"Valves bodies are blank off apart from a small hole. As both ends take the same body the flow rate is restricted so if the radiators in question are not given priority the flow is practically non existent. Drayton's previous TRV which I've had 12 for approx 15 years have a 15mm opening in the lockshield and the TRV valve itself had only the valve with no added restriction of a small 6 mm hole (approx) drilled in the bodyopening. Money WASTED would not recommend and Drayton technical department were as helpful as chocalate fireguard. Balancing the flow was impossible."