Incorrectly fitted TRVs

As part of my getting to grips with the basics of central heating I learned that before the introduction of bi-directional TRVs, they had to be fitted on the flow side of rads.

Having just checked my own (I've been in this house a few months) I found that in every instance the TRV is fitted on the return side. Each is marked with an arrow (which I assume indicates the required direction of flow) and in every case the flow (as indicated by the relative temperatures of the pipes) is in the opposite direction to the arrow.

Questions: why did some TRVs need to be on the flow side? Surely if they are simply closing as a result of the room temperature reaching a certain level it doesn't matter whether they close the return or the flow side? And what is the result of my having my TRVs on the wrong side?

TIA Stephen

The original installer probably got the flow and returns from the boiler mixed up.

If there is only one arrow then they are the wrong way around, what will happen is that they will bang\knock etc as the seating tries to close but you didnt mention this so i assume this is not the case. I cannot comment on the mechanics of an omni directional TRV but if the arrow is the wrong way then assume that the TRV is not working properly/effectivly.

Thanks bster.

There's been no banging or knocking from the valves. They appear to be working fine.

Sometimes they do

Untill they do dont worry, have a beer

Now that's the sort of advice I have no problem with!