Anybody created a pre-heat DHW cylinder

You need to reprogram it so the heat store isn't kept at full temp to make space for the solar energy, or fit a new immersion at a lower point so the solar energy can heat the bottom of the tank.

You could export some of the energy so others benefit.

Dont worry, it wont last. Winter is nearly here.

I haven't run the numbers but expect you'd be better off all round to feed the electric heat into the existing tank. The water wil go above set point, and not need other heat sources until the temp is back down again.

NT

Not worth bothering about. There will be no heat when you need it most. Also heat leaks away from small heat stores before you can use it. Small thermal heat stores are a complete waste of space/money.

A typical sub 4kw domestic array will only be generating a few kwh units ea ch day during the winter months so economically a pointless waste of time, money and effort. You will be better doing your cooking or washing during t he daytime, maybe with a slow cooker and timer.

He could be planning it now to use next summer.

I've considered doing something similar. My thermal store is two cylinders in parallel, with both DHW and UFH being heated by them. But the remaining failed old Nu-Heat UFH is being replaced by radiators (upstairs was done a while ago), so my half-plan was change the stores to series, and make the lower heat exchanger on the first one a heating coil connected to (hypothetical) thermal solar panels. Or, with all the UFH now gone, both lower coils could be repurposed as solar, but in conditions when the boiler will be needed anyway, having one cylinder as lower temperature preheat seems to make sense for the same reasons as the OP.

(The stores do have immersion heaters as backups, so PV panels could also be used in the same way as the OP.)

reprogram what ? ......... there is a simple standard thermostat ... set to 60 degrees.

I could set it to 450 and immersion stat to 60 but then tank would not get warm enough when there is no PV power

today is a particularly damp, grey & gloomy day ... just looked it produced 2.2kWh today

the electric heat into the existing tank. The water wil go above set point,

and not need other heat sources until the temp is back down again.

Can't do that ... max temp I can have store get to is 60 degree ....... any higher and you risk damage to underfloor heating pipes.

How did your Nu-Heat fail ? ............ mine is Nu-Heat and runs really well. Although I went wit the pex system not their earlier triple pipe rubber system.

trying to educate the wife on this .... limited success so far.

Needs to have a large display giving count of free power ... maybe she would use it then.

Yep . Both of mine did 2.1 each.

Tch. You don't need any heat in Summer

No timer so you can use solar during the day and switch in other sources when its going dark?

They should have a mixing valve to prevent the floors getting too hot.

If your ufh runs off a bottom coil you can normally heat the water above it to over 60. Otherwise as you say it's a preheat tank. But a preheat tank brings a few issues:

1. water staying in the unsafe temp zone - ok for most things but not ideal for showers
2. Lots more area for heat loss, but in fairness a foot or 2 of insulation is pretty cheap.
3. Upfront costs that arent there with feeding the power into the main HW cyl.

NT

No the design has a mixing valve on the DHW take off, but the main feeds off for the UFH loop is not via a mixing valve ... the main tank stat prevents it going over 60 degree.

I had a similar installation many years ago but using solar water as the pre-heating. The solar tubes warmed a heavily insulated cylinder in the attic. Incoming mains water went first to the solar cylinder and then to the second cylinder heated by a gas boiler. It was interesting to play with but didn't achieve much. In the summer it provided most of the hot water, in the winter sod all.