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Heat pump outdoor units: breathing your own exhaust

The unit's performance is not the catalogue figure but the figure at the temperature of the air it actually draws. Siting sets that temperature.

A heat pump or air conditioner outdoor unit delivers its catalogue performance only at a particular ambient temperature. The performance achieved in the field is often lower, and the cause is not the unit but the siting.

Short-circuiting

The unit discharges the air it has passed over, heated (in cooling mode). If that discharge finds its way back to the intake, the unit is breathing its own exhaust. The intake temperature rises, the condensing pressure rises, capacity and efficiency fall; on hot days the protection may cut in and the unit stops.

In heating mode the mechanism is reversed but the outcome is the same: the unit draws the air it has just cooled, the evaporating temperature falls and frosting starts early.

Typical siting mistakes

  • Mounted too close to a wall. The discharge reflects off the wall back into the intake.
  • Installed in a recess or a shaft. The warm air cannot rise and accumulates.
  • Several units in a row. The discharge of the front unit becomes the intake of the one behind. It worsens down the row.
  • Wind direction. A prevailing wind can carry the discharge back towards the intake.
  • Louvres and decorative screens. They add resistance and change the discharge direction. See the fan operating point note.

Modelling

The analysis question reduces to a clear quantity: how much higher the mean air temperature at the intake face is than the ambient. That difference converts directly into a capacity loss.

In the model the unit is represented without resolving its blades: a region delivering a known flow rate and heat rejection, plus a fan curve. What has to be solved is not the inside of the unit but its surroundings — neighbouring walls, other units, wind and natural convection.

At low wind speeds buoyancy dominates; an isothermal model misses the short-circuiting entirely.

References

  1. ANSI/AMCA Standard 210 / ANSI/ASHRAE Standard 51 — Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating. AMCA International / ASHRAE.
  2. Churchill, S. W. & Chu, H. H. S. (1975). Correlating equations for laminar and turbulent free convection from a vertical plate. International Journal of Heat and Mass Transfer, 18(11), 1323–1329. DOI: 10.1016/0017-9310(75)90243-4

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