I need to fit a new radiator in our bathroom.

I plan to tap into the pipes (plastic) running under the floor that
feed a single bedroom radiator.

The problem is that I'm don't know which is the hot/outward pipe and
which is the cooler/return.

How important is it that I identify the pipes correctly?

What would happen if I got them the wrong way round?

I don't expect to be using the termostatic control too much as this
new radiator will be used for towel drying etc ..

All advice welcome

Thanks