On Fri, 23 May 2014 08:35:15 +0100, Tim Watts wrote:

Don't forget to factor in ventilation - you should have a small amount
of trickle ventilation available either by design or by accident[1] and
air changes do add to heat loss quite significantly on an otherwise well
insulated building.
snip
... in summer if you are not opening windows, you need to factor in the
heat produced by your body and any equipment your are using.

And solar gain through the windows, particularly any that catch the
sun. I'd say that cooling will be required in the summer.

A heat pump that can work in either direction seems the obvious
choice and if this shed has a couple of south ish facing windows
around a couple of square metres and 3 kW unit may well not be to far
off the size required to keep the place cool.

An air source heat pump won't ice up at below freezing outside air
temps as the air will be pretty dry. The problem is air temps above
freezing to around 5 C when it can have considerable water vapour in
it, that condenses on the collector when it's below the dewpoint and
then freezes when the collector is sub-zero.

--
Cheers
Dave.