I am about to install a new radiator in my apartment; we removed the old one without noting the direction of water flow. So I now have two pipes sticking out of the floor and I don't know which is which. The pipes are now capped.

The new radiator is marked for the direction of flow; through the valve and in at the top of the radiator. Will it make much difference if the new radiator is connected backwards, water going in at the bottom and the valve after the radiator? And if it is important, how do I establish the direction without flooding my apartment?


2 Answers 2


Two thoughts.

  1. If you remember which side the valve was on. That is most likely your input. It could have been installed backwards however.

  2. Crank up the heat and give feel test it. One of the pipes should be stone cold.


Pipes from one radiator lead to another and do not cross, so you orient the radiator the same way as the other radiators on the same floor are oriented. If you only have one radiator, you will need to get into your neighbor's apartment to figure out which way the pipes are going.

  • Thanks @Tyler unfortunately the builders of this block didn't follow that rule; in one bedroom I have two radiators (original installation so presumably correct) with opposite `polarities'. And my neighbour's floor layout is different!
    – NL_Derek
    Commented Nov 15, 2015 at 14:10
  • @NL_Derek Even so, the principle is the same. You must follow somehow a pipe from the replacement seating to the nearest intact radiator and this will tell you the orientation. If one person bangs on the pipes and the other person listens it should be obvious which of the two ends is connected. Commented Nov 15, 2015 at 14:24

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