HEATING AIR © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 Hello, I would like to show you the three most common methods of heating air: 1.Electric element heaters. 2.Reverse cycle or heat pump heating. 3.Hot water or steam coil heating. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 Electric element heating is a very simple way of heating the air being supplied to a zone.  In an installation of this type, the area in the duct surrounding the heaters would need to be constructed of non-combustible materials to offer protection in the case of a fire. A safety thermostat would also need to be installed in the vicinity of the heaters so that should an overheat situation occur, the power to the heaters is cut off thus preventing further damage.  Electric element heating can be applied to most air conditioning systems from the smallest to the largest. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 Reverse cycle or heat pump heating is mostly applied to systems at the smaller end of the range. This method utilises the refrigeration system to provide the heating by installing a reversing valve in the system.  This allows the refrigerant to reverse its direction of flow in most parts of the system with the result that the evaporator (inside coil) becomes the condenser, and the normal condenser (outside coil) becomes the evaporator.  We call the normal evaporator the inside coil and the normal condenser the outside coil. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 The main advantage of this method of heating is the reduction in operating costs when compared to other heating methods. The unit is not producing the heat directly as in electric element heating, but is simply removing it from the ambient air and transferring the heat to the inside air. In effect, the unit is attempting to refrigerate the ambient air.  An efficient reverse cycle heating system could be expected to have operating costs about 50% of an electric element heater of the same capacity. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 This is an example of a reversing valve. These are sometimes called four-way valves because of the four line connections required for their installation. The valve consists of a main body, four line connections and a solenoid valve attached to the main body.  The valve is composed of three basic components: pilot valve, main valve body (including valve slider) and solenoid coil.  The four-way valve slider is shifted by changes in differential pressure in the valve that is actuated by the pilot solenoid valve. The valve slider is specially designed to prevent incomplete change-over and movement with minimum pressure differential. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

Cooling Cycle  The pilot solenoid is de-energised during the operating cycle.  Discharge vapour enters the top connection and pressurises the valve body. This vapour bleeds through the two bleed ports in the slider mechanism and pressurises these two chambers to discharge pressure. The right hand pilot tube is connected to the suction line via the pilot valve, which reduces the pressure in the right hand chamber to suction pressure and the left chamber is at a higher pressure.  The resulting pressure differential causes the slide mechanism to move to the left of the body. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

Heating Cycle  The pilot solenoid is energised during this cycle. Discharge vapour enters the top connection and pressurises the valve body. This vapour bleeds through the bleed ports in the slider mechanism and pressurises these two chambers to discharge pressure. The left hand pilot tube is connected to the suction line via the pilot valve, which reduces the pressure in the left hand chamber to suction pressure and the right hand chamber is now at a higher pressure.  The resulting pressure differential causes the slide mechanism to move to the right of the body. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

Applications  The two main uses for reversing valves are: reverse cycle air conditioning (valve is de-energised in the cooling mode) reverse cycle air conditioning (valve is de-energised in the cooling mode) reverse cycle defrost of commercial refrigeration (not used very often in modern systems). reverse cycle defrost of commercial refrigeration (not used very often in modern systems). © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

Some units may be equipped with a de-icing thermostat which senses the ice forming on the outside coil, causing it to revert back to normal cooling mode until the ice melts before resuming reverse cycle heating.  Some systems are equipped with an electric heater and a low-ambient thermostat. If the ambient temperature becomes too cold for the heat pump to operate successfully, the low- ambient thermostat stops the refrigeration system and supplements the heat by engaging the electric heater.  Reverse cycle systems lose efficiency when the ambient temperature drops lower than about 7 °C. At these temperatures the outside coil tends to ice-up and this leads to a restriction of the air flow. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence

 Hot water or steam coil heating is sometimes used in larger systems where it is economical to do so such in situations and locations where hot water or steam are readily available at a reasonable cost.  The coil is constructed similarly to an evaporator coil and installed in the air passage or duct. © Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence