I have a dual coil calorifier tank - 210l total, of which 90l is the dedicated solar part, so I only have 220l, which is probably a bit undersized, but couldn’t fit anything bigger in! It is the original solar cylinder, not a ASHP one with a big coil area - I decided I would try it on the basis that a new tank was both very expensive, on very long delivery and wouldn’t fit anywhere and still easily couple to the solar thermal. In reserve, I planned to fit a external plate heat exchanger if it didn’t work. In practice however, it will get the tank to 46C; I need 50C for there to be enough water for everyone to shower in the morning, but I just top it up with the immersion heater during my off-peak (Octopus Go) period.
I also get the dip at the end of the heating cycle and from the chart below, you can see that this is because the DHW/CH valve switched back to CH, so you quickly push water out of the cold radiators through the return pipe. You can also see from mine that, at the start of the heating cycle, it actually lowers the tank temperature and heats the ASHP circulating water from the tank.
I fitted a bivalent mixer valve to the input side of my tank (I think - long time ago now) so this doesn’t happen. When the boiler first triggers, the valve forces the water to circulate and only lets the water from the boiler into the tank once it has reached the fixed temperature, otherwise the water circulates round through the boiler until it heats up sufficiently. Works like a charm.
Plate heat exchangers are a much better way of keeping some heat in the thermal store. From my experience with solid fuel a domestic hot water coil within a thermal store rapidly depleted the store. However this plate heat exchanger set up does require 2 additional pumps and a flow switch either side of the plate heat exchangers.
I did sketch up a design for an external plate heat exchanger and tank circ pump if the existing calorifier wasn’t up to the job, but I’m not convinced it’s worth it given the chart above!
