Q&A: Instantaneous hot water

We live in bayside Brisbane in a four-bedroom, two-bathroom house with an old-style 250-litre electric hot water system. We are investigating which way to go if or when the system needs replacing. It is on the south side of the house and it does not appear to me to be a straightforward job to change to a solar hot water system. It is also near the main bedroom and I understand there can be noise issues with a heat pump system.

We installed a 1.5 kW PV system three years ago and have happily paid no electricity bills for that period and are $700 in credit. I have looked at Stiebel instantaneous electric hot water systems which, at the top of the range, seem to be appropriate for two-bathroom apartments and can cost around $1500.

The technology looks smart and I cannot see any regulatory objection to this option in Queensland. Are there any practical or plumbing issues in making this replacement? I am not a tech tragic—I would just like an opinion as to the appropriateness of instantaneous electric hot water for a house.
Ray Barker

The main issue with a whole-house electric instantaneous system is that it needs a 3-phase power supply, as power draw may be up to 20 kW. Many houses have these now but many don’t. So whether you can install one depends on that. Bear in mind that, while energy use will be less than with an electric resistive storage system as there are almost no standing losses, it will still use a lot more energy than a heat pump or solar unit.

There’s no reason why you can’t install a solar unit. You would use a split system with the tank in the same place as the current tank and the panels on the roof on a tilt frame to overcome the south-tilted roof.

As for heat pumps, most are pretty quiet but noise ratings are given in the specs. You can also put them on a timer so that they don’t run overnight, instead kicking in at, say, 7 am. The most efficient domestic unit on the market seems to be the Sanden, which uses CO2 as the refrigerant. See www.sanden-hot-water.com.au
Lance Turner

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