Should TRVs go on the flow or return pipe when fitting to radiators?

Question from Tony Clark, via e-mail:
Should TRV’s go on the flow or return pipe when fitting to radiators?

Answer from Steve Harris, technical services engineer at Wavin:
The vast majority of new TRV’s are bidirectional and can be fitted to either the flow or return, but it is always good practice to fit them to the flow side. If a non bidirectional TRV is fitted on the return side it will create a vibrating sound as the water tries to flow though it. Always check the product for direction of flow indicators before fitting.