Joule is built for this kind of versatility, ensuring you can draw energy from whichever source is available at the time.

Homeowners often ask whether it’s better to go for a twin or a triple coil cylinder.

In most cases, a twin coil is sufficient – particularly if you’re combining solar with a boiler.

A triple coil only becomes necessary if you plan to add a third heat source, such as a solid-fuel stove or future renewable technology.

It’s also important to note that a twin coil cylinder can be specified with one of its coils suitable for solid fuel (identifiable by its larger 1-inch diameter, compared to the standard ¾-inch coils for oil, gas, or solar).

Therefore, a triple coil only becomes necessary if you genuinely plan to integrate three distinct heat sources where a standard twin coil configuration wouldn’t suffice.

Capacity: 200 Litres vs 300 Litres

One of the most important decisions when choosing a hot water cylinder is its capacity – in other words, how much hot water it can store.

The right size ensures you have enough supply for your household without wasting energy heating water you don’t actually use.

The correct choice largely depends on two factors:

• How many people live in the home
• Your household’s hot water habits (quick showers vs long baths, number of bathrooms, and how often hot water is used at the same time)

200 Litre cylinders

A 200 litre cylinder is well suited to smaller households, typically those with two to three people.

If your home mainly relies on showers and has moderate hot water demand, a 200L single or twin coil model is usually more than adequate.

For instance, the Stainless Steel Cylinder Twin Coil 200 Litre is a compact yet efficient choice for families who want solar compatibility without the need for a larger tank.

300 Litre cylinders

For busier households with more people or higher hot water demand, a 300 litre cylinder is often the better option.

This size is designed for medium to large families (four to six people) and works particularly well in homes with multiple bathrooms or where baths are taken regularly.

With the larger storage, you won’t need to worry about the hot water running out halfway through a busy morning routine.

A model such as the Stainless Steel Cylinder Twin Coil 300 Litre provides both capacity and flexibility, ensuring plenty of hot water even at peak times.

It’s natural to wonder if it’s possible to go too big.

While a larger cylinder won’t harm your system, it can become less efficient if it’s far bigger than your actual needs.

Heating water you don’t end up using simply means higher energy bills.

That’s why it’s important to size your cylinder appropriately for your household rather than choosing the largest model by default.

Vented vs Unvented

Another key decision when choosing a hot water cylinder is whether to go for a vented or unvented system.

These terms refer to how the cylinder is supplied with cold water and how the pressure is managed throughout your home.

What does ‘unvented’ mean?

A vented cylinder is the more traditional setup.

It is supplied by a cold water storage tank – usually located in the loft – which feeds water down into the cylinder by gravity.

This system is straightforward and reliable, but water pressure depends on the height of the storage tank above your taps.

That’s why vented systems often produce lower pressure at showers or taps on upper floors.

An unvented cylinder, on the other hand, is fed directly from the mains water supply.

This means it can deliver hot water at mains pressure throughout the home, providing a strong, consistent flow rate – especially noticeable at showers, baths and mixer taps.

If you’re looking for powerful showers without the need for additional pumps, an unvented cylinder is usually the preferred option.

Please note that cylinders designed for mains water supply should never be used with well water, as this can void the warranty and lead to premature system failure (typically within 1-2 years).

Even if your mains pressure isn’t consistently strong, unvented systems can often be effectively boosted by an additional pump to ensure optimal flow throughout your home.

Because unvented cylinders operate under pressure, they must include built-in safety features such as pressure relief valves and expansion vessels.

In addition, a proper installation requires an unvented cylinder kit, like the Joule Unvented Tank Kit, to ensure the system runs safely and complies with regulations.

Compare Joule hot water cylinders at a glance

Before deciding on the right cylinder for your home, it can be helpful to see the available options side by side.

The table below summarises the key features of Joule stainless steel cylinders, including capacity, coil type, suitability for different heat sources, and whether they are vented or unvented.

Use it as a quick reference to compare models and identify which is best suited to your household’s needs and future plans.

With this side-by-side overview in mind, it’s easier to see how each model aligns with your household’s needs.

The table highlights key differences in capacity, coil type, and system compatibility, but choosing the right cylinder also depends on how you use hot water daily, the heating sources in your home, and whether you’d benefit from the improved performance of an unvented system.

The next section will help you weigh these factors and make a confident, informed decision.

Which one is right for me?

Homeowners often ask whether they need an unvented cylinder.

The answer depends mainly on your property’s mains water pressure and whether you want to remove the loft tank.

• If your home has good mains pressure and you’d like to simplify the system by doing away with the cold water storage tank, then an unvented cylinder makes excellent sense. You’ll enjoy better performance at showers and taps, and you’ll free up space in the loft. It’s also worth noting that even with lower mains pressure, an unvented system can still be a viable option, as its performance can often be enhanced by installing a booster pump.
• However, if your mains pressure is low, or if you’re in an older property where upgrading the system would be complex or costly, a vented cylinder may still be the more practical and economical choice.

By understanding how each type works, you can decide which setup best suits your home’s layout, your water pressure, and the level of performance you expect from your hot water system.

Energy sources and compatibility

One of the biggest advantages of modern stainless steel cylinders is their ability to work efficiently with a wide range of heating sources.

This flexibility means you can match your current setup today, while also leaving room for future upgrades tomorrow.

Oil & gas boilers

Whether your home runs on oil or gas, all Joule stainless steel cylinders are designed for compatibility.

The coil layouts within the cylinder maximise heat transfer, ensuring that water heats up quickly and retains its temperature efficiently.

This makes them a reliable choice for traditional boiler systems, whether you’re heating a compact 200 litre single coil cylinder for a smaller household or a 300 litre twin coil version for a family home.

Solar thermal systems

For homeowners with solar thermal panels, or those planning to install them, choosing the right cylinder is key.

A twin coil cylinder allows your solar system to be connected to the lower coil, harnessing free energy from the sun to heat the stored water whenever conditions allow.

The upper coil remains connected to your boiler, which automatically steps in to top up the hot water supply when solar energy isn’t sufficient – for example, on cloudy days or during winter months.

In larger or more advanced setups, a triple coil cylinder provides even greater flexibility, letting you integrate solar alongside a boiler and a third source such as a stove with a back boiler.

This kind of system ensures you’re making the most of every available energy source.

Heat Pumps

When integrating a heat pump (such as an Air to Water system) as a heat source, the cylinder must feature a ‘high gain’ coil.

This specialised coil has a significantly larger surface area compared to standard coils.

This increased surface area is essential because the water circulating through a heat pump coil operates at a lower temperature than water from traditional oil or gas boilers, requiring more area to efficiently transfer heat and bring the domestic hot water to a suitable temperature.

Future-proofing your home

More and more households are looking to reduce their reliance on fossil fuels by incorporating renewable energy.

Even if you don’t currently have solar thermal panels, it may still be wise to consider a twin coil cylinder.

By installing one now, your system will already be ready for solar integration in the future, saving you the expense and inconvenience of replacing the entire cylinder later.

Similarly, if you anticipate exploring other renewable technologies, such as biomass boilers or stoves with back boilers, a triple coil cylinder gives you the flexibility to connect them easily down the line.

In short, choosing a cylinder that matches not only your current needs but also your long-term plans can help reduce running costs, cut carbon emissions, and extend the lifespan of your investment.

Making the right choice

With so many models and features to consider, it helps to break the decision down into a few simple questions.

By focusing on your household’s daily needs and future plans, you can quickly identify the cylinder that will serve you best.

Household size and hot water demand

Start by thinking about how much hot water your household uses in a typical day.

• A smaller home with two or three people, where showers are the norm and there is only one bathroom, will usually be well covered by a 200 litre cylinder.
• Larger families, or homes with multiple bathrooms where baths and showers may be running at the same time, are better suited to a 300 litre cylinder.

This ensures you have enough stored water without oversizing the system, which can waste energy.

Current heating source and future plans

Next, consider how your hot water is heated.

• If you rely solely on a gas or oil boiler, a single coil cylinder may be all you need.
• If you already have solar thermal panels, or you’re planning to add them in future, a twin coil cylinder allows you to take advantage of free renewable energy while still keeping your boiler as backup.
• If you are using a heat pump (e.g., Air to Water), ensure your cylinder has a ‘high gain’ coil for efficient heat transfer.
• For households combining multiple heat sources – such as solar, a boiler, and a stove with a back boiler – you might find that a twin coil cylinder, featuring a solid fuel compatible coil (1-inch diameter), provides sufficient flexibility without necessarily needing a triple coil. A triple coil cylinder is best suited for integrating three truly distinct heat sources.

Even if you don’t have solar yet, many homeowners choose a twin coil model to future-proof their system and avoid replacement costs later.

Water pressure and system type

Finally, decide whether a vented or unvented setup is right for your property.

• If you’re happy with the current loft-fed system and your water pressure is adequate, a vented cylinder remains a practical and economical option.
• If you’d like to eliminate the cold water storage tank and benefit from powerful mains pressure at your taps and showers, an unvented cylinder with the correct safety kit may be the better choice. It is important to note that, if your water supply is from a well rather than the mains, a cylinder specifically designed for well water applications is required to avoid warranty issues and premature failure.

Bringing it all together

By weighing up these three factors – household size, energy sources, and system type – the right solution quickly becomes clear. For example:

• A family of five with solar panels and good mains pressure will be best served by a 300L twin coil unvented cylinder.
• A couple in a smaller home with a traditional oil boiler might only need a 200L single coil vented cylinder.
• A homeowner planning to add multiple renewable sources could choose a 300L triple coil unvented model, or a specialised twin coil ensuring maximum flexibility for the future.

In short, there is no one-size-fits-all solution.

The best cylinder is the one that fits your household’s daily needs while giving you the confidence that your system will perform reliably – both now and in the years ahead.

Final thoughts

Choosing the right hot water cylinder is about more than just picking a size from a list – it’s about ensuring your home runs smoothly, efficiently, and reliably for years to come.

The best cylinder will give you enough hot water when you need it, work seamlessly with your heating system, and be flexible enough to adapt if you decide to add solar or other renewables in the future.

By thinking carefully about your household size, your current and future energy sources, and whether you’d benefit from the performance of an unvented system, you can narrow down the options with confidence.

At MyBuildingSupplies.ie, we know that investing in the right cylinder isn’t just about today – it’s about comfort, energy savings, and peace of mind for the future.

That’s why we supply trusted Joule stainless steel cylinders, unvented tank kits, and expert guidance to help you choose the right system for your home.

Whether you’re replacing an existing tank, upgrading to solar compatibility, or planning a complete heating system overhaul, our carefully selected range and knowledgeable team are here to support you every step of the way.

Your hot water system should work as hard as you do – and with the right cylinder, you can enjoy reliable performance, lower running costs, and a home that’s ready for the future.