Reply To: Heatpump Hotwater Cylinder Programming

June 10, 2020 at 5:12 pm #7832

Participant

Hi Virginia

How to operate a Heat Pump Water Heating optimally is somewhat of a complex problem and I’m not really happy with the current info available. At BRANZ we are just kicking off a research project looking at innovative ways of heating water (largely thru PV but will include a CO2 hot water heat pump) which will take a few years to run.

I think the first contact should be the supplier of the heat pump. What is their recommendation about how it should be used efficiently? What performance levels do they say you expect? Unfortunately many products are sold based on idealised performance (high water draw-off when the outdoor unit is at 7°C ) that isn’t achieved in use (the boost element wouldn’t come on in this case). I’ve seen one heat pump wh unit that a retired couple were using (low water use) that used more energy that would be required if they just used an electric cylinder. Heat pump water heaters are not the best solution for a household with only a low water heating requirement (1-2 occupants).

Product suppliers are generally not involved in the ongoing energy performance of their product. If the HPWH doesn’t leak and it delivers hot water then it is seen as successful. A related example from electric water heating is where many hot water cylinders are now preset at the factory to 70°C This is not efficient but reduces the likelihood of call backs to the home if the occupants run out of hot water (cylinder was sized incorrectly).

I had a quick look at the model you were working with. This unit looks a bit different from the other common case units and is more like a split system with the heat pump unit delivering heated water to the cylinder. The unit also has ‘boost’ element which is a resistive heating element and seems to have a ‘one-shot’ control (to heat water more quickly) as well as automatic use for cold/frosting conditions . Many other models of HPWH (especially those for use in the North Island Zone1/2) do not have these sorts of elements in them and rely on the heat pump heating. From an efficiency point of view you really want to minimise the use of this ‘boost’ element. The manual describes the programming (I didn’t read it) but doesn’t really suggest how to control it for efficiency – the frost protection just seems to switch on the electric boost when the temperature is below 0°C To avoid boosting ,the option would therefore be to ensure that the cylinder is above the thermostat temperature when the outdoor temperature is below 0°C. The heat pump is quite slow to heat so may need to run for some time to get to temperature but operating at cold times would reduce the efficiency of the system so its not clear how good this solution is.

Traditional refrigerant heat pumps can struggle with getting a high temperature lift from the cold water to the hot water. Many units struggle to get to 60°C for Legionella control and there are various ways of controlling the return water (recirculating)

Keeping an eye on how much electricity the HPWH and varying how you are operating the system may give some guidance as to how best operate the system

Cheers
Andrew