Air source heat pumps....

are a bit useless, as when you need most heat, they work less well and at about minus 5, pack in altogether. This down to the refrigerant gas used.
However there is a new technolgy one out now, runs on CO2 refrigerant and works to much lower outdoor temperatures. Sanyo.
http://www.airconwarehouse.com/acatalog/SANYO_CO2_ECO_Brochure.pdf
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No it's not, it's down to the outside unit (evaporator) icing up. Mine won't work effectively between about +5C and 0C outside because it has to keep running defrost cycles, but once it's got down to -2C and lower, it again works fine, because most of the moisture has already dropped out of the air by then as frost or frozen fog, and doesn't clog the evaporator. IIRC, I tried it down to something like -8 or -10, and it still chucks out lots of heat inside.
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Seems to me that a DIYer could make a heat pump by just compressing air inside and letting it expand outside. It doesn't even have to recycle the air - just get some fresh air.
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wrote:

It needs to be a closed cycle to pump heat from one side to the other.
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wrote:

It needs to be a closed cycle only if you want to reuse the refrigerant. If it's air at high prssure, just let it go outside and get some more air.
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On Wed, 22 Jun 2011 01:57:26 -0700 (PDT), Matty F wrote:

I can see the thinking but it can't be that simple can it?
Yes when you compress air it gets hot so extract that heat before letting it go outside. But where has the energy that has made the air hot come from? Do you get significantly more energy out of your "hot air cooling radiator" than you have put in via the compressor?
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Dave Liquorice wrote:

well there's the rub!
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wrote:

I believe we are on to something here. The motor of course uses energy. I'd have a motor that uses about a kilowatt and compress air from inside the house and run that through radiators, then outside to a nozzle where the air escapes and gets very cold. Need to allow for icing up, and the hissing noise it would make. My house is leaky enough that more air will come back in. Or air could be piped in past a radiator. I don't need a lot of heating. It's winter here and I have not used a heater so far this year.
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Matty F wrote:

you ill indeed see gains. The clue is that the air going out is colder than outside.
It is in fact an air sourced heatpump.
Instead if the 'warm' air heating the regfrigerant you simply use the warm air outside itself.
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In article

Which will enter from the outside and so will be cold.
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Tim Streater wrote:

But not as cold as the escaping air from the outlet.
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wrote in message

See, I said you didn't do the physics as is obvious from your posts. Now go and look at what heat pumps do and stop answering with wrong answers.
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dennis@home wrote:

talking to yourself again?
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wrote in message

No.. to collect heat from the outside it has to expand and cool below ambient temperature. It then heats up in the heat exchanger that is outside.
You then compress it which makes it hotter and push it through the internal heat exchanger where it cools.
Then you expand it again, repeating as required.
Now it would be possible to take in fresh air, expand it to cool, heat it in the heat exchanger and then compress it and release it in the inside exchanger. However it has problems, including..
no lubrication for instance unless you plan on throwing away the oil every cycle. condensation inside the expansion vessel as it cools. noise caused by the release of the compressed air. more difficult to control the flow rates so the air expands/compresses in the right place.
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In the OP the refrigerant was CO2. Does that condense? There are a number of compressor types that don't need oil.
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There is no need for it condense. If you expand a gas it cools (Boyles law), it is then colder than the outside air so will absorb heat from the outside. Then you have to pump it into the inside and compress it, this heats it up and it releases the heat it had absorbed. If you don't expand it first you wont get much heat transfer.
It is somewhat easier to make a system work if it does condense as it is easier to control the flow rates, all you need is a capillary tube.

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On Wed, 22 Jun 2011 22:06:36 +0100, dennis@home wrote:

not Boyles law - that relates P and V noT involved lol
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wrote:

So which one is PV/T = constant then?
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Ghostrecon wrote:

Dont worry dennis' pretty little head over actual facts.
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Matty F wrote:

its a simple physics problem. There are three paths into and out of the system.
air in at outside temp. air out, colder than outside. electricity in.
Total heat is electricity in + (specific heat of air times air flow rate times temp difference between inlet and exhaust air). As long as that difference is there, then some heat has been pumped from outside to inside.
How you do it, is only a matter of efficiency.
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