Could well be the case. Is the ASHP coil below the immersion? Some tanks have a circulation pump that helps even out these variations

@alleyfish I would assume that the immersion element (back up heating device) sits slightly higher up in the tank as it plays second fiddle to the heating coil, thus it only  gets to heat a smaller volume of water.

Posted by: @allyfish

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I've noticed that my hot water cylinder temperature stays warmer for longer if it is heated using the ASHP. When I heat using the immersion, it seems the heating effect in the cylinder is not uniform, and the cylinder cools quicker once HW is drawn off, and even when not. Even drawing a small mount of hot water off drops the cylinder thermostat display several degrees, whereas this does not happen if the ASHP has heated it via the coil.

The conclusion I draw is that the 3kW immersion provides uneven water heating resulting in a lower average water temperature as the surface area contact between heating device and water is much smaller compared to a quick recovery coil. Anyone else noticed this? My cylinder is a Grant 250l pre-plumbed quick recovery.

Heating by ASHP usually takes about 3/4hr on the timer. Thermostat demand is satisfied at 50degC. The maximum heat input from the ASHP is around 8-9kW (10kW unit up to 55degC flow in HW mode)

Heating by solar PV diverter - usually about 2-3hrs based on power diverted to 3kW immersion. Thermostat inside immersion heater unit satisfied about 53degC indicated temperature on the HW cylinder stat. Thereafter, the immersion cycles on the internal thermostat.

The most probable reason is that the immersion heater is positioned higher in the hot water cylinder than the heating coil for the heat pump, therefore the immersion heater is heating less of the tank volume before being switched off by the thermostat. Also check the setting of the thermostat for the immersion heater, which should ideally be set at 60C, thereby achieving almost constant disinfection at this time of year.

@derek-m thanks. The immersion heater thermostat on Grant cylinders is pre-set to 65degC and it cycles on/off on that. However, the separate cylinder thermostat sensor, at the same elevation on the tank as the immersion, (about 1/3 height), indicates 53degC maximum. That tends to suggest the heating of the water is laterally uneven, so the effectiveness of using the immersion for anti-Legionella control may be questionable. At draw off location, top of tank, water could be 60degC+

Either way of heating, ASHP or solar PV & diverter, I've never had the water run cold on me yet. 🙂

@allyfish at this time of the year if you have copious PV you can turn the immersion stat up to maximum thereby dump the max amount of free energy into the cylinder. the greater temperature differences may then be enough to create internal circulation and warm more of the volume. after a long sunny day my whole cylinder is at 70C. but my immersion is low down, just above the bottom of the coil.

@iancalderbank Yup, tried that, but the tank maxes out at 55degC indicated on the cylinder. The immersion thermostat cuts in and it then cycles on that. I think that's due to immersion location vs tank thermostat sensor location, immersion can never reach thermostat set point on tank if it's above 55degC. I've definitely realised heating with the ASHP is most energy efficient, and I can export more at 15p/kWh to Octopus when simply leaving hot water charge c/o the ASHP each day.

@allyfish mm ok, there must be something else making it cut out because you should be able to go as hot as the immersions  limiter, maybe grant have done something clever in your wiring that stops this. In my case being on a deemed export tariff, its better for me to keep the energy and use as much of it as possible, even if at a lower efficiency. which means for me, that an immersion heated tank (whole tank to 70) lasts a lot longer than an ASHP heated one (same tank to 50).  I do not need any ASHP reheat of DHW in summer. But the key point is that you get 15p for your actual export, so in your case its better to export , then re-import at higher efficiency when you need it. Oh the intricacies of it all....

Posted by: @iancalderbank

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@allyfish at this time of the year if you have copious PV you can turn the immersion stat up to maximum thereby dump the max amount of free energy into the cylinder. the greater temperature differences may then be enough to create internal circulation and warm more of the volume. after a long sunny day my whole cylinder is at 70C. but my immersion is low down, just above the bottom of the coil.

If you live in a 'hard' water area, I believe heating the hot water cylinder much above 60C can cause limescale to be deposited on the internals of the tank, so care should be exercised.