Not a term I'd come across before, but a quick search suggests that "high heat retention" simply means that they have better thermal insulation (so they keep the heat inside for longer when it's not needed) - so a feature that's likely to be in LOT 20 devices, but LOT 20 looks at overall efficiency and will tend to include lots of other strategies as well (such as open window detection).

   - Andy.

Thanks Andy.  I was hoping that was all it was. Every time i searched the LOT20 heaters kept popping up.

Another way of looking at "high heat retention" is "low output".

Another way of looking at "high heat retention" is "low output"

Or controlled heat output? - doesn't leak out when it's not needed, but presumably can still be extracted (e.g. by fan) when it is.

   - Andy.

Chris Pearson said:

Another way of looking at "high heat retention" is "low output".

The modern storage heaters have a fan in them.  With the fan off, they keep the heat in.  If it gets chilly in the evening, turn the fan on to waft the heat out, when you want it.