You can measure the high side pressure to find out. The condensing temperature will follow the high side pressure and vice versa. The corresponding values can be looked up on a PT chart for the refrigerant being used. Any superheated gas leaving the compressor will not fall below its condensation temperature until it has all changed phase. Out of 1000 btu's of heat transfer, 950 or more of those btu's flow as the gas condenses.

On the water side of the heat exchange, it matters which way the water flows versus the refrigerant in the HX. Counterflow yields more heat transfer. Usually, the water will exit a few degrees cooler than the entering refrigerant.

In ChetT's proposed mod to his existing system, a BPHE could be rigged between the outlet of the slab and the inlet of the boiler. When the system first fires up, the water entering the BPHE would be slab dwell temp, and the heat pump would run at max energy savings to preheat the water entering the boiler. As the HP ramps up, the boiler could bridge the gap in capacity. Once the HP provides enough capacity, the boiler could stop for the rest of the cycle. Short cycling the boiler would not cause any adverse side effects.

Sorry , do not know .

My MS's do not have an outside service port ( do not know about inside the cabinet ) for the small line ( high pressure when cooling ) . Otherwise you could measure the pressure and interpolate a rough temp range .

God bless
Wyr

This gizmo is also known as a desuperheater. They work better in cooling season than heating season.

Been there before. See post #8 in this thread. Most all mini-split units only have one service port per zone. Each port floats at whatever pressure the indoor unit is operating. Since the metering device (or devices if multiple circuits exist to connect to multiple indoor units) is inside the outdoor unit, both lines leading indoors are at the same pressure, minus any small pressure drop the indoor HX induces.

http://www.securusair.com/images/sol...ow-diagram.jpg

As you can see in the illustration, the indoor HX is plain jane as can be. The only elaborate component is not shown: the control system. The two sensors and fan speed control do all the magic indoors. As discussed before, rigging the fan control to run a water circulator and placing the thermometers to jinx the outdoor unit is highly possible. In heating mode, defrost sensing of the indoor coil is a moot point. The vane motor is usually a slave device, and the "pipe" sensor tells the outdoor unit if there is an airflow problem: if insufficient airflow exists, the discharge temperature will not taper off, due to lack of heat dumped. At a certain limit, the control trips on over-temperature.

Jeff,
Maybe you don't need to move any temperature sensors. You can use the existing radiant slab thermostat. When the system calls for heat you could fool the inside head unit of the mini/s by faking the ambient temperature sensor input with a low temp resistance. Then the head unit would instruct the compressor to turn on.