Heat Pumps and Geothermal Heat Pumps are excellent cooling systems. In the heating mode these systems are not as good as other systems. When there is a great demand for heat in your home, an electric heater inside the unit comes on. These systems are not recommended for places that have very cold winters.
Though many will argue with me. Many of the same problems that exist for combination units apply here. Furthermore a Geothermal unit is the most expensive unit to install than any other unit. Geothermal unit are excellent in areas where they have warm weather most of the year and use heat infrequently, like Florida and Georgia.
The normal heat pump works just like your refrigerator. It extracts the heat from the outside air by using a liquid refrigerant that is cooler than the air outside. As the liquid refrigerant obtains heat from the outside it begins to vaporize. Hence the term evaporator-coil. Then it goes to a compressor where the refrigerant is pressurized into a high-pressure gas that becomes hot.
This hot gas is then led into a coil inside the home where it gives off heat because it is hotter than the air passing over the coil. As it does this, the gas begins to liquefy. Hence the term condenser-coil. The liquefied refrigerant then goes through some type of metering valve that reduces the pressure on the refrigerant causing it to drop temperature, which is sent to the outside evaporator coil. The cycle is then repeated.
In the cooling mode the cycle and coils function are reversed. The theory here is that opposing temperatures want to equalize. In other words, let's say the vaporized refrigerant captures 100 BTU's of heat from the outside air. When the gas is compressed, it becomes hotter than the air inside the home. When this gets to the inside coil, it expels the 100 BTU's of heat to the home. The reverse is true in the cooling cycle. The biggest problem with an air to air heat pump in the winter is ice. Cold air cannot hold a lot of humidity so its humidity levels are usually high, such as 70 to 80%. What this means is that it does not take a lot of temperature drop for condensation to occur.
So as the evaporator-coil extracts the heat from the outside air, the air around the coil start forming condensation on or near the evaporator-coil. Because of the extraction of heat from the refrigerant, the air around the coil usually drops below freezing and ice forms on and around the coil. It is good to note that ice on a heat pump either in the heating or cooling mode is not suppose to be there.
Unfortunately in the heating mode ice is sometimes unavoidable. Heating elements or reversing cycle usually used to get rid of the ice. In the cooling mode, ice is usually a sign that something is wrong and the unit should be serviced. One of the factors that affect the heap pump in the heating mode is the amount of BTU's available in the air outside during the winter. As the temperature outside begins to drop, so does the amount of BTU's of heat available in the outside air.
However, as the temperature outside drops, the demand for BTU's of heat increases inside the home. In other words, as the outside temperature drops the temperature difference between the refrigerant and the outside temperature decreases but the inside demand for heat increases. The temperature difference determines the amount of heat extracted from the air outside. This causes an electric heater inside the heat pump to come on to meet this demand. Your fuel bills increase dramatically as a result. This is why heat pumps do not work very well in cold climates. There are instances where your only choice is a heat pump. In the case of high rise condos where they utilize several through the wall units.
To me this is a very poor choice. If I were to design a heating system for this application and since the options limited to heat pumps, the type that I would choose would be an air to water heat pump. The distribution system would be a radiant floor system with an electric hot water booster to supplement the heat pump if need be.
The reason for this particular type of system is because radiant floor heating systems have the lowest operating temperature of all other types of distribution systems and the heat is distributed where you feel it the most. The way this system would be designed is the outside coil would capture the heat from the air with a vaporized refrigerant and then sent into the home. This gas would go through a compressor that causes it to become very hot. This will then go into a coil located inside a tank filled with water. The water then acquires the heat captured from the outside, the liquefied refrigerant is then sent to an expansion valve where the cycle begins again. The water in the tank is distributed throughout the home via a pump and series of pipes located in the floor. Then the water is sent back to the tank but before entering the tank the water goes through an electric hot water booster. The booster comes on depending upon the temperature of the water in the tank.
The unique aspect of the booster is that it will assists in the defrosting of the outside coil. And at the same time maintain the temperature of the water in the tank. The reason why the radiant floor system is used instead of baseboards is their operating temperature is lower than for baseboards. This system in the cooling mode is accomplished by diverter valves on the hoses that go to and from a cooling coil.
The geothermal heat pump uses the ground for its source of BTU's. Theoretically, there are unlimited amounts of heating BTU's in the earth. If you understand how this system works, the term thermal mass plays a major factor. Thermal mass is anything that can hold and collect either heating or cooling. In the heating mode, the cool refrigerant is sent into the ground. Where the temperature is constant and not affected by the temperature above ground. Since the refrigerant is cooler than the ground, it extracts BTU's of heat from the ground and brings it into the home. This extraction of BTU's are considered cooling BTU's left in the ground. The fields in which these BTU's are extracted become a thermal mass. At the beginning of the heating season there is a low demand for heating BTU's in the home. This thermal mass of cooling BTU's dissipates easily. As the winter goes on, it takes longer and longer to dissipate. At the peak of the heating season where the demand for heating BTU's will be at its most, this field will probably be at its lowest.