Hi David - have you considered making your own? There are lots of examples out there on t'internet. It can be made to fit your available space, and I have seen tall examples, although most are squat. You can have a two section tank, feeding the solar hot water into one for DHW until it reaches the desired temp, then divert any surplus into the next one for use with the UFH system.

Essentially an insulated plywood box, lined with foam insulation and pond liner, all connections brought in at the top, above the water level. The problem I can see is the fitting for an immersion heater might have to break the integrity of the liner, unless you devise an internal watertight box or compartment for it.

Thank you both for the feedback.

Posted By: SteveZThe problem I can see is the fitting for an immersion heater might have to break the integrity of the liner, unless you devise an internal watertight box or compartment for it.

Both the tanks I mentioned are polypropylene with all connections through the lid. I think they've avoided immersion heaters for the reasons you describe & because one mounted in the lid might become uncovered if the water level is wrong.

David

Posted By: davidfreeboroughI think they've avoided immersion heaters for the reasons you describe & because one mounted in the lid might become uncovered if the water level is wrong.

You could install a little float switch or a liquid sensor that only cut in the immersion if the water level was high enough.

I think of this story when people tell me that vented systems are inherently safe!

Any immersion heater should have independent over-temperature protection & those in smaller heater vessels or plastic tanks should also have a thermal fuse. It shouldn't rely on the main thermostat because it can easily be put in the wrong place or get dislodged.

David

If a tank is going to have hot water in it, does that exclude plastic tanks?

Interesting.
I maybe should have been clearer, what I meant was using an ordinary PE tank as a drainback tank without any special reinforcement.

I once heard the story about plastic cups (was at the factory that made them for vending machines).
Then they were initially designed they where thicker, but the almost boiling water was causing them to soften and distort. One solution was to use a more expensive plastic, but they ended up using the same plastic but a lot thinner as this allowed the drink to cool before it had time to distort the cup.
And that is why we have hopelessly thin, nasty brown cups filled with dreadful tasting tea.

Posted By: SteamyTeaI maybe should have been clearer, what I meant was using an ordinary PE tank as a drainback tank without any special reinforcement.

Depends on the working temperature of the specific plastic, and the maximum temperature it can encounter under (a) normal conditions and (b) fault conditions. So PE is not so good, PP better, EPDM even better and silicone betterer yet.

One solution was to use a more expensive plastic, but they ended up using the same plastic but a lot thinner as this allowed the drink to cool before it had time to distort the cup.

And the consumer's answer of course is to use two nested cups, with the layer of air providing insulation.

And that is why we have hopelessly thin, nasty brown cups filled with dreadful tasting tea.

I stopped drinking the coffee from such machines very early in my career. The place where I worked had varnished parquet flooring around the coffee machines. I spilled some coffee one day and it stripped the varnish really quickly. I really didn't like the idea of that inside my stomach!

Looks very interesting OK David.

If you download the brochure for the HPSU it mentions that the heat pump is manufactured by a group company of which Rotex is a part and when you look at there "International" contacts page many of the companies including for the UK are Daikin.

So I would imagine that you could be confident that you are getting a good solution and not something cobbled together by a third party.

Posted By: davidfreeboroughRotex offer a wider range of solar thermal drainback tanks than I realised. Most are available with optional top mounted 3kW immersion heaters.

More interestingly, one of their solar thermal drainback tanks integrates the indoor unit of a split system air source heat pump:

http://en.rotex-heating.com/products/heat-pump/heat-pump-for-new-build.html

It provides domestic hot water & low temperature heating & includes everything you require to connect the refigerant lines from the ASHP outdoor unit, the flow/return to solar drainback panels & the flow/return to underfloor heating. Domestic hot water is provided by a 6m2 internal heat exchanger coil. The outdoor unit seems to be a re-badged Daikin box.

Does anyone have any experience of this system?

David

Did you ever enquire any further about the Rotex tanks and heat pumps David?