How hard a heat pump compressor has to work is directly proportional to the "temperature lift" it has to produce. The greater the temperature lift, the harder the compressor has to work and the more power is needed to run it. Looking at the compression ratio the compressor has to achieve to produce the desired temperature to deliver the heating or cooling required is the compression ratio of the compressor.
The first two charts (in Imperial units and metric units) show the conditions of a 4-ton (14 kW) water to water heat pump system designed to deliver 120°F (48.9°C) to heat a building and 42°F (5.6°C) chilled water to cool a building. These hot and chilled water temperatures are commonly used to deliver heating and cooling with air handling units and/or fan coil units. Dividing the heat pressure of the compressor by the suction pressure calculates the compression ratio of the compressor. In the first two charts the heat pump delivering 120°F (48.9°C) water has a head pressure of 480 psi (3,309 kPa) and a suction pressure of 73 psi (502 kPa). This equates to a compression ratio of 6.57.
The third and fourth charts (in Imperial and metric units) show the operating conditions of the same 4-ton (14 kW) water to water heat pump designed to deliver 85°F (29.5°C) to heat a building and 66°F (18.9°C) chilled water to the building. These temperatures are appropriate for delivering heating and cooling to a building using a radiant floor heating and cooling system or for cooling with radiant panels or chilled beams. The head pressure in the heat pump delivering water at these lower temperatures is 307 psi (2,116 kPa) and the suction pressure is 74 psi (507 kPa)...a compression ratio of 4.14.
The lower compression ratio imparts less stress on the compressor components and a much lower energy input...2,793 Watts compared to 4,296 Watts...a reduction of 35% when the heat pump is producing hot water, and 2,926 Watts versus 3,528 Watts...a 17% improvement when cooling.
Note that the heating capacity of the heat pump increases by approximately 6% and the cooling capacity increases by 58% with a higher water delivery temperature to a chilled beam system and a lower GHX temperature.
These charts are based on the operating parameters of a water to water heat pump system. The same is true of a water to air heat pump system. Lower air flows mean the heat pump will deliver higher air temperatures in heating and lower air temperatures when cooling. This translates into lower operating efficiency, lower capacity and higher compression ratios. And higher compression ratios eventually mean earlier compressor replacements.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.