Metal pipework, such as cast iron, is suitable.
For smaller hot water systems, BRE Information Paper 8/07 indicates that discharges can be made to PVCu stacks, provided that:
relief discharge is from domestic unvented hot water storage systems only – not combi boilers or sealed system boilers.
storage volumes do not exceed about 210 litres.
stacks are fully ventilated (ie. no stack cap or air admittance valve).
pipework complies with BS EN 1329-1:2000 or BS 4514:2001.