Heat Pump ThermostatsHeat Pump Thermostats - 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.

It has more switches to control emergency heat, and some even have lights, which will indicate that the auxiliary heat 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.


Heat Pump Thermostats

These two stages in the HP (heat pump) thermostat for heating give the thermostat the ability to run two different sources for generating heat. Typically only with air source systems which are more common and prevalent than water to air heat pumps or geothermal heat pumps. All thermostat manufacturers offer single and multi-stage heat pump thermostats, including Honeywell, White Rogers, Robertshaw, Lux, Maple Chase, and the many other off-the-shelf brands. It does not matter what brand you get as long as you get a heat pump thermostat.

A heat pump thermostat for an air-source heat pump system is needed to make the heat pump function properly. If you have a two-stage system, then you will need additional terminals and control from the thermostat. Additionally, some digital thermostats have unique settings. If it is not set up properly, it is possible the system will not run properly. See our article about two-stage control problems.

Staging Defined | Heat Pump Thermostats

The normal 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 source 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 source heat pumps are equipped with a secondary method of generating heat. Common heat pump secondary heat is usually electric heat but there are other types of providing secondary heating for air source systems 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 HP condenser calls for the defrost cycle to be engaged. The HP thermostat controls all this except for the defrost cycle.

Defrost Cycle | Heat Pump Thermostats

When an air to air HP defrosts, the cycle engages the refrigeration circuits to change over from generating heat to 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. That is not controlled by the HP thermostat but by a solid-state circuit board, which is typically in the condenser.

Many different methods have been used to engage the defrost cycle in the typical air-source heat pump. However, the most current common method is to engage the defrost cycle based on a preset amount of run-time. Many HVAC manufacturers offer different times, which can be adjusted by changing a jumper setting on the solid-state control board inside the HP 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 wintertime, ice or thick frost is noticed on the HP unit, the defrost time needs to be narrowed to prevent icing and frost. The setting for defrost timer is not located in the HP thermostat but on the internal control board.

Heat Pump Thermostats - Controlling Heating and Cooling | Heat Pump Thermostats

heat pump thermostat wiring diagramA heat pump uses the process of 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 fail-safe will fail to heat. The reversing valve is controlled from two different places in the system. For the defrost cycle, the defrost control board controls the reversing valve, and in normal operation, is controlled by the HP thermostat.

Reversing Valve Operation and Control | Heat Pump Thermostats

This reversing valve solenoid, partially controlled by the HP thermostat, is generally energized in the cooling mode (except as noted above). A wire (usually orange wired to the “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. That 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 HP 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 the 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 HP thermostat plays a key role in the sequence of operation of the heat pump system.

Smart WiFi Thermostats for Heat Pumps

There are many new thermostats on the market that offer lots of new features. Many of these thermostats are a breeze to install and set up. Here are some of the features you can expect from some of the newer smart WiFi heat pump thermostats:

  • Easy thermostat installation for most new smart WiFi thermostats.
  • Easy scheduling using a smartphone app or browser
  • Energy use tracking
  • Geofencing. What is geofencing? You enable this in the settings and set a diameter in miles (or kilometers) from your home. When you travel outside of that circle the thermostat automatically goes into setback mode. When you return inside of that circle the thermostat will return to normal settings. That way, when you return home, it is comfortable according to your settings.
  • Remote monitoring and control. You can open the app on your smartphone and check the temperature inside your home. You can also make changes.
  • Touchscreen control
  • Weather forecast displayed on the screen
  • Motion sensors to turn the display on when you approach

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Heat Pump Thermostats