Interesting to see how Trystan’s system (with coils) performs compared to my system (heat exchange) when the temperature of the tank is warmer than the starting temperature of the ASHP loop.
With my system, the return temperature is actually higher than the flow temperature for the first few minutes, as ASHP circuit steals (borrows) heat from the cylinder until the compressor takes over.
The overall COP (2.8) is lower than if the bottom of the tank starts colder (3.6) (excluding standby).
- July 27th, 20°C ambient, water heated to 52.5 °C, 0.9 kWh electricity => 2.5 kWh heat
- July 28th, 20°C ambient, water heated to 47.5 °C, 1.4 kWh electricity => 5.0 kWh heat
This phenomenon is discussed in the thread Improve efficiency: Heat the hot water when it’s cold.
I might tweak my system so that it doesn’t try heating DHW at all if the bottom of the tank is still warm…
Do you see the same thing happening with Mixergy?