DHW To Reheat or Schedule? What's the most economical?

The ‘reheat setpoint’ is the temperature to which the system reheats the water. 
The ‘hysteresis value’ is an installer setting, it can be altered from the default value of 10 degrees, but the system work’s satisfactorily at 10.

The reheat function kicks in when the tank temperature drops below the ‘reheat setpoint’ minus the ’hysteresis value’, ie 40-10 for me, ie it reheats from 30 degrees to 40 degrees.

Comfort and Eco setpoints are used for scheduling. The shower symbol on the graph just indicates when a tap is run, I suppose they used a shower as it would indicate more water used than washing up might. If they’d used a bath tap of course that would generally suggest even more.

They have made it a bit complicated ! But they have designed the system to allow the user to personalise the setup. The general advice is to adjust the hot water to the lowest temperature which you find comfortable and means you don’t run out.

Time to experiment !

Thanks

so my reheat is set at 30, so this means when the temp drops to 20 it reheats 30???

My tank temp is set 55 comfort and 50 eco. Does that only operate when I have a time scheduled. When a time is not scheduled and there is no solar, will the water on my heat up 30degrees, as above?

Ooh… now you’ve got me !!! I’m not sure what happens if you don’t have a schedule set !!! @hydrosam has the same controller as you so can help here please 🙏🏻 

We are on Octopus Go with Battery. Solar Panels and ASHP… We reheat over night to 45 degC using the ASHP and then boost to 60degC using the electric immersion using the cheap rate tariff (we are heavy users of DHW in the morning)…. during the day we reheat when the tank temp fall below 35degC and also boost back upto 60degC on sunny days when the batteries are full….

What make and model of heat pump are we talking about here?

On my Altherma heat pump, the reheat hysteresis is fixed at a 5C differential. So if you target is 40C, the heat pump will reheat the water if it drops below 35C. The later Altherma models allow this difference to be configured, and 10C is a much more sensible figure. 

Whether you use reheat, schedule+reheat, or schedule only, and the temperature you heat too depends on how big your DHW tank is, and how much hot water you get through. 

In my house we have a 300ltr tank, 4 adults, 2 teenage girls. We run schedule+reheat. Schedule at 2pm daily to 46C (about the most efficient time of day for heating the water, cos it’s warmest outside, and heat pump is all about extracting heat from outside air, if its an air-source heat pump), with a reheat setting of 30C, so if tank temp falls to 25C, it’ll raise the tank temp up to 30C (at the sensor) which usually gives hot water at the top of the tank. We rarely trigger the reheat option, maybe twice this year I think. I found this setup to be the most economical for us.

@juliamc is suggesting you might have the same heat pump as me, to change from reheat to schedule you need to go in to the installer settings, which are hidden away, but I can talk you through finding them if you want to go down that route.

FYI Batteries and heat pumps are a great mix. Not cheap but payback is more shorter than solar PV. Mine will payback in about 4 years I think, currently saving me about £2000 a year. I charge over night at 7.5p/kwh which usually lasts the whole day. Again that’s with my use, it will different for others.

@Boots and Bens ’s controller looks like your @hydrosam as seen in