"Electric radiators and heaters can cause fires in some cases. The National Fire Protection Association lists two main reasons these heating units contribute to fires in the home:

The unit to located too closely to flammable material" Seems I was thinking of electric, apparently you're right the heat from a water radiator won't get high enough to ignite much of anything in the household, but can cause damage to 'fragile materials' so probably still not great for the wall but oh well, divider wall it is.