Direct & Indirect Cylinders

When discussing Hot Water Cylinders, the terms “Direct” and “Indirect” are used to describe whether a Cylinder’s heat sources are hydraulically separated from the stored secondary water. If any of them are separated, the Cylinder can be described as “Indirect”, and this separation is usually provided in the form of a coil or other heat exchanger.

Direct and Indirect Cylinders can be either Vented or Unvented, but on this page, we are only looking at the Vented variation.

Direct Cylinders

Direct Cylinders were originally designed to be used in conjunction with solid fuel appliances with copper (non-ferrous) back boilers, but today they are more commonly used with an electric immersion heater as their main heat source. Direct Cylinders can have two immersion heaters to take advantage of lower rate Economy 7 tariffs (or similar).

Still available for the replacement market, these Cylinders can be specified with either high-flow or low-flow Direct boiler connections.

A Direct Cylinder diagram.
An Indirect Cylinder diagram.

Indirect Cylinders

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.

The coil provides necessary hydraulic separation, as the primary water within the coil would otherwise contaminate the potable secondary water stored within the Indirect Hot Water Cylinder’s body.

Economy 7 Cylinders

Economy 7 (also known as Maxistore) Cylinders and combination tanks 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 in the Economy 7 hot water Cylinder.

Economy 7 Cylinder example drawing

Slimfit Cylinders

Slimfit Cylinder Diagram

Although we had been manufacturing slimmer Cylinders for many years, the 2010 update of Part L of the Building Regulations brought with it the requirement for Cylinders to be supplied with thicker insulation, even for replacement Cylinders. We knew, however, that this would mean many homeowners would find difficulty in replacing Cylinders which reside in tight ariring cupboards. This prompted us to develop a new range of ‘SlimFit’ Cylinders.

The main aim of the Part L update was to greatly reduce the amount of day-to-day heat loss from Hot Water Cylinders, helping to reduce CO2 emissions in the longer term. Another key benefit we introduced into these units as standard is a multi-bore high recovery coil, designed to heat the unit faster than a traditional coil. In doing so (and with the addition of an aqua control thermostat), a quicker heat-up time is achieved and a saving of fuel is attained.

Since October 2010, all replacement vented Cylinders have been required to have a heat loss rating so significantly reduced, that we have since had to apply a nominal thickness of 40 to 50mm (as we have always done on our solar Cylinders, and heat pump Cylinders), 50mm being more than the requirements for replacements and the minimum thickness for new build installations. Inevitably, thicker insulation results in a larger diameter and with the restrictions that some airing cupboards impose, we believe that it may be preferable for the increase in the overall physical size of the unit to be taken up in the height, rather than the diameter.

Indirect (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 – which 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 though, 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

combi combination cylinder diagram

Combination Cylinders (also known as Combis) are intended to save space by incorporating the feed and expansion tank into the top of the unit. This removes the need for a separate header tank, which is useful where there is little to no loft space. The external tank is pre-installed on the unit and connected with the cold feed and vent pipes, further reducing installation time.

Although the unit sacrifices some height that could otherwise be used for extra hot water storage, we can manufacture combination Cylinders up to 600mm in diameter, allowing for a hot water storage volume which is sufficient for most domestic properties.

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