Optimising Hot Water Tank Once per Day

I’ve been setting up my hot water and I’ve a new puzzle.

(note I’m already aware of the problem with my heat meter due to air in the system, but I think this one is independent of that).

I’ve followed the general practice of “let it get cold and heat slowly to a lowish temperature”.

I set up my hot water to run once per day(middle of the night to make use of a cheap tariff). I went for what I thought was a large tank in a 300L Heat Geek Newark Cylinder (I think they now call this their super cylinder). I haven’t yet moved over my showers so the only current use during the day is hand washing and a little washing up.

What I found, which surprised me, was that by mid afternoon the water was cold and the tank temperature reading low so I had to add a 2pm heat up as well. To test this properly (without adding any cold water) I found a day when no one was in the house (zero water use) to see what happened. Thursday 4th July.

Hot water target temperature 50oC.

According to the specs the tank should lose 1.64kWh/24hr (65oC store and 18-25 oC ambient). It’s summer and the tank is in an unheated loft. Lets assume conservatively 1.64kWh *(10/24) energy lost in the 10 hours between my overnight heat and afternoon heat (~0.7kWh or about 2oC lost).

I don’t have a temperature reading for the tank at 2pm annoyingly. Can I use the flow and return to estimate it? I would assume from the flow and return that it looks like the tank is cold or certainly not ~48oC. Which backs up what I’ve found in reality.

It’s annoying me that I can’t figure this out. What am I missing?

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Where are you / the heat pump measuring the tank temperature from?

If it is too low in the tank it will be in the cold water being introduced when there is still loads of hot water at the top.
When the heat pump runs a hot water cycle it will mix everything up so the cold stuff a the bottom will bring down the over all temperature of the tank until it is heated again.

Stratification is much better than I ever expected before having a tank. I trigger reheats with a temperature probe near (but not too near) the top of the tank.

Water out of the tap was cold? What temperature was the tank reading?

You can use the return temperature to infer the temperature inside the tank while heating it; return is typically a few degrees higher.

Also look at the total heat energy (kWh) produced during that cycle - compare that to how much energy you guess may have been used or lost.