BS Box - 3 Pipe & 2 Pipe Systems.

Please excuse my ignorance.

But I have another few college / improver questions.

On a COOLING only system, I understand the 2 pipe principal - Suction/Discharge.

But on the 3 pipe heat pump system, I am getting slightly confused.

I felt 3 pipes on the return pipe run to the unit a Daikin VRV the other day, and am I right in saying that one is suction, one is hot gas and one is liquid ?.

Is this termed as MIXED phase ?.

On the 4 way reversing valve does it have input from the Hot Gas,Liquid and Suction as described above

In the ceiling voids I was told that there were BS boxes, can anyone explain the term BS,and how these actually work within the system please.

Thanks everyone for any help.

On a 2 pipe VRV system the BS (Branch Selector) boxes are not required, as the outdoor unit will be in either heating or cooling, so refrigerant flow will only be in 1 direction at any time.

On a 3 pipe VRV system the outdoor unit supplies circulation pressures for hot gas, liquid and suction vapour, depending on what the indoor units want.

The 3 pipe circuit connects the outdoor unit to all of the indoor BS boxes. The function of the BS box is to allow the indoor unit(s) connected to it to select either cooling or heating.

If an indoor unit is in cooling then liquid refrigerant is allowed to pass through the BS box from the 3 pipe circuit into the evaporator and then suction vapour is returned to the 3 pipe circuit.

If the indoor unit is in heating mode then hot gas is allowed to pass through the BS box into the evaporator and liquid is returned into the 3 pipe circuit.

If 50% of the indoor units are in heating and 50% are in cooling, then the outdoor coil is not utilised and the compressor just provides circulation. If any one of the indoor units changes mode from this state, then the outdoor coil is brought in to play to handle the imbalance.

Clear as mud??? :D

Thank-you eversomuch Frank, that was the answer I was looking for.
All the very best.
Mike