[jeff5may]

It all depends on the air or water flow and the temperature gradient driving the heat transfer through the exchanger. With a water loop or well outside, the ground temperature and pump size pretty much dictate the minimum size for that HX. Indoors, the main function of the unit will dictate what size and type of HX to employ. For example, if your main goal is hydronic slab heating, a larger plate HX and circulator pump will extract many more Watts of heat at a lower condenser temperature than if you're making hot tap water. The hundred Watts or so of pump power used to circulate slab water (versus a DX coil in a hot water tank) is usually well offset by the increased COP of the refrigerant cycle.

With an air source HX, the outdoor unit should have an expansion valve in it when it is being employed as an evaporator. This allows the unit to draw much more heat in when outdoor air temperature is higher. A cap tube reacts to changing conditions much more slowly. Whenever outside temperature drops below freezing, the TXV can get you the extra heat gradient needed to drive heat flow. A cap tube is optimized for a very narrow set of conditions. Outside of these conditions, the cap tube system is leaving potential efficiency on the table.

However, adding expansion valves and check valves adds complexity to the refrigerant cycle. There are sensors and moving parts included that a cap tube rig simply doesn't have. When things go wrong, a cap tube rig is much easier to troubleshoot. With a small capacity system, the energy saving potential is not as significant, so most manufacturers use cap tubes to minimize cost and complexity. When the real world conditions stray outside the unit's design window, the units simply shut down.

On the other end of the spectrum, lots of newer units employ electronic expansion valves. The refrigerant cycle becomes less complex, as the EEV can control flow in both directions, so there is no need for extra mechanical components. The EEV serves the same function as a TXV, but it is electronically controlled. Instead of a pressurized sensor bulb balancing the opening and closing of the valve against spring pressure, an EEV relies on a digital controller. The digital controller connects to temperature and/or pressure sensors, interprets the observed conditions, and adjusts the EEV accordingly. As you may suspect, the controller can be programmed to behave however you want it to.

In many modern heat pump units, the programming of the controller is the main (and sometimes only) cause for differences between units. This is increasingly prevalent in mini-split and inverter-compressor driven units. Many ecorenovators have purchased different size mini-split units of the same brand, and found little to no difference between the outdoor units. Some of them even had the same part numbers on key components, or identical outdoor units.