Posted by: @willcb2023

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Hi everyone. I have had a Grant ASHP installed (17Kw R32 Aerona) retrofitted onto my gas boiler for controls, with a 300 litre water tank. I also have solar PV which provides up to 2 KW (but generally a bit lower). No batteries installed.

If I had an Iboost installed, I would get hot water provided from the solar panels, but this is not possible due to the legionnaire override switch using the immersion (and having no spare immersion slot on the tank).

However the heating runs about 8kw over a 2 hour period on a daily basis, and so the solar panels cannot cover the usage. Is there any way to slow the heat pump down to better utilise the solar power provided and get an equivalent result from an Iboost type solution?

One possible way to maximise the output from your solar PV and minimise your heat pump usage would be to install an interposing relay or contactor.

When the relay/contactor is de-energised, the immersion heater would be connected to the output from a power diverter, being fed from your solar PV system, thereby making use of any available solar PV output.

If there is insufficient solar PV energy available, the heat pump controller could then be used to energise the relay/contactor to power the immersion heater directly from the mains supply. Though it would be more efficient and cost effective to run the heat pump in this situation.

Alternatively it may be possible that the proposed Iboost actually has a boost capability, that allows it to supply the immersion heater at full power from the mains supply. This should only be used when absolutely necessary.

When you state that your heat pump is using 8kWh each day, is that just for DHW heating? Is the heat pump actually running or just powering the immersion heater?

What size is your PV system?

Unless you use vast amounts of hot water it would probably be most cost effective to heat the water from solar PV via a power diverter. When there is insufficient solar PV then running the heat pump should be the next most cost effective. Running the immersion heater direct from the mains supply should only be used when absolutely necessary.

@derek-m The heatpump is called by the Hot Water tank between 12pm-2pm each day and runs (immersion heater is only used once weekly for a legionnaire cycle to bring the water to 60 degrees C). The PV system is 10 panels I assume each with capacity 250W (It was already installed (2015) when I moved in, in 2023). I hope that that explains, I am new to all this.

@willcb2023 I have a solar Iboost which heats my hot water from the electricity produced from my PV panels. It is connected directly to the immersion heater in my DHW tank. There is no connection to the immersion heater from my Samsung ASHP as this was disconnected when the iBoost was fitted. There is no need for a legionella cycle during the summer as the solar PV electricity heats the water to over 60°. During winter the solar PV electricity doesn’t get the temperature up as high. If I want a legionella cycle, I top up the water temperature to 55° with the heat pump and then use the Iboost to heat it up to 60° using electricity from the grid.

I see no reason why this should not work for you in the same way.

Posted by: @willcb2023

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@derek-m The heatpump is called by the Hot Water tank between 12pm-2pm each day and runs (immersion heater is only used once weekly for a legionnaire cycle to bring the water to 60 degrees C). The PV system is 10 panels I assume each with capacity 250W (It was already installed (2015) when I moved in, in 2023). I hope that that explains, I am new to all this.

If you are considering having a power diverter installed, I would suggest that you select one with a scheduled boost facility. That could then be used to perform the legionella cycle.

To minimise power usage by your heat pump then lower the desired DHW temperature to the lowest setting that meets your needs.

If you do install a power diverter then change the schedule on your heat pump to 2pm to 4pm, which is often the warmest part of the day, so the heat pump should be more efficient or may not be needed at all if the DHW is hot from the solar PV. 

Not sure what facilities the IBoost unit offers; the MyEnergi Eddi has such facilities that you can have scheduled boost and they also have a optional relay board (I have one in mine) that can monitor water temperature via one of two temperature probes that can be attached to the relay board. The system can be programmed to do all manner of functions this way. I use the Eddi’s main circuit to supply PV / Grid power to the Sunamp Thermino and the relay board has been programmed to provide grid power in the evenings to a dual fuel towel rail. Regards, Toodles.