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Heat 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
A 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
Heat Pump Thermostats
You say that the thermostat does not control defrost, but the thermostat can stop defrost if outdoor compressor is in defrost cycle or mode! My Honeywell thermostat does that which is very annoying. My heat pump has not completed the defrost cycle and is blowing out cool air via vents then suddenly I hear a click from thermostat shutting down the heat pump! WTF!
The defrost control circuit board in a heat pump is in the condensing unit. It sounds like you have a problem with either the defrost control board or the heat strips in the air handler. The heat strips are also controlled via the defrost control board in the condensing unit when the board timer calls for the defrost cycle.
Question, I just replaced my old Honeywell thermostat with a Honeywell RTH6360D1002 on my upstairs heat pump with emergency heat. I took a picture of the original wiring configuration to make sure the wiring matched. So, R=red, O=orange, E=beige, W2=white, Y=Yellow, G1=green, C=black. My problem is that when my A/C kicks on, it blows out hot air. Any help would be appreciated.
Not sure as there is not enough information here to give a definitive answer. The yellow wire controls the cooling part and if it is broken or not connected properly then it will not work. Additionally, the orange wire needs to function also for most systems as it controls the reversing valve on a heat pump and activates the reversing valve when it calls for cooling on most units with a few exceptions. So you need to make sure the orange and the yellow wire are terminated properly and are not broken. Check the wiring carefully and make sure to turn the power off.
question. I have an older heat pump unit, over 20 years old, still running and very efficient on the A/c side with a 18 degree split. it has shut down in in summer a few times , but I believe that was either to due to icing or thermal overload. it came back on with in the hour. Summers are brutal here with heat and humidity in 80’s and 90’s. Although my unit is in a shaded side of the house the heat is intense
my real issue is I had an old Honeywell like the one you show in photo on this page. I deiced to upgrade and replaced with a digital honeywell T Sat. Problem is I didn’t realize until after I replaced it the new one was not set up for heat strips. Also never anticipated temps dropping below freezing out here, in Myrtle Beach sc.
I don’t believe there is any way to jump in the additional wires so what T stat do i need to buy to fix my mistake.
Second problem is i didn’t take photos or have the original wiring diagram from the back of the T stat so have a few wires in the wall that will need to be connected. Do you have any suggestions on wiring based on that old honeywell T stat.
I do recall that the old honeywell T stat had a separate switch to turn on emergency heat. Would that mean my system would not automatically switch over to heat strips?
Most heat pump thermostats have a switch on them for Auxilliary Heat or Emergency Heat. That is the switch for the heat strips and turns them on when you want manual operation for whatever reason. If something happens with the refrigeration part of the heat pump and the customer calls me at 10 pm at night I advise them to turn it to emergency or aux. heat and I will see them the next day. This way they do not have to pay an overtime charge. Also, if I needed a part it would be difficult or impossible to get that late. So it’s better to turn it to Aux or Emergency heat until the next day.
In your case, you would not have heat and the thermostat likely has not been automatically engaging the auxiliary heating when needed. The best advice I could tell you is to go to the air handler and find out which color of thermostat wire turns the heat strips on. Hopefully, there is a terminal strip that is labeled. On the terminal strip, the wire attached to the “W” is your Auxilliary heating. That is the wire you need to attach to the “W” terminal on the heat pump thermostat. The wire is likely the white wire, however, don’t guess lest you cause further problems. If there is no terminal strip, you need to figure which wire goes to the heat strip relay. This will be a relay or sequencer in the air handler and it engages the heat strips when the temperature offset is achieved in the thermostat (typically ~3°).
Your system has been using the heat strips on occasion but only when it turns into defrost mode. That is not helping you now. The control for this comes from the condensing unit. When the defrost timer in the condenser calls for defrost the reversing valve switches and the heat strips in the air handler turn on. It runs like this for a while and then it goes back into normal heat pump heating mode and the reversing valve goes to heating position and the electric heat strips turn off.
As far as your compressor overheating, see our compressor overheating troubleshooting page to possibly resolve that issue.
My Honeywell thermostat only activates heat strip if indoor temp is below 2 degrees the room temp is set at when heat pump is in defrost mode. I don’t like this. Usually the defrost cycle ends before I get warm air. Then what really sucks occasionally thermostat will shut off HP while in defrost mode, thus not completing the defrost!
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