new to diy wrote:

Ok, I've just bought an IR thermometer with laser guide (IR89 -
awesome) and have began balance my rads in earnest.

I have a choice of 2 rads which could be considered closest to the
bolier. One rad has a 4 degrees C drop across it (20-16) but the
other, get this, has an increase across it (21-27)? Have the builders
put the wheelhead/lockshield valves on the wrong way round? Would the
central heating system still work ok if this were the case?

Any advice greatly received as always.

Like all tools, an IR thermometer needs to be used properly.

I found that mine gave inconsistant results, even though I used black
tape as a target.

Then when I looked more logically, I discovered that there was a
parallax problem where at close distances, the laser target wasn't the
same spot that the IR was reading. The dot might be on the pipe, but the
IR is pointing at the wall behind the pipe.

If yours allows continuous readings while the trigger is pressed, try
holding it sideways and scanning across a hot pipe. On mine ( Maplins
£30 ) at a range of a couple of inches, the temperature reaches a peak
reading when the laser dot has passed the pipe and is shining on the
wall.

I now scan across a small target and look for the peak. It's much more
reliable.