Why does the outdoor fan on a heat pump need to run on heat mode?

[fourthindiana]

The outdoor fan on heat pumps always runs when the compressor is pumping refrigerant when the heat pump is in heating mode unless the heat pump has defect(s). I don't understand why the outdoor fan on a split system heat pump needs to run when the heat pump is in heating mode.

When a split system heat pump is in heating mode, the outdoor coil is the evaporator coil. When a split system heat pump is in heating mode, the outdoor coil absorbs heat from the outdoor air. I don't see how the outdoor fan would help the outdoor coil absorb heat.

If anything, it seems to me like the outdoor fan would blow heat away from the outdoor coil. I cannot think of any reason why it would make a heat pump more efficient or otherwise help heat a house or building to have the outdoor fan running on the outdoor coil when a heat pump is in heat mode.

Why does the outdoor fan on a heat pump need to run when the heat pump is in heating mode?

 

[Averagesupernova]

Why do you assume that the evaporator coil will lose heat when operating with the fan running? The evaporator coil MUST be colder than the ambient air in order to take heat from the ambient air. Blowing air over the coil that is warmer than the coil will warm the coil, which is what we want. Why would we NOT want to blow air over the evaporator? The reason we call this type of refrigeration absorption refrigeration is because we ABSORB heat and move it elsewhere. Sometimes we want the heat, sometimes we don't. In the case of a heat pump, we want the heat and move it indoors.

 

[CWatters]

+1

The evaporator extracts heat from the air. That cools the air making it harder to extract more heat from it. The fan replaces the cold air with warmer air.

 

[russ_watters]

Both the indoor coil and outdoor coil have fans and both run in both modes, for the same reason (to increase heat transfer through the coil).

 

[cjl]

As you pull heat from the outdoor coil, it gets colder. Blowing air over a cold coil helps warm it back up.

 

[DaveE]

Still air is a poor conductor of heat, it will act sort of like insulation between the coils and ambient air. One reason for the fan is to keep the air moving to physically transport the heat away instead of allowing it to diffuse through still air.
For example, think about how it feels to be outside on a cold day with or without a breeze blowing.

 

[Rive]

Just one more point of view.
The efficiency of the heat pump depends on the temperature difference of the end points, while the heat pump itself is actively working to make it worse (taking heat from the cold side making it colder: deliver heat to the hot side, making it warmer).
To prevent the efficiency fall the temperature of the end points must be maintained, forcibly if nothing else works well enough. Thus: fans and coolers.

 

[russ_watters]

Still air is a poor conductor of heat, it will act sort of like insulation between the coils and ambient air. One reason for the fan is to keep the air moving to physically transport the heat away instead of allowing it to diffuse through air.
For example, think about how it feels to be outside on a cold day with or without a breeze blowing.

Typo, but otherwise yes.

 

[russ_watters]

Just one more point of view.
The efficiency of the heat pump depends on the temperature difference of the end points, while the heat pump itself is actively working to make it worse (taking heat from the cold side making it colder: deliver heat to the hot side, making it warmer).
To prevent the efficiency fall the temperature of the end points must be maintained, forcibly if nothing else works well enough. Thus: fans and coolers.

I'll add to that, that fans use energy too and thus add their own inefficiency to the system. A balance has to be maintained to find the optimal efficiency; too much airflow and the fan energy loss is higher than the heat transfer gain.

One way to improve things is with larger heat exchangers. They provide more effective heat transfer and less resistance to the airflow, enabling more heat transfer with lower fan power. This is why, if you've recently replaced an air conditioner, you notice the new unit is physically larger than the old one.