You will only heat the full 300L from Stone Cold. Thereafter, you will be topping up and heating what you use. If the DHW tank is well insulated, I would personally leave it as is.  This, however, is a personal opinion, and I'm no expert.  I do suspect that the cost of swapping the tank will be hard to recover unless there is a value/market in your second-hand tank.

I agree with @morgan. 

You can turn down the target tank temperature so you aren't diluting the water when it comes out of the tap/shower.  40C is probably about the lowest you can go and so a good starting point, but you may need to turn it up a few degrees depending on pipe runs and the relationship between when you heat the water and when you use it.  The lower you can get it the lower your running costs (and heat losses) for a given amount of hot water delivered.  Do run a legionella cycle weekly though.

There’s no second hand market for DHW tanks (I was told) so you would never get your money back and waste the materials in the tank.

@jamespa has good advice on temperature settings. Our 200L tank is set at 48C for 2 of us, so 45C may be plenty for you?

Posted by: @martingale

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Is there any strategy I can use to only heat say half the tank to the specified temperature?

Hmmm.

You would need to part-fill the cylinder using a material with a lower Heat Capacity than water (4186J/Kg/°C)
and a higher density, so that it sinks to the bottom of the tank. The density of water is 997Kg/m³.

That material would also need to prevent the ingress of water of course.

There are very few contenders for such a strategy.

Glass marbles are a possibility, provided they don't fall into any connected pipework.
The cold water inlet is usually at the bottom of the tank.
Glass has a Specific Thermal Capacity of 837 J/Kg/°C and a density of 2500Kg/m³

Better than that would be Gold with 129 J/Kg/°C and a density of 19,350 Kg/m³
or spent uranium from a nuclear reactor:  thermal capacity of 116 J/Kg/°C and similar density to gold at 19,050 Kg/m³

British Nuclear Fuels would probably provide you with their spent fuel free of charge, which gives you a better price point than gold.
They've been looking for somewhere to store nuclear waste for decades.

Posted by: @transparent

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British Nuclear Fuels would probably give you their spent fuel free of charge, which gives you a better price point than gold.
They've been looking for somewhere to store nuclear waste for decades.

That sounds like change of use from domestic to mixed domestic/industrial (or possibly sui generis) so would probably need planning consent.

Posted by: @transparent

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Posted by: @martingale

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Is there any strategy I can use to only heat say half the tank to the specified temperature?

Hmmm.

You would need to part-fill the cylinder using a material with a lower Heat Capacity than water (4186J/Kg/°C)
and a higher density, so that it sinks to the bottom of the tank. The density of water is 997Kg/m³.

That material would also need to prevent the ingress of water of course.

 

There are very few contenders for such a strategy.

Glass marbles are a possibility, provided they don't fall into any connected pipework.
The cold water inlet is usually at the bottom of the tank.
Glass has a Specific Thermal Capacity of 837 J/Kg/°C and a density of 2500Kg/m³

Better than that would be Gold with 129 J/Kg/°C and a density of 19,350 Kg/m³
or spent uranium from a nuclear reactor:  thermal capacity of 116 J/Kg/°C and similar density to gold at 19,050 Kg/m³

British Nuclear Fuels would probably provide you with their spent fuel free of charge, which gives you a better price point than gold.
They've been looking for somewhere to store nuclear waste for decades.

If you could get hold of a Stainless steel container of Vitrified High Level waste, then it has a heat output of around 2KW.

There are difficulties storing these containers due to the massive quantity of radiation shielding that you would need to hide them behind - typically 2m thick concrete walls.

Transporting them is something of a game, you would need flask handling facilities to remove them from their storage containment and move them (behind shielding) into their new home.

The other minor issue is finding a way of dumping heat that it surplus to requirements, the existing Sellafield Storage facility (the Vitrified Produce Store) is cooled by natural convention up through and away into a very tall discharge stack, this is backed up with FD fans of pretty significant size - I expect that it might be a tad tricky to get planning permission. 😀 

On balance, an ASHP is cheaper, and a helluva lot safer.

Hi @martingale, welcome! You can't really have too big a DHW cylinder, but you can have one too small. If it's well insulated modern unvented cylinder, as it should be with a ASHP, the standing losses (the W/h heat lost through the insulation) will be very low assuming it is located indoors and at room temperature, or perhaps in an airing cupboard that is usually a bit warmer. As @morgan says, you're only reheating what you use, plus a tiny amount of energy lost from standing losses. 

If you don't use much DHW then set the time clock or ASHP control to charge it up just once a day, ideally early afternoon when the outdoor daily temperature is highest, or during a cheaper rate if you have a variable price electricity tariff. Ensure it's not recharging several times a day, as that's not very efficient or necessary, and since DHW usually takes priority over CH with ASHPs and you'll not be heating the house whenever the DHW is recharging.

Storage temperature and cylinder set point: whatever you prefer, ASHPs will normally provide water at up to 55degC, which will efficiently heat the DHW cylinder to 50degC. Some R290 refrigerant ASHP models can go higher. Legionella sterilisation at least once a week to +60degC for 1hr via the 3kW immersion. This heats the upper part of the cylinder contents, where water is drawn off, to above 60degC. We increased our tank temperature from 50degC to 55degC, with the ASHP set to supply at 60degC for recharging, and we've not noticed any increase in energy costs or any longer duration for DHW cycles. The cost per year of DHW is very small compared to the cost of heating the house. My wife likes the hot water, er 'hot'!

It's a common misconception that the whole hot water tank is heated to the indicated temperature, but the bottom of most DHW tanks and any contents below the height of the immersion heater wont be. The water at the bottom is usually cooler and, at the very bottom of the tank, at or near cold water mains temperature. The contents are held stratified, cold at the bottom, hot at the top. Mixing of contents is used in commercial and healthcare settings, but rarely in a domestic setting.

PS: if you have a secondary pump circulating hot water continuously around your HW system, which is sometimes the case with larger properties, try switching it off. They are very wasteful of energy, and not necessary unless you are very insistent on having instant hot water at the tap a second after you turn it on.

Posted by: @allyfish

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Legionella sterilisation at least once a week to +60degC for 1hr via the 3kW immersion.

Cross references for @martingale 

Background discussion on the risks of succumbing to Legionnaires Disease is here

And here's an investigative discussion on reasons why the anti-legionella cycle may not be operating as required
with a link to a previous topic where an incorrect immersion heater was fitted, which includes my calculation of the energy required to raise water by 10°C

Many Members here do not run an anti-legionella cycle,
but if you do, then it needs to be effective!

I do not think that a 300l cylinder is too big for a two person household. I live in a 1 person household and have a 250l cylinder, which is just about big enough. I heat the cylinder in one two-hour window every day when the electricity price is lowest (I use Octopus Agile). The cylinder then provides enough water for washing up under running water and showering for 10-15 minutes at 50C.

If you use a tariff such as Octopus Agile, with a large cylinder, you can produce 100% of your hot water from the cheapest electricity price in a day, giving a significantly lower average electricity price for hot water heating (one of the main uses of energy in a home) than you would get otherwise.

If your ASHP has a Home Assistant integration, you can automate the process of heating the water at the lowest price times so that you do not need to think about it.

@jamespetts If you have the space for a high capacity DHW tank then, this may be utilised as a sort of battery with TOU tariffs. If as you say, a high proportion of one’s energy consumption is for hot water, then this is a heat battery and could be very much cheaper than a sizeable battery to store that energy. Regards, Toodles.

Posted by: @jamespetts

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I do not think that a 300l cylinder is too big for a two person household. I live in a 1 person household and have a 250l cylinder,

The size of DHW tank affects the anti-legionella cycle.

... unless of course, you've decided not to run it!

The usual arrangement is for the heat-pump to raise the temperature to 50°C
then the immersion heater takes over to bring the water up to 60°C, where it is held for around 10-mins.

Assuming you have a 3kW immersion heater, you can calculate how long that will take....
... which is why I posted a link to the topic where I presented the calculations.