Why is my Mitsubishi Ecodan not heating up the tank?

I must admit I was puzzled by the the scale trap but the arrangement for heating the hot water (if I’ve got the right version) is different from a gas boiler as you can see below.

In a system gas boiler there is a coil inside the HWT which comes from the boiler and it has a fixed volume of water that doesn’t contain any hard water because it is pre-treated and just circulates.

In the Ecodan fresh (hard) water goes into the tank but instead of a coil, water is taken from the tank, goes through a heat exchanger, through the scale trap (the order is important) and back into the tank. So scale will build up in the hot end of the heat exchanger and scale trap. Hard water is circulated in these components whereas in my gas boiler the scale accumulates on the outside of the tank coil. I suppose doing the direct water in/out of the tank avoids the second heat exchanger (the coil I have in my gas boiler) and being a lower temperature system this is probably important because each heat exchanger involves a temperature drop and you are trying to minimise high temperatures in an ASHP system.

​@juliamc I’m thinking you may not have a scale trap if your ASHP feeds a coil in your hot water tank. I do know that at least some ASHPs do it that way (they have a bigger coil than a gas boiler version does) but perhaps operate at a slightly lower efficiency. The diagram does say that scale traps are only used on Mitsubishi Electric made Hydroboxes/Cylinder.

Peter