Which type and size of hot water system is best for your home

Providing hot water accounts for around 30 per cent of the total energy costs of a typical New Zealand household – so it's important to consider how much heated water you need and what energy sources you have available.

Dulux hot water systems.
ARTICLE Nick Paris

Providing hot water accounts for around 30 per cent of the total energy costs of a typical New Zealand household – so it’s important to consider how much heated water you need and what energy sources you have available.
We’re often guilty of taking hot water for granted, until that morning when we discover the hot water element has blown, or the cylinder has developed a leak. When this happens, it is a good opportunity to take the time to discuss with your plumber the hot water options available to you. If you are renovating, then now is the time to start thinking about your hot water needs and expectations.
To decide which type and size of hot water system is best for your home, you first need to think about which criteria are important to you and your family. These include the size of your household, where you live both in terms of climate and your available water heating energy source (solar, natural gas, LPG or electricity), your budget and where the system will be located (inside or outside). 

Electric storage water heaters

Storage hot water heaters are the most common type of water heater in New Zealand. Until around the 1960’s, low pressure hot water installations used a simple gravity feed header tank located in the ceiling to supply cold water to the cylinder at low pressure. Over time, the header tank has been replaced with modern pressure valves that deliver water to the cylinder at a low pressure.
When renovating, low pressure electric cylinders are generally upgraded to mains pressure cylinders (a Building Consent is required for this). The latter offer several advantages including reduced pressure or temperature spikes when another tap is turned on and compatibility with most modern tap mixers, which are designed for mains pressure only. Mains pressure cylinders typically need to have a 9L shower rose installed to prevent the cylinder from running out of hot water too quickly.
Originally, hot water cylinders were not well insulated and therefore radiated significant heat. To make good use of this heat, the cylinder was located in the lining cupboard and used to dry out damp washing. Today, advances in foam insulating technology mean that very little heat is lost through the outer casing of a hot water cylinder, resulting in a more energy efficient storage cylinder. Many renovators are relocating the cylinder to the outside to free up the cupboard space. 

Electric hot water heat pumps

Utilising the same technology as a heat pump uses for warming a room, a water heat pump transfers heat through a heat exchanger, within the inside of the tank. The use of energy is very efficient compared to that of an electric water heater, with the expansion and contraction of refrigerant gas removing energy from the air and transferring it to water.
For a heat pump water heater to work correctly, the heat pump compressor must be installed outside. Many modern systems have the cylinder and compressor combined as one unit. The disadvantage of a heat pump water heater is that the colder the outside temperature, the slower the recovery time of the heater. To overcome this, heat pumps are generally 250L or larger in capacity to ensure plenty of water is on hand.

Gas continuous flow water heaters

Never run out of hot water! Gas continuous water heaters deliver hot water on demand by passing water through a highly efficient heat exchanger that is activated as soon as a hot water tap is turned on. Modern gas continuous water heaters are technically advanced in design with electronically controlled gas and water control so that only the right amount of energy is used to heat the water.
Continuous water heaters are typically installed on an outside wall, in close proximity to a kitchen or bathroom. The water that is heated is pre-set to 50°C, and the range of unit sizes are based on the ability for the unit to heat the inlet cold water to this pre-set temperature. Typical sizes of gas continuous flow heaters are 16L/min, 20L/min, 24L/min and 27L/min. The ability of these types of heaters to deliver water at the maximum rate is governed by the temperature of the cold water inlet.

Solar storage water heaters

If you are looking at reducing your carbon footprint, then a solar water heater is the best option. To provide hot water on cloudy days or in times of high hot water demand, solar water heaters come with either a gas or electric booster. If you live in a home that does not have suitable roof space and orientation for the installation of solar collectors, then a heat pump water heater is the next best option if you are looking to replace an electric storage heater with a more energy efficient alternative. 

If you have reticulated mains natural gas available to your property, then a high efficiency gas storage or continuous flow water heater can be a good option, providing good efficiency and a simple and flexible installation for your plumber.
When deciding whether to have electric, gas, solar to a heat pump, consider the benefits of investing in an energy efficient water heater that will delivery hot water at an affordable price. When it comes to low cost or free energy, you can’t look beyond solar or a heat pump for a long term solution.

Hot Tips

1. Look to free up your hot water cupboard space and specify a hot water cylinder designed to go outside.
2. Get your plumber to correctly select the right cylinder for you.
3. Ensure the copper hot water pipes are insulated as this will help reduce your water heating cost.
4. Keep running out of hot water? Install a modern 6L or 9L per minute shower rose.
5. Ensure that your hot water cylinder is well insulated or wrapped up in a cylinder wrap.
6. If you have a continuous flow water heater, enquire about having temperature controllers installed in your bathroom and kitchen. Your unit will then only heat water to the temperature you need, not the pre-set 50°C.
7. Check out the energy star rating of the continuous flow unit and compare it to others. The more stars, the more energy efficient.
8. Pre-set the thermostat of your hot water cylinder to 65°C to kill any Legionella bacteria, which can flourish in a hot water cylinder if the water temperature is too low. Install a tempering valve to prevent a hot water burn to your skin.
9. If you have children or elderly relatives living with you, check your water temperature in the bathroom. If it is higher than 55°C, have a tempering valve installed. You can purchase a 45°C tempering valve that is set to 45°C for greater protection within the home.
10. Make sure that your hot water cylinder is firmly held in place. Every hot water cylinder must have at least two seismic straps firmly fixed into sold timber to prevent toppling.

You might be interested in reading: A healthy home hot water systems in your home.

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This article by Nick Paris featured Issue 004 of New Zealand Renovate Magazine. New Zealand's first and only magazine solely dedicated to home renovations.


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*All information is believed to be true at time of publishing and is subject to change.

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