I am always wary about Nettaheats. Went to a site once where one had blown up.
It was on an open vented system with the cold feed taken directly into the back of the boiler. The cold feed fitting had corroded internally and stopped the system water from being able to expand or make up. Theoretically I suppose it should have vented out, before it went bang, but it never for some reason. Probably on the basis that if the cold feed fitting had corroded there was a reasonable chance the vent fitting would have as well.
And yes it was a mess.
And they had only just been serviced, but being fair there was no way you could tell there was a problem if you did not check for expansion and make up down the cold supply from the header tank or how high the water was expanding up the vent.
Lets be honest, the heat of the boiler conducts up the surrounding pipework as well as the hot water and so feeling the pipes alone could make you think it was okay. But the make up water in the header tank should have been checked.
A lesson learnt.