About 3-5% of new water heaters installed are the instantaneous or continuous flow type. They are typically used:

• for outlets that are a long way from the building’s main hot water storage system
• where hot water use is low and inconsistent (such as in a holiday home or very small household)
• to boost hot water supply from systems that can’t always meet demand (such as solar water heating)
• to boost hot water supply from systems with limited hot water storage, or to fittings having high hot water demand (e.g. showers).

Continuous flow systems are can be heated using electricity, gas or LPG.

Advantages and disadvantages

Advantages of continuous flow systems over storage systems include:

• they don’t run out of hot water, as they rely only on a cold water supply
• there’s no requirement to keep water hot when it’s not being used, which means there are no standing losses and no energy input to maintain the temperature of stored water
• they don’t require a cylinder, so they can be useful where space is limited
• they can heat directly to the required outlet temperature, without having to heat to 60C and then temper with cold water
• the temperature can be easily adjusted
• they can be located close to the hot water outlet.

Electric units may be more energy efficient than gas, but gas has a higher maximum heating capacity and is better able to impart sufficient heat to mains pressure water.

Drawbacks of these water heaters include:

• both are responsible for greenhouse gas emissions (electric units are responsible for emissions during generation, and gas units produce emissions during combustion)
• gas units have flue emissions
• electric units may not heat water sufficiently where water flow rates are high
• electric units may require heavy duty wiring and cannot use off peak electricity supply rates
• for gas heaters that are ignited by a pilot light, wind can extinguish the pilot light – specifying electronic ignition is recommended.

Calculating size

The required flow rate is used to size continuous flow systems, and this is based on the number of outlets served by the unit.  Obtain specific flow rates from the supplier or manufacturer. Flow rate recommendations for a gas continuous flow water heater from www.energysmart.com.au are:

  

Connecting with other water heating systems

Continuous flow water heaters can operate in conjunction with hot water storage systems, to use the advantages and overcome the disadvantages of each system.

One option is to have a smaller volume hot water storage cylinder for the general hot water supply, and a continuous flow water heater running off a cold water feed to provide supplementary heating for outlets that have high hot water demand (for example, showers and washing machines). The hot water supply should be mixed at the outlet, with the flow rate maximised from the continuous flow water heater and minimised from the hot water storage cylinder.

A second option is for the continuous flow water heater to act as the booster system to storage systems which have limited hot water capacity or intermittent energy supply (such as solar systems or heat pumps).

Installation requirements

LPG and reticulated gas heaters require good ventilation for the exhaust gases and for this reason are usually installed on the building exterior. Where installed within the home, the space must be well ventilated and exhaust gases flued to the outside.

Electrical heaters are compact and can be installed close to the hot water outlet e.g. often with cabinetry.

For your clients

To help your clients make decisions about water heating options, and to help them find ways of reducing their hot water bills, refer them to www.smarterhomes.org.nz/energy/water-heating and www.smarterhomes.org.nz/energy/solar-water-heating.