What is a reversible valve?

When a Thermostatic radiator valve is termed reversible it can be mounted vertically or horizontally. It also can be applied to a valve when it is fitted on either the flow or return. In all cases it is suggested that a bypass valve is fitted to control the velocity and flow of water when a TRV is fitted on the return. It is advisable for TRVs to be fitted on the flow into the radiator to ensure correct heat up.