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Right House - A new approach to comfortable, healthy and energy efficient homes

Water Heating

Friday, September 22, 2006
Water heating accounts for about 30-35% of a home’s energy consumption (electricity and gas). Large reductions in hot water energy use are feasible by improving the thermal performance of the hot water system.
Unless a cylinder is relatively new and has an ‘A-grade’ sticker, it is likely to be under insulated.

A cylinder wrap provides an easy, inexpensive solution with a one- to two-year payback period. (The use of a wrap precludes the traditional use of the hot water cupboard as an airing cupboard. And while it is more energy efficient to use a clothes dryer for a few minutes than to allow a constant leakage of warmth from the cylinder, it’s even more eco-friendly to dry your clothes outdoors, or under shelter.)

Lagging the hot water pipes with a neoprene sleeve also saves heat loss - wrap the entire pipe, not just the first metre out of the cylinder. Where possible, the cylinder should be close to utility areas such as the kitchen, laundry and bathroom to reduce the volume of standing hot water in the pipes.

Water Temperature

The water stored in a hot water cylinder needs to be hot enough that the cylinder doesn’t become a breeding ground for Legionella bacteria (no less than 60deg celcius), yet not so hot that it scolds the skin at the point of delivery. Water at 60deg celcius takes about one second to scold the skin of a child.

For this reason, regulations require that hot water for personal hygiene in day-care centres, schools, nursing and retirement homes and similar institutions, does not exceed 45deg celcius at the point of delivery e.g. shower, hand basin, bath.

For personal hygiene in other buildings, such as domestic residences, the water temperature at the point of delivery must not exceed 55deg C (though it may be delivered to kitchen sinks and laundry facilities at between 55deg celcius-65deg celcius).

If a tank thermostat is set to heat water to 60deg celcius - hot enough to prevent Legionella propagation and for delivery to sinks and laundry fixtures - temperature regulation of the hot water supply to the personal hygiene sanitary fixtures must be provided by way of an in-line tempering valve set to either 45deg celcius or 55deg celcius, depending on the use of the building.

Tempering Valves

Hot water cylinders heated by supplementary means such as solar panels or fireplace wetbacks can reach temperatures in excess of the safe levels normally controlled by thermostat devices. In these situations a tempering valve between the point of use and the tank reduces the risk of scolding at the point of delivery, but maximises the efficient storage of hot water in the cylinder.

Size plays a role here too: a 135-litre cylinder storing water at 75deg C holds the same amount of heat energy as a 180-litre cylinder with water stored at 55deg C. The bigger cylinder with its lower water temperature is a safer option for users.

Turning the cylinder off is only an energy saving if you are away for more than two weeks.

Demand Or ‘Tankless’ Heaters

Instantaneous or ‘demand’ hot water systems are modern versions of the old fashioned (and rather scary) gas caliphont water heating devices which perched over the cast iron baths in many New Zealand homes and exploded into life when the hot tap was turned on.

By applying heat at the point of use demand heaters avoid the need for a hot water tank. They have the ability to deliver an endless supply of hot water, but at a limited flow rate. Where the demand for hot water is extensive, or demand occurs at several fixtures simultaneously, more than one unit may need to be installed. Demand heaters can also promote lengthy showers, simply because the hot water doesn’t run out.

Demand heaters can be electric or gas operated. Electronic ignition eliminates the need for a pilot light in gas models and digital remote controllers allow an exact water temperature to be selected without the addition of cold water.

The units are compact and generally externally mounted. Gas models use natural or bottled LPG gas, both of which are an unsustainable source of energy.
 
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