[jeff5may]

This rig you have going will be an awesome test bed to aid you in understanding all the different factors at play in the system. You already have some time under the hood with different compressor capacities and the effects of changing cap tube size on the system as a whole. These pieces of the puzzle are of great importance, and knowing what to expect when changes happen is vital.

When you swap out that cap tube for a txv, try not to forget about watching those temperatures and pressures in the various places in the system. The constant flow properties of a cap tube make it fairly easy to get the system charge close to where it needs to be with some trial and error. With a txv in the system, it automatically changes its flow to keep the evaporator happy. This behavior is not always good for the rest of the system.

With the heat pump water heater, you had a design goal of a certain high temperature the unit built up to slowly. As the water tank temperature rose, the COP of the unit fell. You found a cap tube length that didn't flood the evaporator and adjusted the charge to produce hot discharge gas. Not so difficult, right?

With this setup, you should have a large enough evaporator to move as much heat as the compressor is able. The txv will make sure the compressor is always getting fed well. However, on the condenser side, the goal is not necessarily high discharge temperatures and pressures. Changing the system charge upwards (adding gas) will yield you higher discharge temperatures that you may not need or want. After a certain point, your compressor will be drawing extra power without moving any more heat.

The thing is, the txv can sense the extra pressure from the added gas, and adjusts the flow rate to compensate. It doesn't care how much energy is being wasted on the high-pressure side, it is taking care of the evaporator.