Right with the cold weather now with us. I have a small 500x500 single radiator in the hall way.

1. I could get a double one but how much extra heat would it give out

2. I've priced up a vertical one 1800x500 and I'm not going for that as its too expensive.

3. Can I fit a horizontal one, but set it up in a vertical position, but doing this will the water still flow around it ok

Thanks in advance

Number 3 is out since the fins on the rear will be in the wrong direction to promote convection so it just won't work very well.

Double ones do make quite a difference, in one room I replaced a single with a slightly smaller double and it made a noticeable improvement.

Double ones double the surface are therefore are doubleish

i..e a single 1600x600 rad gives out 1658 watts

a double same size gives 3109 watts

Please be aware that the radiator might be purposely undersized if its the 'Bypass' radiator for the system (i.e no valves and always on) and there is a room stat in the hallway..

And fitting a rad tge wrong way round would cause it to fall off the wall as the brackets are on the wrong way.

If you have a salvage yard nearby you will get one cheaply. Many plumbers throw them away when doing a new system.
Try the bay for a s/hand unit.

Thanks for the answers fellas. Think I will try a double one first

Ta