Domestic Hot Water Heating

There are two ways in which a warm water central heating system can service a domestic hot water supply:

■ Combi boilers are designed to produce sufficient quantities of hot water to service the central heating system and produce instantaneous hot water for domestic use. In the case of storage combi boilers, the domestic hot water cylinder is incorporated into the boiler unit itself.

■ In system boilers, the water pumped around the central heating system can be used to heat the domestic hot water indirectly. It is diverted from the radiators or underfloor pipes and into an internal coil within a cylinder. The coil is heated by the water passing through it, and this heat is in turn transferred to the water stored in the cylinder. These cylinders are known as indirectly heated mains pressure cylinders. The domestic hot water is never in direct contact with the water used for central heating.

Indirect mains pressure hot water cylinders
 - supreme efficiency and economy!

No instantaneous water heater or standard hot water cylinder can match the performance of an indirect mains pressure hot water cylinder. A standard hot water cylinder employs a 3kW electric element, while an indirect hot water cylinder uses an internal coil to transfer 26 kW of boiler heat to the water. The reheat time for a 210 litre indirect hot water cylinder is just 20 minutes!

Because the performance of an indirect mains pressure cylinder is eight times greater than a standard electric cylinder, much smaller cylinders can be used. A 210 litre Baxi hot water cylinder is suitable for high demand applications such as multiple-bathroom households with five or more adult occupiers. A 300 litre cylinder is typically used in very high demand and commercial applications.

In a typical household, the boiler would heat the domestic hot water for an hour in the morning and an hour in the evening. These heating times and the stored water temperature can be adjusted easily to suit any household or lifestyle.

Baxi Hot Water Cylinders

Central Heating New Zealand distributes the Baxi brand hot water cylinders. They are made of duplex stainless steel which offers superor resistance to corrosion, especially in aggressive water areas, and without the need of a sacrificial anode. Coupled with Baxi’s state of the art automated welding techniques and post weld processes, Duplex ensures the longest possible working life for every Premier Plus water heater.

Features of Baxi Hot Water Cylinders:
■ Optional 3kW electric element
■ Unique ‘coil in coil’ heat exchanger
■ 50mm thick insulation means minimal heat loss
■ Indirect thermostat and over temperature thermal cut-out for boiler connection and control.

Ring Main Plumbing

Ring main plumbing is used extensively in commercial applications, but its benefits for the domestic market in New Zealand have been undersold. It is commonplace in homes overseas. It provides instant hot water, at every tap, at any time of the day – suiting the unique requirements of the occupants.

A flow and return line is plumbed into the house close to the bathrooms and sink taps. It is completely lagged, and has a small electric circulating pump in line which transfers water from the cylinder, around the “ring”, and back again. The pump can be controlled by a timer to cover the three typically busy periods of the day, or by a motion sensor in the bathrooms. Each “leg” from the ring to a basin requires a tempering valve, so that the ring can circulate at the cylinder stored temperature of at least 65°C.

The energy and water savings that result from not running off cold water while waiting for the hot to arrive, are significant. Typically between one and five litres of run-off occurs every time a hot tap is used. 
 

Hot water cylinder versus instantaneous water heater

A hot water cylinder will always provide a much higher water flow than an instantaneous water heater. They are ideal for the rapid filling of baths or simultaneous use of multiple showers.

An instantaneous water heater works by raising the temperature of incoming water by a certain number of degrees. The outflow rate is good if the incoming water is not too cold. The lower the incoming water temperature, the slower the flow rate in order to produce the desired water temperature.

Most instantaneous water heaters, being combustion appliances, are installed outside. Flueing is very simple. However, combination gas boilers are installed indoors and so their position must take into account the flue and pressure relief requirements. These boilers draw air for combustion from outside, so no oxygen is depleted from the home.

Some storage water heaters are positioned outside. Our recommendation is to install a storage cylinder inside the house, as it is more efficient to store water within a 20°C home environment than outdoors.

In general we recommend a combi boiler for a single bathroom dwelling, and a storage combi boiler for a two bathroom dwelling. For high demand, multi-bathroom dwellings we recommend a 200 or 300 litre storage cylinder with ring main plumbing.

Find out about Solar Hot Water Heating.

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