[jeff5may]

I'm going to relate this situation to cars.

How you set up your machine depends on how you want to use it. If you are trying to make a top fuel dragster that will do a super fast quarter mile, it will end up much different than one designed for ultimate fuel economy. Every component selected will serve different design goals.

If you want to go fast quick, you will run higher compression and a super-rich fuel mixture. It will drink something like nitromethane, which is uncommon and expensive. The drivetrain and wheels will be sized for maximum torque and power transfer to the road. If at some point during that quarter mile, you feed a little too much fuel, flames may shoot from the exhaust and the engine may explode. That may be OK as long as it happens near the finish line.

For an ultra-efficient fuel saver, you may find that running a higher compression ratio is a good way to develop enough power. That way, you can use an engine that would normally be too weak for your car. With the boost in performance, it uses less fuel than a larger, stronger engine would in the same car. A cheap, abundant fuel will do the job nicely. The drivetrain and wheels will be specified that keep the engine in its "sweet spot" for efficiency at the cost of raw output power. The machine may travel 100 miles or more on the same amount of fuel as the dragster.

Running high discharge (head) pressure and filling your condenser up with liquid is much like the drag racer. You will end up with hotter air coming out of your condenser, even if only half of it is actively exchanging heat, if you slow your air handler down enough. The added compression ratio will kill the efficiency (and maybe longevity) of the compressor, more so if you feed it high suction pressure. More total mass will flow, and lots of liquid subcooling in your condenser will transfer a few btu to the space being heated. Presto! Hyper heat mode has been achieved! Expect a higher electric bill.

On the other end of the spectrum, running lower head pressure will reduce the power draw of your compressor, due to the reduced compression ratio. Somewhat cooler air will be coming out of your condenser, due to the lower saturation temperature. However, due to the reduced refrigerant charge, less liquid in the condenser means more active surface area to change phases and less subcooling. The air handler can be sped up, and the increased airflow carries more btu's to your space being heated. Expect longer run time.

If your indoor temperature is low and the hx is effective, running less charge and head pressure will save you lots of energy in the compressor. In the same manner, running a higher suction pressure and less superheat in the evaporator flows more mass with the same amount of displacement. The newest, super high SEER split units employ a huge outdoor hx and a computer-guided electronic expnsion valve to try to keep as much liquid in the evaporator as it can handle without flooding the compressor. The variable-speed compressors are guided to keep the head pressure high enough to effectively shed heat in the condenser, but low enough not to waste energy.