Waste Water Heat Recovery vs. COP

Asked this over on FB heat pump group but would be interested to hear the take on it here too.

Would the addition of WWHR onto the shower improve the performance/COP of DHW or would it actually do the opposite as any excess preheated water going back into the tank would increase the temperature and mean the pump would run less.

Was thinking about retrofitting something like this (purely for the eco benefits, I realise this is not going to suddenly save me £££££) - Recoup Pipe HEX - Vertical waste water heat recovery for showers

I realise this will obviously reduce the cost as there will be less water to be heated and would be more energy efficient but wanted others take on the idea.

Does it not depend on whether the preheated water goes only to the shower (B) or the cylinder (C)?

Installation - Installation methods for WWHRS - System A, B or C

I was thinking more of a option A install as this is supposed to be the most efficient (from a water usage perspective).

My guess is that the bottom of the tank would become warmer, and COP would be slightly worse as a result, but overall would require less energy to reheat the cylinder.

Or, depending on the heating strategy and coil configuration, it takes longer for the bottom of the tank to get cold, and the middle of the tank is also cooler, so that heat pump runs less often and gets better COP.

If the warm inlet water comes into the tank slowly, then it might rise above the existing cold water rather than mixing with it.

So really, I’ve no idea! :laughing:

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You’ll reduce the hot water heating COP but you’ll reduce the energy input needed too.

Say sCOP 4.5 now. Say the drain water gives 25% heat recovery.

COP 4.5 for 45 litres means 20 litres of electricity for 90 litres of hot water. If you get 22.5 litres of hot water for free and then a (slightly reduced because of the preheat) COP 4 for the other 67.5 litres that means 15 litres of electricity used. 15 litres of electricity into 90 litres of hot water gives an effective COP of 6 for the whole system.