Coolant Heating System

[Da Kat]

Hi All,
I have a Volkswagen Marine VW TDI 150 5 cylinder turbo engine. After a recent haul out I have found that running the engine no longer heats the hot water system. To clarify, I am fairly certain this only happened after the haul out. The element side of the system works as generator and shore power do heat the water.

A few points and questions:

Does the coolant transfer heating system pump only come on when the coolant is at a certain temperature or does the pump run all of the time?
If yes, it only comes on at a set point temperature, what do I need to look at/change that may have failed to turn the pump on?
If the pump should run continuously, given that it is pumping coolant, what sort of pump should I buy to replace the existing one as the pump is not coming on at all at this stage? (It is a 12 year old yacht and so the exact same pump is probably wishful thinking).

Could it be an airlock in the coolant system?
If yes, can I just run the engine with the coolant filler cap off to fix the airlock issue? I do not see a bleed valve as mentioned in another post for bleeding the system but did see mention of running the engine with the cap removed. (Yes I have trawled through the posts first hoping for an answer &#128512.

Hopefully I have provided enough information and that the info combined with the picture will soon see me having hot showers 🤣🤣

Paul

[goboatingnow]

Check the thermostats and the circulating pump.