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?

Fevric J. Glandules wrote:

Can a heat pump (i.e. air conditioner
in reverse) work in sub-zero temperatures? Won't it just
ice up horribly?

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
--
Chris J Dixon Nottingham UK


Plant amazing Acers.

On 22/05/14 23:29, Fevric J. Glandules wrote:
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.

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.

On Friday, May 23, 2014 8:35:15 AM UTC+1, Tim Watts wrote:
On 22/05/14 23:29, Fevric J. Glandules wrote:
I am planning a work-shed. Garden office, if you prefer.
Fully insulated for year-round use.

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.

A little crossflow exchanger could much reduce ventilation losses


NT

Chris J Dixon wrote:
Fevric J. Glandules wrote:

Can a heat pump (i.e. air conditioner
in reverse) work in sub-zero temperatures? Won't it just
ice up horribly?

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

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.

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.

"dennis@home" wrote in message
eb.com...

Dennis, could you check your computer's clock or timezone? Messages from you
seem to come through with the next day's date.

James

On 24/05/2014 14:46, James Harris wrote:
"dennis@home" wrote in message
eb.com...

Dennis, could you check your computer's clock or timezone? Messages from you
seem to come through with the next day's date.

James


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

Dave Liquorice wrote:

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.

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?

"Fevric J. Glandules" wrote in message
...
Dave Liquorice wrote:

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.

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?

Depends how long you think you'll live
Personally I'd pay the £75.

bm wrote:

"Fevric J. Glandules" wrote in message
...

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?

Depends how long you think you'll live
Personally I'd pay the £75.

More a case of how long I have to carry on working to keep
Fevric Junior and Fevrica fed and clothed...

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

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.


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.

On 25/05/2014 11:33, Fevric J. Glandules wrote:
bm wrote:

"Fevric J. Glandules" wrote in message
...

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?

Depends how long you think you'll live
Personally I'd pay the £75.

More a case of how long I have to carry on working to keep
Fevric Junior and Fevrica fed and clothed...


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.