There’s lots to think about when considering high-efficiency hot water.
By Richard Keech, May 2018
Energy efficiency is using no more energy than you absolutely have to to achieve a good outcome. This is very true for domestic hot-water systems which typically account for between 1/5 and 1/4 of a home’s total energy needs.
I’ve written previously about hot water ‘Hot Water, more than meets the eye‘. The essence of this was:
- Hot-water systems generally use a simple thermostat to keep water above 60C for biosafety (ie killing Legionella bacteria);
- Regulations in Australia now allow clever regulation such that water is kept at a lower temperature (say 50C) for much of the time, with a periodic (say weekly) boost to higher temperature for sterilisation;
- Solar hot water system owners often boost manually, which means they probably fail to meet minimum biosafety requirements;
- Legionella can sometimes grow in cold-water pipes in hot weather.
So what next? How to reduce the energy you currently use heating your water?
If you currently have an old and inefficient hot-water system an you want to use less energy then the most obvious way is to replace it with a new and efficient unit. If this applies to you then read on.
Go electric. I’m a big fan of all-electric systems because the use of fossil gas is problematic as a fuel in our homes (see http://media.bze.org.au/nogas.pdf).
Northern Australia. If you’re in the north of Australia (say Brisbane and places further north) then a simple close-coupled solar hot-water system is probably the way to go since the sun’s energy is sufficient most of the time. Close-coupled systems have the tank and the solar collector together on the roof. They don’t need a pump to operate which helps make them reliable and simple.
Southern Australia. In cooler parts of Australia solar hot-water can’t get enough solar energy for a few months of the year so the winter boost-energy needs are large. On the other hand heat-pump hot water systems use much less energy all year round. They have the additional benefits of being easier to install, and they don’t compete with solar PV for valuable roof space.
Choosing a heat-pump system. The best unit on the market is the the Sanden Eco Plus, although it is not cheap. I like it because:
- The thermal performance is the best available;
- It’s reliable Japanese-made with a quality Australian-made stainless tank;
- It uses a refrigerant with a very low global-warming potential;
- It performs extremely well in cold weather; and
- It’s amazingly quiet.
Sanden Eco Plus heat-pump hot-water system (image courtesy of Sanden)
Sizing. When replacing your system, consider how big it has to be. The larger the tank, the more energy you need just keeping a volume of water at the set temperature. This is called the ‘standing losses’. So be sure to get a system no larger than you absolutely need. A factor here is ‘recovery time’ – the time required to warm up the water if you’ve run out. If a system has a good (ie short) recovery time then you can, in theory, cope with a much smaller tank since the boosting from an empty tank can be quick enough that you don’t need to ‘bank’ a lot of hot water against the chance you’ll run out.
If you don’t want to rip out your current system, what can you do to make it use less energy? Also these measures might still apply if you replace your system.
Use less water. The less hot water you use, the less energy you need. So very-low-flow shower heads are a good way to save. I’ve heard very good things about the shower units from Methven (https://www.methven.com/au/showering/) available from the ATA here.
Methven Kiri low-flow shower head
Water temperature invariably drops between the tank and the outlet. For most homes this temperature drop is more than it needs to be because there is inadequate pipe insulation, aka ‘lagging’. There are three main considerations:
- If there is less heat lost in the pipe then you’ll need less hot water in a water mixing situation like a shower, so less energy needed;
- If there is less heat lost in the pipe then you can possibly regulate the tank at a slightly lower temperature to achieve the same delivery temperature (so long as the unit allows for it and biosafety needs are satisfied);
- If heat can be retained in the pipes long enough, then you’ve got almost instant hot water for second and subsequent use from the same (or nearby) outlet, within obvious limits.
In Australia our regulations (AS/NZS 3500.4) require pipe insulation as follows:
|Region A (hot)||Region B (temperate)||Region C (cool), excluding alpine||Region C (cool) alpine|
|External||R0.3||R0.6||R0.6 or R1.0 (depending on length)||R1.0|
The Regions (A,B,C) in the standard correspond to climate zones as follows:
|B||4,6||Melbourne, Mildura, Albury, Dubbo, Kalgoolie|
|C||7||Canberra, Hobart, Ballarat|
To get this level of insulation you’ll need the thickness given as follows:
|R value||thickness of closed-cell foam (mm)|
Suitable products are Armaflex and K-Flex from a plumbing supplies or good hardware store.
The pressure release valve on your tank should also be insulated. A good product for this is the Valve Cosy available from the ATA here.
Further reading and references
Wright, M., 2015, ‘Slash your hot water bill by 86%’, The Australian, https://www.theaustralian.com.au/business/business-spectator/slash-your-hot-water-bill-by-86/news-story/707a96bfaefd13d941038d6629a690be