Hot water thermostat

[Luke29]

Hi guys. After some advice please. 
I have a pre plumbed cylinder and heat only boiler. The heating is zoned and we have nest stats. My hot water stat on the cylinder is set to 38 degrees as we have young children who can reach basins etc. Bath has blending valve fitted. However a few occasions now the water has came out a ridiculously high temp steam coming off it the lot. I’m worried the kiddies may get the raff of this if they turn a tap on etc, any suggestions? Cheers guys 

 

[ProDave]

If the cylinder stat is set to 38 (barely tepid, surely you mean 48?)  and the water is coming out scalding, then either the stat is not working or there is a motorised valve that is sticking.

More details please of how it is wired and plumbed.

 

[Murdoch]

Did it work properly before the Nest stats were fitted?

 

[Geoff1946]

Is the hot water a zone itself, with its own motorised valve controlled by the cylinder thermostat?   If so, either the 'stat isn't opening, (its contacts)  or the motorised valve isn't closing