I had a thread during the winter on my issues with caviation and over heating. https://hearth.com/talk/threads/cavitation-issues.166597/ I've finally started tearing into my system to see if I can find the flow issue on my boiler side. I was seeing huge delta Ts on the boiler side of the FPHX. First thought was my pump was damaged from cavitating. The impeller looks good. So next thought was my FPHX scaled up. I pulled it out of the plumbing today and from looking in the ports i couldn't see anything. Looks like the day I installed it. My only other restriction could possibly be the themomix valve being stuck semi closed and allowing bypass of boiler flow. I didn't get a chance to pull the valve ti look at it but other then that what could be my loss of flow? I went from delta Ts of 10-15°F at the beginning of the season to 40-45°F towards the end.