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Air-con: recommended min usage to keep in good order.

Discussion in 'Car Repairs / Maintenance' started by mointainwalker, 18 Mar 2013.

  1. alan333

    alan333

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    Manual air conditioning and climate control are the same thing. It's just that one is controlled via manual controls by hand, and the other via automatic sensors.

    I stick by my original post that A/C can be used hot or cold and is very effective at demisting windows (except if the ambient temp is below 3 degrees or so when it doesn't work, even tho the light comes on).
     
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  3. mointainwalker

    mointainwalker

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    alan

    Please help me with my ideas. I have only ever used a/c for cooling

    1) As far as I know the a/c i.e. compressor only produces cold air - is that correct ?

    2) Since the air seems rather cold, blending it with the max available heat in the heating system, would result in a fairly anaemic heat, if any - agree or not ?

    This may be fairly academic for me- at the moment anyway - for December to March will usually be below + 3 C here, ( N Alps ). Hopefully April will give us some warmth.

    EDIT

    Are you saying that a/c will not take moisture out of the interior air when exterior is at 3 C or less ? ?
     
  4. Peter.N.

    Peter.N.

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    I would think that 7% is specific to one particular car because its obviously not going to be the same with a 1000 cc car weighing 15cwt as it is with a three litre weighing 2 tons.

    Peter
     
  5. mointainwalker

    mointainwalker

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    @ Peter

    No not specific, just a rule of thumb in the same way that roof-bars are "said" to also add 7% or open windows 4%.

    Just intended as an indication of magnitude.
     
  6. alan333

    alan333

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    I'm not remotely an expert but my understanding is that air con works by changing liquid to gas and back again.  Liquid absorbs heat and then is compressed to a gas state where it turns colder therefore releasing the heat it just stored.  The cycle is then repeated endlessly.  So the system doesn't really "produce cold" but rather "removes heat".  Gas also doesn't retain as much moisture as liquid so I assume this is where it dries the air as it goes along.  I don't quite understand the same point as you tho ie why doesn't the cooling effect contradict the heating effect - but it doesn't. The system can be reversed somehow to produce heat, like refrigeration units on delivery trucks, or heat pumps for central heating/cooling.  Perhaps the answer lies there?

    My knowledge of whether A/C works below a certain temp comes from a car I previously owned in which the owner's manual said it wouldn't work if the ambient temp is under 3 degrees (I haven't read the Avensis manual closely enough to know if it states similar).  However that would contradict the above suggestion of reversing the system.  I've also heard that having A/C on when the engine is cold causes heat to be produced quicker by the same "reverse" procedure.  Perhaps this would only work when ambient temp is high enough tho.

    Sorry I can't be more helpful but my experience of A/C is that it made no difference whatsoever to fuel consumption on any car I've owned, and that it is excellent for drying the air.  Try it on a damp night when your windows begin to mist slightly.  Turn it on and you'll find it defogs even the rear window in seconds.
     
  7. Burnerman

    Burnerman

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    As I understand it, the A/C only works to cool when the interior fan is on, and the cabin temperature is turned down as far as it will go.
    Turning the heat up stops the cooling process but does allow the circulating air to dry (hence the pool of water underneath the car when it stops).
    When the ambient temperature is low, the drying effect will be much less - as per the domestic dehumidifier.
    John :)
     
  8. mointainwalker

    mointainwalker

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    alan

    " The system can be reversed somehow to produce heat, like refrigeration units on delivery trucks, or heat pumps for central heating/cooling. Perhaps the answer lies there? "

    Have to disagree with you there. This is a very basic system and could no more be reversed to pump heat into the car than your fridge could become a cooker.
     
  9. Peter.N.

    Peter.N.

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    We have a heat pump for heating our bedroom, by reversing the direction of gas flow you can make the evaporator become the condenser and vice versa. its the energy used to compress the gas into a liquid, that causes it to get very hot - bicycle pump effect - the heat is then lost in the condenser in front of the radiator in a car, the liquid then goes into the evaporator which is in the heater in a car, through a very small tube, as it leaves the tube it turns back into gas again and therefore takes most of the heat that was lost in the condenser.

    Any liquid takes heat to make it evaporate, spill some meths or petrol on you hand and blow air on it, it feels cold, that's why you feel cold when you are wet and why sweating cools you, its all the same principle. If you were to lower the air pressure sufficiently you could make water boil at room temperature but it would lose heat rapidly.

    This is the principle of refrigeration. A heat pump is a very efficient way of heating because as already mentioned it doesn't produce heat but moves it from one place to another. Our bedroom heat pump under ideal conditions uses about 700 watts of electricity to produce 2500 watts of heat but when the temperature outside is near freezing it has to work much harder and its efficiency drops considerably. Now think how hard the heat pump (aircon) has to work in your car on a very hot day and you can see why it uses a lot of power from the engine.

    Peter
     
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  11. mointainwalker

    mointainwalker

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    @ Peter N

    Thanks for the basic physics lesson but I really don't need to be told the principles of a heat pump.

    What I am pointing out that as they are constructed and normally used neither a car a/c unit nor a fridge can be reversed in function .
     
  12. Peter.N.

    Peter.N.

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    You may not but others might.

    Peter
     
  13. alan333

    alan333

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    Ok
     
  14. Avocet

    Avocet

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    I've done a bit of work on AC systems. I don't know whether the following might be of any use but here goes:

    1. The 7% figure will have come (almost) out of a hat! Aircon systems are like domestic fridges and they have various thermostats in them. Different systems work in different ways, but they all have some means of preventing the compressor from running if the temperature of the evaporator (the bit that gets cold) drops low enough to freeze the condensation on it. IF that happens, it quickly turns into a block of ice and you don't get any air into the cabin until it has defrosted. Because of that, the pump constantly cycles in and out - cutting in when the evaporator temperature gets above about (say) 5 degrees C and cutting out when it gets down to (say) 1 degree. As a result, it goes without saying that in cold weather, the aircon system won't use anything LIKE 7% more fuel! In hot weather, it's likely to use more than that. Most are designed to cope with about 40 degree ambient air temperatures, so none of them work at full capacity in the UK. Also, as has been said, driving with the windows open also carries a fuel consumption penalty, so it's a trade-off. Again, that penalty varies with speed, but general consensus amongst manufacturers is that at motorway speeds, even in hot weather, you're probably better off using the aircon rather than opening the windows.

    2. No you don't get "anaemic heat"! Because of the way the system cuts in and out, the air coming off the evaporator is never likely to be less than about 1 or 2 degrees. I can't think of any healthy car whose heater couldn't cope easily with that sort of incoming air temperature, because even if they don't have aircon, we regularly see that sort of temperature in winter. Simple thing is just to try it!

    3. My last car (Peugeot) owner's manual tells me the system needs to be run for at least 15 minutes a month. This is partly to keep the seals lubricated but also because the problem with the current refrigerant is that it doesn't mix well with its lubricating oil. The oil tends to separate-out if the system isn't used, so the compressor suffers abnormally high wear on startup after a prolonged period of inactivity. Having said that, it's worth not getting too hung-up about it. In a hard winter, even if you keep the "switch" on all the time, the anti-icing feature mentioned in 1 will probably prevent the compressor from engaging if the ambient air temperature is below freezing, so we might THINK our aircon is running, but actually the compressor isn't turning! Remember that they have aircon cars in places like Sweden where the ambient temperate won't get above freezing for months at a time!

    4. Yes, it's great for demisting because warm air can hold much more moisture than cold. As the moist air passes though the evaporator, it cools and dumps the surplus condensation that it can no longer carry. It is then drier when it passes over the heater matrix (where it is warmed up again) so that it carries much less moisture into the cabin. The downside (as has been said) is that the condensation gets dumped into the heater box - which is supposed to have drain holes, but you do (especially on cars where the aircon is used all the time in humid climates) end up with musty smells on startup. You can buy various cleaning kits from Halfrauds and the like.
     
  15. mrcabrach

    mrcabrach

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  16. Burnerman

    Burnerman

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    Useful tip about spraying stuff into the pollen filter housing....I guess that Dettol spray would be even more scientific!
    Nice one
    John :)
     
  17. Avocet

    Avocet

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    I'd be a bit wary of things containing bleach - depending on where the pollen filter is relative to the other parts of the vehicle. Some have foam seals on the various flaps that direct the air and I've a feeling bleach splashes (or concentrated vapour) might damage the foam in the long term.
     
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