Our experience with heat pump hot water

Or the long winded title: Our experience with a 350L Sanden heat pump hot water system, how to program block-out times to minimize energy with tariff 93 & solar panels.

This post is part of my Sustainable House Day submission.

Bear with me here… Conventional hot water systems use gas or electricity to heat the water. For electric models the resistive element is only 100% efficient. The specific heat capacity of water is 4,180J/kg.K, and to raise 1 litre of water by 1 degree takes 1.16 Watts, or for example, raising 40 litres of water by 50 degrees will need 2.3 kiloWatt-hours (kWh) of electricity. That may be your hot water heating for the day. Here’s a useful reference.

This doesn’t take into account heat losses through the insulation of the hot water cylinder or pipes to the hot water taps. That may double the amount of electricity needed!

The amazing point here is that a heat pump hot water system may be up to 400% efficient, so the electricity usage could be one quarter!

Now if you have solar panels, and aren’t using all of the energy generated during the day, it is worth diverting the excess power to heat your hot water. For Tasmanians you only get 8 cents/kWh exporting your power, but will pay 15 or 17 or 27 cents/kWh (depending on the tariff & time of day) importing power to heat your water.

Below is the daily energy to heat our hot water for the last 8 months. It ranges from 1kWh/day in summer to 1.6kWh/day in winter.

blue=kWh/day to heat our hot water, red=14 day average

Even on cloudy winter days a 5kW solar system can probably generate 1kW around the middle of the day, so an hour or two is sufficient to keep our water hot, without paying anything.

Then on the occasional days when the sun isn’t shining (Hobart is the 2nd sunniest capital), we import electricity at the off-peak rate of 15 cents/kWh by using tariff 93. See my post regarding this time-of-use tariff.

So the final piece of the energy minimization puzzle is to ensure your hot water is only heated during the middle of the day when

  • importing power is the cheapest, or better
  • your PV panels are generating the most power

For this you need a timer on the hot water system, or program the built-in timer. For our Sanden system its called the block-out time setting, and it’s rather fiddly to get right.

WARNING: turn off the power on your first attempt, or be really confident the fan isn’t going to turn on!

Firstly remove the heat pump top cover by unscrewing 4 screws:

The control panel is tucked away at one end. Undo the single machine screw shown:

The control panel is at the end of a wiring loom & tricky to wriggle out of the metalwork. In the photo below is slides to the left then upwards. Be patient:

finally accessible

There are many versions of the service manual on the internet, and the instructions in each are subtly different. Here’s the link for the manual for our model GAU-160FQS.

Here’s the programming flowchart:

First set the time-of day by this sequence:

  • press any button to enter Clock Display Mode
  • press the Up &/or Down button to set the time

Then set the block out time:

  • hold both the Up & Down buttons until the display changes
  • press the Down button until “bo” is displayed
  • hold the Enter button to change the time
  • press the Up &/or Down button to set the time to 1412 – this will set the time so heating can only occur between midday & 2pm; you may want a longer period if using more hot water

That’s it. Observe the compressor fan turns on at midday to check the setting is correct. And it should stop no later than 2pm.

The other non-technical but no less important thing to do is ensure there is insulation (pipe lagging) on all the pipes to minimize heat loss. For example:

Happy energy savings.

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