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

Unfortunately we don’t get a cheaper tariff at night. On my controller it has “Reheat Temp” it is set at 40. Does that mean when the tank temp drops to 40 it will reheat, or does it it will reheat to 40 when the tank temp drops to a certain temp. In the first scenario what temp would it reheat too, how would I control this? In the second scenario how would I control the temp it drops to before reheating? We do have solar and an  i boost which is great for heating the water. Unfortunately we don’t get much sun in the Highlands, especially Oct to April.

Hey @Boots and Bens,

Moved this topic into the ‘smart home’ category so it get’s more traffic. 

Just tagging in some fellow heat pump users to see if they can help @Chip370 @Nick Challacombe @Old Timer @RAJ @christianedward @M.isterW 

I believe that reheat is more efficient but that's just anecdotal. However, if you have a tariff that offers cheaper electricity at certain times of day that makes it more economical to schedule the water heating. There are loads of tariffs that offer this so it's worth shopping around, particularly if you also have a battery.

If your reheat temp is 40, when the water around the sensor reaches that temp the heat pump will heat the cylinder back to your chosen temp (that is normally 50 or 55). Remember it only takes the temperature from a single place in the cylinder so you might have plenty of hot water even when the sensor is reading low.

Thanks, I maybe will lower the reheat temp to 30.  No reduces tariff with So Energy. What can you tell me about batteries? I am guessing they are very expensive and you need to live somewhere very sunny for them to be economical?

That's a good reason to move to a different energy company.

Batteries are pricey but you don't have to be in a sunny place for them to be worthwhile. It depends how much electricity you're generating. We are mostly self sufficient in the summer because the battery gets us through the night. In the winter it allows us to use all of our generated electricity instead to sending some to the grid. I sometimes charge the battery on cheap rate electricity to get us through the expensive periods, which saves us money. If you want to make your battery work harder, there is at least one tariff on the market that actively encourages you to charge your battery when there's excess electricity and discharge it to the grid at peak times. So there are plenty of ways to make it financially viable.

This excerpt from the Daikin manual explains schedule +reheat. The hysteresis value is 10 degrees by default so when the tank temp falls below reheat - hysteresis (40-10=30 in my case) the reheat function is triggered. 
The example of 60 deg for hot water is unnecessary high ! Mine’s at 49.
You also have to bear in mind that the heat pump can only do one thing at a time so the space heating goes off when it’s doing the hot water. 
 

@Boots and Bens if you set reheat to 30 it’ll only ever come on when the tank drops to 20 !!

Ok so set at 40 it will reheat at 30, why the 10 degrees difference, what the point in setting for a reheat temp you want and then it does not do that until it is 10 degrees below that?

?  You say it will heat up to whatever the set temp is, is that the set comfort temp or eco temp? Or is a temp that cannot be changed?

Sorry dont really understand the graph I do have the manual with that in, what to the  shower symbols mean, when a person showers.  We don't use the shower that often, at most once a day one person and a couple of days a week not at all. Probably use more water for washing up. So my thinking was if I changed the set reheat temp form 40 to 30 then it would not heat until it dropped to 30 and hence not as often, Thank you very much for advise and patience

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