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For the best efficiency you should be aiming for 45 degrees. HPs are pretty inefficient at anything over 50 degrees. The newer models are engineered to produce higher temps (I guess to mimic gas boilers so reduce the need to modify existing rads, and user behaviour) but they’re still more efficient the lower you go.
For hot water that means you need a bigger cylinder as much more hot water is being used. Instead of mixing 60 degree water with cold to shower at 40 degrees, it’s almost all coming from the cylinder at 45. That just seems like a no brainier anyway, why heat water to a higher temp than you need only to dilute it. Additionally, a tank at 60 degrees looses much more energy per hour than a tank at 45.
ASHP controllers have a disinfection cycle built in where the HP takes the cylinder up to ~55 then the immersion kicks in to hold it at 60 for ~20 mins. Ours is one a week but if I could, I’d set it to once a month. The risk of legionella’s is actually pretty low, especially if you’re cycling almost all the water out the cylinder on a daily basis.
You could drop the flow temp for HW by five degrees every week or so, see how you get on. The limiter will be the cylinder size.
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dbr
@rhowe
If we lowered the flow temp of our boiler to 55 degrees, would that provide a reasonable approximation of how a heat pump would behave?
We have an unvented cylinder and 55 is maybe a bit low to prevent bugs, but we could run the immersion heater occasionally to boost the temp and kill of any nasties.