Evohome and Mixergy

[Gandi]

I wonder if anyone can help.

I've recently had a new Sapphire oil-fired boiler installed, connected to my existing Evohome now via the Opentherm interface. At the same time I had a Mixergy X tank installed.

The boiler, for central heating seem to be working fine now, following advice here. This was to remove Central Heating BDR91 and replace that zone valve with one that is normally open.

The problem I now have is with hot water and the Mixergy tank. This works fine using electrical heating and uses the clever Mixergy logic, but when I switch it to using the boiler (necessary now this time of year), I have problems. Both the Evohome and Mixergy seemingly want control differently, which has resulted in several overheating events. I'm guessing this is where the boiler is fired by Evohome while the HW valve is closed or the Mixergy tank stat is not requesting heat...

I've temporarily overcome the issue by setting the tank to heat to 100%, and allowed the Evohome to control demand, but this is far from ideal.

Mixergy have advised that only with their Mark 2 Heat Pump interface will I resolve the issue - but so far with little explanation as to how and why. With Opentherm being almost unique to Sapphire in the world of oil, my installer has stepped back from the problem - somewhat understandably as I specced it all!

Any thoughts?

[Richard]

You need the Mixergy cylinder to give a 'satisfied' and 'not satisfied' demand to the CS92A. This can be done with a resistor box with a couple of resistors and a relay attached the Mixergy cylinder.

I'm assuming this is what the Mixergy MK2 Heat Pump Interface will give you? Any reason why the standard heat pump interface won't work?