My first post, have been reading the posts and you guys know your stuff....
I like the idea of a solar panel for hot water and CH use. My current system is 210ltr unvented. I think the lowest cost to do this would be to swap the immersion heater for a heat exchanger that is fed from the panel (assuming this can withstand the system pressure) and have some sort of high temp dump (and finding a reputable cerified engineer)?
However, to get the most from the solar panel a design that also supports the CH would be better, to minimise costs can my unvented system be converted to a thermal store system?
New to this so just exploring options and would welcome your ideas.
The best is a solar heat bank/thermal store. To convert the unvented to one is very difficult and I would say 210 litres is too small.
All is not lost. An unvented cylinder can be converted to a heat bank by fitting a DHW plate heat exchanger, pump, TMV blender, flow switch, small header tank and make spreaders out of copper pipe. The boiler would heat the cylinder directly. Not difficult at all. You now have a spare coil in the cylinder. This can be used to heat the rads, (it "should" be man enough to do it on 210 litre cylinder depending on house size) using TRVs on all rads and a Grundfos Alpha auto variable speed pump. This will now do the CH and DHW.
Unless you buy a solar cylinder, or a direct one with the tappings where you need them, and build it yourself (much, much cheaper), you will need a separate solar cylinder. The solar cylinder would be heated directly or indirectly by solar panels. Have its draw-off pipe to the cold feed connection of the now heat bank. The primary water outlet of the DHW plate goes to the cold feed of the solar cylinder (plates are so efficient they can drop water of 70 to 80C to 25 -30C). So now the water runs from the top of the heat bank, through the plate and into the bottom of the solar cylinder, then out of the top of the solar cylinder and into the bottom of the heat bank. A full loop through both when DHW is called.
But there may be very useful water in the solar cylinder. So, a 3-way blending valve with remote temp sensor is fitted supplying the plate on the primary side. The hot input port is off the top of the heat bank and cold port off the top of the solar cylinder, the temp senor on the DHW outlet above the plate. It is set to DHW setpoint say 45C. A normal blender is also on the DHW outlet that takes the outlet of the plate and the cold port from the cold mains, set to 44-45C. Now the solar cylinder takes priority is mixing of water with the majority of hot water there mixed before being exhausted and then the heat bank starts to get depleted. Even then the hot heat banks water is being mixed with cooler solar cylinder water.
If possible a shunt pump circuit can be fitted. Take a pipe from the top of the solar cylinder to a point above the CH coil in the heat bank (if one is there of course) and one from the bottom of the heat bank to the bottom of the solar cylinder. A pumps is fitted on the flow. A differential controller senses if the solar cylinder has more heat than the heat bank and brings in the pump when needed to pump hot water from the solar cylinder to the heat bank. This may be not needed as with constant DHW use, each time a tap is run water from the solar cylinder enters the bottom of the heat bank anyhow.
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