Combining Domestic Hot Water Systems with Central Heating

A Unique and Efficient Heating Combination

Unlike many other methods of heating your home, our central heating systems are able to meet not only its space heating requirements, but also its entire hot water supply for baths, showers and the like.

Baxi combination ("combi") boiler

Baxi combination boiler

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 (suitable for homes with up to two bathrooms).
  • 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.

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.

Hot Water Cylinders vs Instantaneous Hot Water Systems

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. 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.

Baxi hot water cylinders

Baxi hot water cylinders

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 domrequirements 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.

Domestic hot water can be heated in one of two ways: a hot water cylinder or instantaneous hot water, each of which offers its own unique benefits depending on your living situation.

Domestic hot water for everyone in your family.

Domestic hot water for everyone in your family.

A hot water cylinder will always provide a much higher water flow than an instantaneous water heater and is consequently ideal for the rapid filling of baths or simultaneous use of multiple showers. Central Heating New Zealand uses indirect hot water cylinders exclusively due to their rapid heating time—just 20 minutes for a 210 litre cylinder—and the separation of the heating water flowing through your central heating system and the heated water coming out of your tap.

Because an indirect mains pressure cylinder is eight times more powerful than a standard electric cylinder when connected to a boiler, much smaller cylinders can be used. 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. 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, where we offer cylinders up to 2000 litres.

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.

If space is a concern for you, or if you do not use the hot water regularly as for a vacation home or similar, instantaneous heating can be a great alternative to a hot water cylinder. As part of a combination gas boiler system, an instantaneous water heater works by raising the temperature of incoming water by a certain number of degrees without requiring the temperature upkeep of a hot water cylinder, which requires additional energy to maintain its heat. It should be noted that the lower the incoming water temperature in an instantaneous hot water heater, the slower the flow rate will be in order to produce the desired water temperature.

Most instantaneous water heaters, being combustion appliances, are installed outside, and flueing is very simple. However, combination gas boilers are installed indoors and we position them for ideal functionality, taking into account the flue and pressure relief requirements. These boilers draw air for combustion from outside, so no oxygen is depleted from the home.

In general for domestic hot water, 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.

To find out more, download our brochure on our range of Hot Water Cylinders.

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