Heating / cooling an insulated shed

I am planning a work-shed. Garden office, if you prefer. Fully insulated for year-round use.
I'm looking at 2m x 4m (ish) floor plan and 2m walls. 90mm thick Celotex on walls and ceiling, and as much as we can squeeze in on the floor.
My back of an envelope calculation is that a perfect Celotex box of those dimensions needs 250W to maintain a 25 degree temperature differential. IOW in theory one person and a computer can stay comfortable when it's zero degrees outside.
Obv., nothing is perfect, and there will be a number of double-glazed windows and doors.
But I figure a fairly low-power heater would be sufficient.
However... my eyes were caught by various offerings of evaporative coolers at ~70 quid. And I thought it might be nice to have a cool office on those few days of the year that it actually gets hot.
But it turns out that by most accounts, those things are useless. OTOH for a bit more wodge one can get a proper real air conditioner, albeit one where you need to pipe hot air out of a window, using a pipe and a (supplied) blanking plate. And then in the winter you can turn the thing round and use it as a heater.
But then for a bit bit more wodge, you can get a real proper wall-mount air conditioner with external heat exchanger.
The big attraction is that these things can (allegedly) operate in reverse, so that you can get ~3kW of heating for ~1kW of input. Which could, in the long term, save quite a lot of money.
So... has anyone got any experience of this sort of thing? What's the *realistic* cost of heating a shed-like-this over the winter months? Can a heat pump (i.e. air conditioner in reverse) work in sub-zero temperatures? Won't it just ice up horribly?
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Fevric J. Glandules wrote:

I have had them at work. Yes, it does ice up, so the control system runs it in reverse for a while - it melts the ice, but blows cooler air indoors. On balance it seemed to work, though it might be less comfortable in a smaller space.
Chris
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Chris J Dixon Nottingham UK
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Chris J Dixon wrote:

    I have one in the workshop. They work well apart from 0 to 4C. The efficiency drops off below freezing but it keeps the place at a nice healthy 26C IME. The AC is useful in hot weather, a couple of days a year. Very painless to operate.
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On 22/05/14 23:29, Fevric J. Glandules 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.
[1] door and windows frames are likely to leak unless you've put uPVC in or added excellent seals.
I'd work on 1kW peak at the worst times of the year - in or out.
In winter I'd look to maintain it at 10C with rapid rise when required (good convector or fan heater) and 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.
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On Friday, May 23, 2014 8:35:15 AM UTC+1, Tim Watts wrote:

A little crossflow exchanger could much reduce ventilation losses
NT
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On Fri, 23 May 2014 08:35:15 +0100, Tim Watts wrote:

<snip>

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.
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Dave.
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On 24/05/2014 12:01, Dave Liquorice wrote:

I have only had mine since November but it hasn't frozen yet. The temp drop through the outside unit is only about a degree so its not going to get to freezing point very often. If it does freeze then the unit will heat the outside just enough to melt the ice so it can drip off. It makes you wonder why they don't coat it with that water repelent stuff so it all runs off before it can freeze. I might buy some of the spray on stuff and try it.
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Dennis, could you check your computer's clock or timezone? Messages from you seem to come through with the next day's date.
James
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On 24/05/2014 14:46, James Harris wrote:

OK, thanks. Its a new computer and I must have slipped when I set it.
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Dave Liquorice wrote:

Yes, looking at south-facing French windows / doors and another small-ish window on that face. Also east and west facing windows to catch morning and evening sun.
Thanks to all for replies. It's beginning to look like an A/C unit might not just be an indulgence, but might actually be a sensible choice. But I'm particularly interested in running costs / efficiency.
If it takes, say, 500 Watts over the winter months to keep the room warm for, say, 50 hours a week for 30 weeks, that's 750 kWh or ~75 quid. If the heat pump cuts that to a third then that's a saving of 50 quid a year. Over ten years it pays for itself.
Is that a realistic calculation?
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bm wrote:

More a case of how long I have to carry on working to keep Fevric Junior and Fevrica fed and clothed...
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On 25/05/2014 11:33, Fevric J. Glandules wrote:

If its a damp environment a dehumidifier will warm it a bit and dry it out at the same time. You can get quite a bit of "free" heat from drying damp air.
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