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Best way to heat and store hot water with ASHP and Solar - am I being stupid?

Domestic Hot Water & Cylinders

Last Post by Mars 1 year ago

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nickmorgan

(@nickmorgan)

Active Member Member

Joined: 1 year ago

Posts: 16

Topic starter 07/02/2025 9:29 pm [#2087]

We're about to start renovations and will be installing an ASHP with Solar PV and batteries. For the hot water, I've looked into solar diverters, Mixergy tanks and phase change systems, and no doubt there are other systems to consider.

BUT, aren't all those options irrelevant if you've got an ASHP?

If the ASHP is 300-400% efficient then for every 1KW of input i'll get 3-4KW of output, whereas I believe all the other systems are more like 1KW of input for 1KW for output and so I believe it's as simple as getting the best cylinder I can afford and connecting it only to the ASHP?

Or am I being stupid and missing something or misunderstanding something?

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Gary

(@gary)

Reputable Member Member

Joined: 3 years ago

Posts: 350

08/02/2025 10:00 am

You are correct, ASHP is the most efficient method, solar diverters were popular before batteries became widely available and prior to any heat pump uptake.

Phase change like sunamp is as inefficient as direct heating the only advantage is if you have no space for a cylinder its a smaller unit.

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Judith

(@judith)

Prominent Member Member

Joined: 3 years ago

Posts: 537

08/02/2025 7:22 pm

@nickmorgan you are not missing anything at all and when you dial in the import and export rates on electricity tariffs then a diverter is even more uneconomical.

For example Octopus export is 15p per kWh and import on cosy ~13p per kWh and the effective COP for DHW is ~3 so heat DHW in cosy cheap periods and export all excess. Early afternoon is both warmer so higher COP and possibly your PV could be powering the ashp and exporting the rest.

If you have an EV then you might use other tariffs which could work well too.

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP >4) open system operating on WC

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4709

08/02/2025 10:01 pm

@nickmorgan you’re absolutely spot on with your thinking. It took me a while to fully grasp this, but a few years ago, it clicked for me too, and we removed our solar diverter to let the heat pump handle hot water.

The only “quirk” with hot water production is that the heat pump has to work harder to deliver it. For example, we heat our hot water to 45°C, but to achieve that, the heat pump needs to run at a flow temperature of around 55-60°C. This is because you need a higher flow temperature to ensure effective heat transfer through the coil or plate heat exchanger in the cylinder. If you ran it at 45°C, there’d be virtually no heat transfer, and the water wouldn’t get hot enough.

You can see this in action in the attached image. The two spikes correspond to our morning showers and an evening bath.