I have recently tried to balance the central heating system in my house as there are three rads which only get slightly warm. Some rads have trv valves and some Dont, To find out which was the in and out on each rad i closed the valves on both sides of the rads then when the system was up to temp I opened both vales on each rad together to see which side was hottest first. I have found that on several rads that when i open both valves together the end that gets hot first does not always have the trv valve on!! should these only be fitted on the input to the rad? they seem to be on different sides. is that a problem. thanks.