TM TOPIC: Heat Pumps GLOBAL PROPERTY INSPECTIONS TECHNICAL BULLETIN August 1, 2002 This is the eighth in a series of technical bulletins in which an over-all examination of the home will be discussed as viewed through the inspection process. Some of the items in this series will not be able to be seen through the visual process of inspection, but will be presented to give a better understanding of how a home is constructed, the mechanical and electrical systems and items to address during the inspection process. In this series of bulletins we have discussed the basic components and construction practices for building a home starting with basic foundations and following through with floor, wall, and roof framing. In this bulletin we will briefly discuss basic types of Heat Pump Systems, how they operate and what an inspector looks for during the course of an inspection. A heat pump is a single heating/cooling system unit that has the capability to provide warm air in the winter months for areas that have colder winter temperatures as well as cool air for the summer months like an air conditioning system. A heat pump is basically a system that has the capability to work in reverse. A heat pump has similar or the same components as a centralized air conditioning system with the exception of one major component, a reversing valve.
Heat Pump Evaporator Coil at the Outside Unit In the heating mode heat is collected and absorbed into the evaporator coil at the outside unit. Condensing Coil at the Inside Unit
Remember: An air conditioning unit collects and removes heat from inside the home. Through the heat transfer process cool air is dispersed back into the home. With a heat pump system heat can be collected and transferred from outside air even when outside ambient temperatures are colder to produce heat to the inside of the home. Key Component Reversing Valve The main difference between an A/C unit and a heat pump unit is a component called the reversing valve. The reversing valve changes the direction and flow of Freon (Puron) gas within the system. Inspection Tip: When conducting a visual inspection and determining the difference between a standard A/C unit and a heat pump, look for the reversing valve. They are typically located inside the outside unit.
Heat Pumps and Efficiency A heat pump can be a very efficient unit based local (area of the country, northern and southern climates) and outside temperatures. Heat pumps located in southern climates can be more energy efficient than a standardized gas or electric forced-air HVAC system. When the outside ambient temperature falls to the 32 degree F. and the temperature starts to fall inside the home, a heat pump begins to become less efficient and more dependent on an auxiliary back-up unit, which all types of heat pump systems have. At 70 degrees inside the home a heat pump will begin to run constantly in order to maintain a comfortable temperature. As the outside temperature falls to 32 degrees and even colder, a heat pump is running and relying on the auxiliary heating unit to provide and maintain comfortable indoor temperatures.
Various Types of Heat Pump Systems All Electric Heat Pumps With an all electric heat pump unit the back-up or auxiliary unit consists of electric heating elements, the same or similar as an all-electric forced-air furnace. The cost to operate these back-up units can be expensive because of higher energy consumption. They are typically smaller in size and have to run more constantly to adequately keep the home at comfortable temperatures. These types of units are classified as air to air systems. In the winter mode they can collect and convert heat from outside air temperatures and convert it and transfer it to the inside to heat the home. In the summer mode a heat pump functions an operates just like a standard A/C unit removing heat from inside the home and converting and recirculating it to cool air back inside the home. Water Source Heat Pump Water source heat pumps works on the same principles as an air-to-air system heat pump with the exception that instead of collecting heat from an air source, they collect heat from a water source that is typically run underground either in an open or closed loop system. These types of
systems can be more energy efficient because of constant 40 to 50 degree underground temperatures, which allows less dependency on an auxiliary back-up unit. Remember at 32 degrees an air-to-air system becomes more dependent on back-up heat. On the previous page an open-loop water ground system utilizes to separate wells to collect and disperse the water into. Heat energy can be collected from the temperature of the water. Ground Source Heat Pump Ground source heat pumps operate on the same principles as a water source heat pump with the exception being the water source is contained in a closed loop continuous piping system. Water is circulated through the system by a compressor and heat energy is collected at the heat exchanger unit typically found inside the home. Ground temperatures are typically 40 to 50 degrees year around at 4 to 6 feet in depth, which just like a well source system, can collect and transfer heat to the home. Closed loop piping systems can also be installed so that the pipe can be run in a vertical position. The Defrost Cycle Because outdoor air is relatively cool when a heat pump is running in the heating mode, the outside coil is acting as evaporator. Frost can form under certain on the coil under certain types of weather conditions, relative humidity and ambient outside temperatures. The frost can effect the efficiency of the heat pump and must be removed through a defrost cycle. During certain intervals a heat pump will reverse to the cooling cycle while in the heating mode. This is a temporary but necessary function that must occur in order to remove the frost build up from the coil. The fan at the outdoor unit stops when the system switches over to the cooling cycle, which can speed up the process of melting the frost build up. The system remains in the cooling cycle until the temperature has risen to 57 degrees F. The time the unit runs in the defrost cycle depends on how much frost build up is present. During this period the indoor blower continues to blow cool air. With the usage of the back up heating element or adding an additional heating element, the cool air can be warmed and circulated back through the home.
A timer, relay and thermostat controls the defrost cycle. The defrost thermostat is located at the bottom of the outdoor coil where it can respond to changes in temperature. Heat pumps can be a reliable and efficient way to heat and cool a home. They may seem to be a complex system to understand, but is relatively the same type of system as a central airconditioning system with only a few added components. Inspection Tip: Just like an air conditioning system, in the winter time, NEVER operate or test a heat pump in the cooling mode when outside temperatures have not been higher than 65 degrees for a consecutive period of time. This technical bulletin has been drafted to be general in nature and not technically exhaustive.