Heat pump pipe sizing - why bother?

[alexdouglas1969]

HELLO!

For example, on a typical split where the expansion devices are located on the outdoor unit, why is there a difference in pipe size to the indoor? Surely if they are both potentially the suction line then any difference in diameter would impact on the operating pressures?

And if not then why not have the same size throughout the system?

[Brian_UK]

Because the outgoing pipe is part of the "expansion" device.

If it was a larger diameter then the refrigerating effect would start somewhere along hte pipe and not at the evaporator.

[nh3simman]

They also operate at different temperatures.
In heating mode, the outdoor unit is cold.

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Don't they put expansion devices at both units? (with a non-return bypass)

[mashan]

If You draw the refrigerant flow diagram You will see that in both of modes (heat/cool) the tighter pipe is pipe with liquid refrigerant, and other pipe is pipe with gas phase. Density is reason for difference of pipe size.
If You take other sizes, You will change the capacity and velocity...

[lana]

Hi everybody,

In heat pump system, every calculation - including pipe sizing - are based on cooling mode. Then when switched to heating mode everything is relatively easier
Cheers

[Peter_1]

I think Mashan is right: the smallest is carrying liquid with some flash gas in it while the largest one is only carrying pure gas. So density is the answer.