Heat Pump Thermostats
Two Stage Heat Pump Thermostat
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To the average person a heat pump thermostat can be a very complicated thermostat compared to the typical thermostat which serves an air conditioning system with gas, electric, or an oil furnace. Of course the heat pump thermostat typically has more wire terminations than the regular thermostat. The heat pump has more switches to control emergency heat and some even have lights which will indicate that the auxiliary heat and/or emergency heat is functioning. Most heat pump thermostats are two stage thermostats for the heating mode and a single stage for the cooling mode. These two stages in the heat pump thermostat for heating gives the thermostat the ability to run two different sources for generating heat (typically only with air to air systems which are more common and prevalent than water to air heat pumps). All thermostat manufacturers offer single and multi-stage heat pump thermostats including Honeywell, White Rogers, Robertshaw, Lux, and Maple Chase.
Heat Pump Thermostat Staging Defined
The normal heating sequence of operation for a heat pump in heating mode allows for normal operation generating heat using the refrigeration cycle. When the ambient temperature outdoors falls the refrigeration method for providing heat becomes less efficient and it is normal for air to air heat pumps to fail to keep up with the heating needs of the occupants depending on this outside ambient temperature. For this reason many air to air heat pumps are equipped with a secondary method of generating heat. Common secondary heat is usually electric heat but there are other types of providing secondary heating for air to air heat pumps such as a gas or oil furnace. In other words, the secondary heating system is only engaged as necessary when the refrigeration method of generating heat is inadequate and/or the outside heat pump condenser calls for the defrost cycle to be engaged. The heat pump thermostat controls all this except for the defrost cycle.
Heat Pump Defrost Cycle and the Heat Pump Thermostat
When an air to air heat pump defrosts cycle engages the refrigeration circuits change over from generating heat to actually engaging the cooling cycle just as the thermostat was set to air conditioning. It becomes necessary to counter-balance this cooling effect by turning on the secondary heat. This is not controlled by the heat pump thermostat but by a solid-state circuit board which is typically in the condenser.
There are many different methods which have been used to engage the defrost cycle in the typical air to air heat pump however the most current common method is to engage the defrost cycle based on a preset amount of runtime. Many HVAC manufacturers offer different times which can be adjusted by changing a jumper setting on the solid-state control board inside the heat pump condensing unit. Depending on your geographical location and the skill level of the installer or start-up technician will depend on what this time will be. If, in the winter time, ice or thick frost is noticed on the heat pump unit, the defrost time needs to be narrowed to prevent icing and frost. The setting for defrost time is not located in the heat pump thermostat but on the heat pump control board.
Controlling Heating and Cooling with the Heat Pump Thermostat
Honeywell Visionpro can be used for Heat Pumps or other HVAC Equipment combinations
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A heat pump uses the process of HVAC refrigeration to provide heating and cooling. All heat pumps are equipped with a reversing valve to facilitate this reversal from heating to cooling and vice versa. All manufacturers, except a few (Rheem and Ruud among a few others), energize the reversing valve for the cooling cycle. The importance of this is that if the reversing valve solenoid fails it will fail to the heat mode. Heat can be considered far more essential than air conditioning (cooling) so therefore the failsafe will be fail to heat. The reversing valve is controlled from two different places in the system. For defrost cycle the reversing valve is controlled by the defrost control board and in normal operation is controlled by the heat pump thermostat.
This reversing valve solenoid, partially controlled by the heat pump thermostat, is generally energized in the cooling mode (except as noted above). A wire (usually orange wired to "O" terminal on the heat pump thermostat) runs from the thermostat to the condenser where the typical air to air heat pump has a reversing valve. When the thermostat calls for cooling the wire carries (typically) 24 volts to engage the reversing valve solenoid. 24 volts is also applied to the compressor contactor and a relay inside the air handler. This causes the system to come on in the cooling mode and cool the zone until the thermostat satisfies. The reversing valve also engages when the solid-state control board calls for defrost mode only when the heat pump system is in the heating mode. As the board energizes the cooling mode to defrost the outdoor heat pump condenser it also energizes the secondary heat mode.
In the heating mode, the reversing valve is not energized with 24 volts (except as noted above). When the thermostat disengaged the cooling mode it de-energized the reversing valve solenoid and spring inside the valve forced the valve mechanism back to the heating mode so it is not necessary to energize the reversing valve in the heating mode (unless the manufacturer of the unit is as mentioned above). The heat pump thermostat plays a key role in the operation of the heat pump system.
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Thermostats 
