Taking the heads off stops the pin from creeping down, but it doesn't change the fact that modern TRV valve bodies themselves have narrow internal ports compared to full-bore valves, as the photos demonstrate. That said, we risk over-focusing on TRVs and getting bogged down in discussing 50-year-old domestic plumbing. The reality here is almost certainly compounding issues. Even revising down to 1,500 hours a year over 7 or 8 years puts the Webasto at around 12,000 operating hours. For what is essentially designed to be a small vehicle heater, that is massive mileage. However, because you’ve had problems from the beginning, it’s clearly not just high runtime. It’s heavy usage sitting on top of underlying installation, coolant, or flow issues from day one. Continuing to speculate from our keyboards won't solve it. To get to the bottom of it, you really need a competent marine heating engineer on board to inspect the whole system properly under load.
