Solar HW and immersion backup control

[peter warm]

Looking at Solar Thermal hot water, with immersion backup (the development is off-gas). My question is: has anyone got experience of ensuring the immersion doesn't end up heating the tank too much? Presumably controls would need tuning to the individual occupants needs?

Any comments/experience of this gratefully recieved!

[Peter Bayer]

Just some thoughts.
Keep the volume of potable water stored as low as possible to avoid legionella.
Only heat enough water by immersion heater to cater for immediate demand.
Set the immersion heater temperature as low as possible to maximize solar component of water heating.
Given the above, a properly optimized thermal store seems a more likely bet than a hot water cylinder as it avoids the need to heat the full volume above 60C to kill bacteria.

[Peter Starck]

I considered doing this and my solution was to input the solar hot water to the top of the tank to use stratification. I would have used a Willis Solasyphon. One immersion was to be in the side of the tank half way down with another at the bottom of the tank. The idea was to top up the heat with the higher immersion at the end of the day if needed. The lower immersion was to be used to heat the water to 60C once a week if needed.

[David OLIVIER]

If no gas supply, seems to me there are still lower CO2 alternatives than elec 0.59 kg/kWh plus solar 0.00 kg/kWh.

Like LPG 0.27 kg/kWh plus solar.

Or if desperate a heat pump.

[Dave Howorth]

There's a lot of discussion of this issue on the Navitron forum. From my failing memory:
(1) yes it depends on the occupants usage, because
(2) a thermal store is better for the reasons given by Peter Bayer
(3) and stratification is valuable so
(4) the auxiliary heating should be towards the top of the tank and designed to heat just enough water for one period's use if the solar hasn't done enough
(5) while the solar should have access to the entire tank, but heating from the top first as Peter Starck suggests
(6) BTW, one possibility to avoid is the idea of an auxiliary post-heater fed from the thermal store as a pre-heater, since that doesn't heat the water long enough to kill bacteria reliably.

Cheers, Dave

PS Sorry about not being called Peter, but I guess David is the second best choice

[Jon Walker]

I'd agree that two immersion at different heights is a good idea. If you then get one of the more sophisticated Resol controllers you can use a 2nd (3rd, 4th etc) solar controller relay to operate one or more of the immersions e.g. suppress the lower immersion if solar system is still working.