Most of the new TRVs are bidirectional and can be fitted to either the flow or return (there is a bi-directional arrow on the TRV), but it is always a good habit to fit them to the flow side. If a non-bidirectional TRV (normally old ones) is fitted on the return side it will create a vibrating sound as the water tries to flow through it. Always check the product for the direction of flow arrows before installing.
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Areas we serve are Hemel Hempstead, Dunstable, Luton, Watford, Hemel Hempstead, Welwyn Garden City, St Albans, Harpenden, and surrounding areas.