I've had a sticky goo out of one of mine, fitted by a former owner. It's black and went all over the floor, skirting, valve and pipe.
Are you saying the sticky goo is coming from the head itself, rather than sludge coming through the pin or gland? As these are liquid thermostats, could it be the liquid seeping out?? I presume they no longer work properly once they've dropped their sticky goo? (!!!)
ras c's....
Best valves on the market but if going on a filthy system then valve orifice is prone to blockages due to 8mm orifice , more often than not system doesn't require balancing when using these valves , although if system require a narrow temp differential then radiator outputs will be limited.
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