Hi, I haven't used this particular item, but have installed a plate heat exchanger, and thermal balancing valve to do the same thing. I suspect that's what's inside the black box.

I haven't compared costs for the above 2 items versus the Friwa black box, but if there's a large markup, then I would buy the individual items.

For info. the thermal balancing valve acts to restrict the flow of hot water from the buffer (on the outlet of the PHx) to match the flow of heating water with the fresh water being heated ie. restrict the outflow heating water temp, being controlled to a cool temp, to prevent wasting heating water in lower fresh water flow situations.

Posted By: djhI'm not entirely sure I understand what the module is. Is it something you add on to a plain water tank to turn the tank into a thermal store? I'm not clear why it would be preferable to buying a ready made thermal store?

its something you add on to a buffer tank, to get DHW. The buffer tank (thermal store almost) is heated by various things solar/heat pump/gas/immersion. I guess the flow rate and secondary circulation option make it better than a thermal store with just a mains pressure DHW coil inside.