Direct vs Indirect Hot Water Cylinders: Which Is Right for Your System?
Direct vs Indirect: What’s the Difference?
In a direct cylinder, heating elements sit inside the water you use. In an indirect cylinder, heat transfers through a coil, keeping your heating system water separate from your domestic hot water. This separation defines “indirect,” regardless of how many coils it contains or what heat sources it serves.
Whether you’re replacing an existing cylinder or specifying one for a new installation, understanding the distinction between direct and indirect cylinders is essential for choosing the right solution for a home’s heating system.
The terms “Direct” and “Indirect” refer to how heat reaches the stored water inside the cylinder. In a direct cylinder, heating elements (typically immersion heaters) sit directly in the water that comes out of the taps. In an indirect tank, heat is transferred through a coil or heat exchanger, keeping the heating system water separate from the domestic hot water. This separation is what defines a cylinder as “indirect”, regardless of how many coils it contains or what heat sources it serves.
Both types can be manufactured as either vented or unvented vessels, but this page focuses specifically on vented variations. These open-vented cylinders operate at mains cold water pressure on the inlet and rely on gravity pressure for hot water distribution, making them ideal for properties without adequate mains pressure or where building regulations require vented systems.
Newark Cylinders manufactures the full range of direct and indirect vented cylinders, along with several specialist variations designed for specific installation scenarios. On this page, we’ll guide you through:
- Economy 7 Cylinders – twin-element designs for off-peak tariff installations
- SlimFit Cylinders – compact solutions for tight spaces and airing cupboards
- Gravity Cylinders – designed for older heating systems with gravity circulation
- Combination Cylinders – integrated cold water storage and hot water heating in one vessel
Quick Comparison: Direct vs Indirect at a Glance
| Feature | Direct Cylinder | Indirect Cylinder | When to Use |
|---|---|---|---|
| Heat Transfer | Heating elements inside water | Heat transfers through coil | — |
| Primary Water Separation | N/A | Yes - keeps systems separate | Better for complex systems |
| Installation Complexity | Simple | More complex (piping required) | Budget-conscious = Direct |
| Upfront Cost | Lower | Higher | — |
| Lifespan | Shorter (element degrades) | Longer (separated water) | Long-term value = Indirect |
| Best Heat Sources | Electric immersion | Boiler + solar + heat pump | What's your heat source? |
| Future Heat Pump Ready? | Possible but difficult | Yes - designed for it | Planning heat pump? = Indirect |
| Immersion Heater Options | Top/side placement | Not applicable | Customize with Newark |
| Multi-Coil Upgrade? | No | Yes (Newark option) | Multiple heat sources = Indirect |
| NC Customisation | Non-standard size + element placement | Non-standard size + coil positioning | All options available bespoke |
Direct Cylinders
Direct Cylinders were originally designed to be used in conjunction with solid fuel appliances with copper (or other non-ferrous) back boilers. Still available for the replacement market, these cylinders can be specified with either high-flow or low-flow direct boiler connections.
Today though, they are more commonly used with an electric immersion heater as their main heat source. As part of our bespoke service, immersion heaters can be positioned in the top of the cylinder and/or in the side of the cylinder. The difference this makes is that an immersion heater will only heat the water above the lowest point of its element.
As such, a top entry immersion heater will usually only heat the top 27” (685mm) of the cylinder. This may be perfectly adequate for the user’s hot water demand, but in a cylinder that’s taller than 685mm, there would be a proportion of water underneath the element that can’t be utilised.
A side entry immersion positioned at low level would give the user the option to utilise the full volume of the cylinder, if/when their hot water demand is higher. The same can be achieved by two side entry immersions at different heights – a higher-positioned one for lower demand and a lower-positioned one for higher demand.
Direct Cylinders can also use the dual immersion heater system to take advantage of lower-rate Economy 7 tariffs (or similar).
Direct Cylinders: When To Specify
Use a direct cylinder when:
- Budget is the primary concern (lower upfront cost)
- Heat source is electric immersion heater (simple setup)
- Customer has solid fuel backup (non-ferrous burner, can be direct)
- You need flexibility with element placement (Newark can customise top/side entry)
- You don’t anticipate future heat pump upgrade
Advantages:
- Simpler installation
- Lower cost
- Can customise element positioning for better water utilisation
- No coil maintenance required
Disadvantages:
- Shorter lifespan (immersion heater element degrades)
- Can’t support multiple heat sources
- Not suitable for heat pump integration
- Requires larger cylinder for higher demand (if top-entry element)
Indirect Cylinders
Indirects incorporate some form of hydraulic separation between the clean secondary water that they store, and the potentially contaminated primary water that heats them. This is often in the form of a coil – a helical tube that transfers heat from the primary water within it, to the secondary water around it, via conduction across the wall of the coil. It is also possible to achieve this heat transfer using an external plate heat exchanger (PHE) – we call this “indirect via PHE”.
The heat source for these types of cylinders is most commonly a pumped boiler, but can also be a gravity-circulating, solid fuel burner – more on this below. Other heat sources, such as solar thermal panels and heat pumps, can also provide heat to cylinders using the indirect method, but they require significantly larger coils, so different terminology is used to differentiate and avoid misunderstandings.
Indirect Cylinders: When To Specify
Use an indirect cylinder when:
- Heat source is a boiler (standard practice)
- Customer might upgrade to heat pump (already coil-ready)
- Installation includes solar thermal panels (need separate coil)
System needs multiple heat sources (solar + boiler + immersion backup) - Longer lifespan is priority (separated primary water = cleaner system)
Advantages:
- Longer lifespan (primary water isolated from domestic water)
- Ready for future heat pump integration
- Can incorporate multiple heat sources
- Better performance with advanced heating systems
- Coil design can be customised for specific heat sources
Disadvantages:
- Higher upfront cost
- More complex installation (piping)
- Requires more careful system design
Why Indirect Cylinders Are Heat Pump Ready
Heat pump systems require large coils to transfer heat from the heat pump’s low-temperature output efficiently.
Direct cylinders cannot accommodate heat pump integration without major retrofit work. Indirects are designed with coil capacity for heat pump systems, making them the only practical choice if a future heat pump upgrade is planned.
Additionally, indirect hot water tank can incorporate multiple coils (one for boiler, one for solar, one for heat pump), allowing complex heating systems with multiple energy sources. Direct cylinders, by design, cannot support multi-coil configurations.
Economy 7 Cylinders (AKA Maxistore)
Economy 7 (also commonly referred to as Maxistore) Cylinders are designed to take advantage of the various cheaper-rate night-time tariffs.
During nighttime hours, the water contained in the cylinder is fully heated by converting this cheap rate of electricity to heat energy via an immersion heater at the base of the cylinder.
Any additional hot water demand during the daytime is met by the boost of an immersion heater, which is situated higher up, thus only heating a smaller amount of water.
Gravity Cylinders
As mentioned above, Indirect models feature heat exchanger coils which accept heated primary water from an external, usually “on-demand” source of heat (like a gas boiler). The primary water provided by the heat source is usually pumped around the cylinder’s heat exchanger coil, but sometimes, the heat source isn’t quite so “on demand”, nor is it circulated in the same way.
The primary water, instead, circulates via “Gravity”/thermosyphon/convection/stratification – a natural rising of hot and falling of cold fluids. This slowly circulates the water between the heat source and the cylinder.
Heated primary water generated by Solid Fuel Burners is not typically available during the warmer months, so an immersion heater or other on-demand heat source may be required for those periods.
If a solid fuel appliance has a non-ferrous burner, it can be used like a Direct Cylinder, i.e. without the need for a coil.
Combination Cylinders (AKA Combi)
All vented cylinders require a feed & expansion (F&E) tank, in order to push hot water out of the taps and showers whenever they’re opened. F&E tanks are usually separate from their cylinders and are joined to it by pipework.
Combination cylinders, however, have their F&E tanks combined with their cylinders, i.e. built immediately on top. However, Combi cylinders are less common due to the reduced height between the F&E tank and the cylinder, negatively affecting the hot water’s flow rate. This is why F&E tanks will often be found at the highest point of a house – to maximise the hot water’s flow rate. But some properties don’t have loft spaces (or anywhere else) to site an F&E tank, so a combination cylinder is the next best solution.
Indirect and Direct Cylinders are both available as Combination types.
Frequently Asked Questions
What is the difference between a direct and indirect hot water cylinder?
A direct cylinder has heating elements inside the water you use for showers and taps. An indirect cylinder uses a coil to transfer heat from a separate water circuit (primary water) into your domestic hot water. This separation is what defines a cylinder as “indirect,” regardless of how many coils it contains or what heat sources it serves.
When should I use a direct cylinder?
Use direct cylinders for simple installations with electric immersion heaters, solid fuel backup, or when budget is the primary concern. They’re simpler to install, lower cost, and allow customisation of immersion heater placement. However, they have a shorter lifespan and can’t support multiple heat sources.
When should I use an indirect model?
Use indirect cylinders with boiler systems, solar thermal, heat pumps, or when you need multiple heat sources. The separated primary water gives longer lifespan and is essential for heat pump integration. They’re higher upfront cost but better long-term value.
Can Newark Cylinders customise direct or indirect cylinders?
Yes. All Newark Cylinders products are made-to-order. We can customise dimensions, coil positioning, connection configurations, and immersion heater placement to match your exact specification.
Is my cylinder ready for a future heat pump upgrade?
Direct cylinders are difficult to retrofit for heat pump integration. Indirect cylinders are designed for heat pump readiness and can typically accommodate larger coils. If you’re planning a future heat pump installation, indirect is the better choice.
All of these cylinders can be manufactured to your exact specifications, with custom coil configurations, connection positions, and dimensions to suit any installation. If you need something that doesn’t fit a standard template, we’ll build it.
Need a Custom Cylinder Solution?
Our technical team can design cylinders to your exact specifications. From prototypes to full production runs