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Air conditioner's coil Blower of an air conditioner Blower of air conditioner

I observed the fan on external units of air conditioners does not force air towards the hot coil containing hot liquid to make it cooler. Instead it draws air from from the direction of the coil it and blows it away. In my opinion, it would be more effective if the air from the fan was blown toward the coil rather than the fan is drawing air from the coil and blowing it away.

Then my question, why?

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  • $\begingroup$ Surely it is temperature difference that will make the heat move? $\endgroup$
    – Solar Mike
    Dec 5, 2019 at 7:41

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You are correct. In terms of fan efficiency, it is more efficient to force cool air onto or through a hot zone, than to draw hot air from a hot zone and expel it to the atmosphere.

With the external unit of split system air conditioners (heat pumps) you have to consider other things.

The current arrangement may be easier and/or cheaper to manufacture.

Also, under the current arrangement, the external units of split system air conditioners is compact and thus requires occupies less space where they will be located. Also, the fan is located at the front of the unit, away from the wall. When it expels the hot air from around the coil it does so freely, without any obstacles impeding the air flow.

For the fan to blow cooler air onto the coil two situations are possible. Firstly the fan could be at the front of the unit drawing unimpeded air from the atmosphere. It then blows that air onto the coil forcing the hot air around the coil to move. That forced hot air will have to negotiate any internal obstacles within the unit and could hit the wall of the building, which may restrict the movement of the air away from the air conditioning unit.

The other possibility is to locate the fan at the rear of the unit drawing in air from the narrow gap between the wall of the building and the unit, which is restriction to airflow, and forcing it onto the coil. Again, that forced hot air will have to negotiate any internal obstacles within the unit - a further restriction to airflow - before exhausting to the atmosphere, at the front of the unit.

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  • $\begingroup$ Not forgetting if the fan is pulling air there's the risk of pulling some small birds (not a good idea) and that any close particle in suspension would be pulled too (if by chance pulls something similar to a rope things will get ugly). $\endgroup$
    – Leafk
    Dec 5, 2019 at 15:51
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Putting the fan behind the coil makes it harder to stick a finger into the fan arc. Positioning the fan as close as possible to the coil and shrouding it on the sides minimizes the efficiency difference mentioned in the comments above.

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  • $\begingroup$ Two closely identical answers... $\endgroup$
    – Solar Mike
    Dec 5, 2019 at 18:03
  • $\begingroup$ should I delete? $\endgroup$
    – niels nielsen
    Dec 6, 2019 at 4:13
  • $\begingroup$ Add a few words and make a third... what do you think? How about just keep the best one... $\endgroup$
    – Solar Mike
    Dec 6, 2019 at 5:25
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Putting the fan behind the coil makes it harder to stick a finger into the fan arc. It also directs the air blast directly over the fan motor, improving its cooling. Positioning the fan as close as possible to the coil and shrouding it on the sides minimizes the efficiency difference mentioned in the comments above.

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In my SF Bay Area climate, a mini-split heat pump is mostly providing space heating. We have only a short cooling season of 60 hours per year and a heating season of 3000 hours per year. Also our yards are small and our noise ordinances are tight.

So I think a good Bay Area design would have the outdoor unit be arranged with fan 6"-8" away from the wall, blowing (and warming with "fan heat = watts of fan power usually adding 1-2 degrees F ") the air toward the coils which extract the heat from the air. The coils and fins also muffle the sound of the compressor and fan. In fact the fan (being close to the house wall) is moved another 12" away from the property line where the noise ordinance measures sound. An acoustic tile could be placed on the wall to absorb any sound rather than reflecting it. Compressor heat loss occurs (between the fan and the coils before the air reaches the coils so in this arrangement the coils also capture compressor heat loss to the moving air. Voila! a quieter and more efficient outdoor compressor unit. But it has the small penalty of running the condenser 1-2 degrees warmer for the 60 hour cooling season.

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The primary concern is to protect the unit from environmental damage and groundskeeping rash. So the coils go to the building face. This also helps with noise abatement inside the building. And by pulling the front cover, you can get at the mechanicals. So the general arrangement is fixed by toughness considerations. A suction fan isn't less efficient here given the very low temperature gains associated with the condenser air. And it blows the hot air away from the building.

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