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Direct & Indirect Cylinders
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 cylinder, 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
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). More on these types of Direct Cylinders below.
Indirect Cylinders
Indirect Cylinders 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.
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.
SlimFit Cylinders
A modernised version of the conventional Direct/Indirect Vented Copper Cylinders comes in the form of SlimFit Cylinders. These variations allow for a wider variety of sizes to fit in narrower spaces.
Caused by the requirement of thicker insulation from the 2010 update of Part L of the Building Regulations, the update aimed to reduce the amount of day-to-day heat loss from hot water cylinders. In turn, this would help reduce gas/oil use and CO₂ emissions.
As part of our SlimFit Cylinder design, we introduced a multi-bore high recovery coil, designed to heat the unit faster than a traditional coil. With the addition of an aqua control thermostat, the heat source could run for less time, preventing the burning of considerable amounts of fuel each year.
Inevitably, thicker insulation results in a larger diameter, and with the restrictions that some airing cupboards impose, we believed that homeowners would prefer an increase in height, rather than in diameter. This resulted in the development of a new slimline cylinder specification. These taller, thinner cylinders, with thicker insulation, retain the minimum volume of water that a household needs, yet isn’t any wider than their old cylinder.
Furthermore, this concept was expanded with the ‘SlimFit Ultra’ model. This ultra slim cylinder has a total diameter of 370mm, with a maximum volume of 130 litres.
Gravity Cylinders
As mentioned above, Indirect Cylinders 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 a F&E tank, so a combination cylinder is the next best solution.
Direct and Indirect Cylinders are both available as Combination types.
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.