TECHNICAL THEME
Wetback cylinders David Waite of Rheem outlines the differences between copper and stainless steel wetback cylinders. Copper cylinders
The traditional way to store water heated from a wetback fireplace is using a low pressure storage water heater. This wetback copper hot water cylinder has extra connections, allowing a direct feed from the wetback firebox heat exchanger to the cylinder via flow and return pipes. The gradient on these pipes is important, so G12 and the manufacturer’s instructions must be followed to ensure safety and best performance (see Figure 1).
Above: Figure 1—Low pressure wetback layout.
Stainless steel coiled cylinders
Stainless steel mains pressure cylinders suitable for wetbacks are gaining in popularity (see Figure 2). This type of cylinder is fitted with an internal heating coil. The water heated by the wetback fireplace is circulated through the coil in the cylinder rather than directly into the cylinder. The coil circuit must be open vented because the water in this circuit is being heated by an uncontrolled heat source. The open vent needs to be located from the highest point of the coil, and a header tank or pressure reducing valve must be used to replace the water lost in the coiled system. The water inside the cylinder can operate at normal mains pressure, like a standard mains pressure cylinder. The coil design is important so that the hot water from the fireplace can naturally thermosiphon without the aid of a pump, and stratify in the cylinder. As with low pressure wetback cylinders, it is important to follow the manufacturer’s instructions. With a mains pressure wetback system, the potable water and water in the coil never mix. However, because of the low volume of water in the coil and circuit, the maximum wetback heater output is limited to 3kW. The stainless coiled wetback cylinder has many advantages, including being able to deliver mains pressure water at the outlets, and can be placed higher in the system to get better flow and return from the wetback fire, as the pressure at the tap is not reliant on the open vent low pressure system or header tank.
Inserted inside the cylinder is an inlet copper riser extending almost to the top of the cylinder. This assists with stratification of the hot and cold water. The size of the riser is determined by the size of the cylinder. Just like standard low pressure cylinders, a header tank or a pressure reducing valve must be used to reduce the mains pressure to standard low pressure (76kPa). However, a wetback copper cylinder must be open vented as it is supplied by an uncontrolled heat source, which is directly heating the potable water in the cylinder.
a wetback copper cylinder must be open vented as it is supplied by an uncontrolled heat source
Above: Figure 2—Pressure cylinder wetback (valve tank supply) layout.
April/May 2018
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