Heat Pump Sequence of Operation - Every electrical-mechanical piece of equipment has a sequence of operation. While some manufacturers of heat pumps vary slightly with their heat pump sequence of operation the overall sequence of operation for a heat pump is the same for most manufacturers. So unless you have a special type of heat pump (they are out there – the Acadia heat pump is designed for extreme cold weather temperatures up North like Canada) the sequence of operation for your heating equipment should be very similar to what is described here. This is the basic operation of a heat pump like you will find in conventional heat pumps.
Heat Pump Sequence of Operation
In automatic operation on a call for heat, the thermostat switch closes. This energizes the compressor contactor and the blower motor in the air handler. The compressor contactor closes and the compressor turns on along with the condenser fan motor. The refrigerant inside the refrigeration circuit begins to flow.
The refrigerant in the condenser absorbs heat from the outside air. The refrigerant is transferred indoors to the evaporator coil where the blower is energized. The fan speed is typically slower for heating than it is for air conditioning so the blower inside the air handler most likely has a multi-speed blower or an ECM blower motor and is controlled to a slower speed than when the air conditioning is on.
Heat Pump Sequence of Operation - Industry Differences
There is only a slight difference in the operation if you own a Rheem or a Ruud heat pump system.
As the blower motor fan and the compressor contactor are energized so is the reversing valve. All other manufacturers energize the reversing valve in the cooling mode.
The reason most manufacturers energize the reversing valve in the cooling mode is that if the valve fails they want the valve to fail to the heating cycle as heating is more important than cooling from the air conditioner.
Heating the Air
The refrigerant passes through the evaporator coil while the blower is blowing air across the coil where the heat is transferred from the refrigerant and coil and into the air.
The air flows through the ductwork and is delivered into the spaces via the diffusers. The air recirculated and pulled back to the air handler through the return where the filter is located. This process heats the air providing heat into your home or business.
The thermostat is located near the return. When the thermostat senses the temperature is at the set point it breaks the switch inside the thermostat to turn off the heat. The compressor contactor inside the condensing unit de-energizes and the compressor shut down along with the condenser fan motor. The blower continues to run on a time delay and when the time expires the blower shuts down. That is the operation without including the defrost cycle.
On colder nights when the heat pump runs for an extended period of time the heat pump will collect frost simply because it typically operates below the dew point and so the humidity will accumulate in the form of frost or ice on the condenser coils.
This ice needs to be defrosted. Regular readers who read the Heat Pump pages here at High Performance HVAC will know the defrost cycle method is typically on a timer. When the heat pump runs for a specified time the defrost cycle will kick in and thaw out or defrost the frost or ice on the condenser coils. The time interval in the defrost cycle is manually set by the HVAC installation technician when the unit is installed and will vary from region to region.
Heat Pump Sequence of Operation - Defrost and Backup Heating Methods
When the defrost cycle energizes, the reversing valve switches mode to the air-conditioning mode. This switches the whole heat pump system into an air conditioner in the cooling mode. This is necessary to defrost the condenser coils. It is counteracted through heating strips by the defrost control.
The defrost control energizes the backup electric heat so the heat pump continues to provide heat. This timed cycle ends after a certain amount of time as determined by the manufacturer of the heat pump. Everything switches back to normal heat pump mode.
There are also heat pumps that have gas or oil furnaces for the backup heat. It works the same as with the heat strips described above except instead of electric heat strips providing the heat the gas or oil furnace provides the heat. These systems are referred to in the HVAC field as dual fuel systems and are considered more efficient than heat pumps with electric heat strips for backup heat.
Heat Pump Troubleshooting
If you are troubleshooting heat pump problems you will want to break things down into groups. Is it a control problem or is it a mechanical problem? Mechanical problems include refrigeration issues, airflow issues, and mechanical parts. Control problems include relays, the heat pump thermostat, and anything in the control circuitry of the system. Understanding the heat pump sequence of operation will help you identify the issue quicker, therefore solving the problem faster.
Heat Pump Sequence of Operation
Technical Resource: Refrigeration and Air Conditioning Technology
I have a TVH8 Tempstar(communicating) HP with the NAXA00101DB Daughter Board and Observer Thermostat to allow it to work with a non communicating Thermo Pride OL6 oil furnace. System works but defaults to stage 2 of 5 because of non communicating furnace or coil. System uses the blower in the furnace. A couple of questions….
1. HP puts out about 90-100 Degrees but takes a long time 20-25 min. to get there. As heat is ramping up the circulating cold air through-out the house has brought the temp down which makes the HP run longer. Because defrost is set on a timer, it will sometimes go into defrost before it heats the house. Is there some way I can delay the blower start-up say with a disc thermostat on the coil to start the blower after it gets to about 80 degrees or so. If so where could this be attached.
2. Or, use two separate thermostats. One to communicate with the HP so all stages can be utilized the other for the furnace and will I still be able to use the blower in the furnace for both.
Any thoughts would be appreciated.
I just installed a Rheem RQPMA024JK Package. Everything is outside. Just have ducting coming in the mobile home. Have everything hooked up. A/C working good but can’t get the compressor to run in heat mode. I have 24 volts at the reversing valve, I made sure the reversing valve wire at the thermostat was connected to B terminal at the thermostat since it is a Rheem. I don’t have the brown wire connected to anything outside. Is that the problem? LED 1 and LED 2 codes are both flashing in normal mode.
Double-check the wiring from the wiring diagram from the Rheem to the new thermostat. If everything checks out, call a pro.
Hi ,i have a Goodman GSX14 heat pump with a Amana 10 KW air handler, this morning it’s -22C. My heat pump ran all night without stopping. The thermostat read system on and when the air handler kicks in it reads system on +2 heat is good in the house. However my concern is when the air handler kicks in is it normal that the heat pump still runs? Plus like i said the heat pump never stops. I wouldn’t want to damage anything .
P.S. this system is about one year old.
It’s normal but not that efficient. When the temperature is super low the heat pump will struggle unless it’s designed for operating at colder temps. Heat pumps are great especially if you live in the mid-Atlantic to southern regions.
Thank you for taking time to address peoples questions and concerns. We have a 5 year old Lennox XP25 System with a CBX40UHV Air Handler. Our only concern is that during a defrost cycle in heating mode the air handler is running at high speed blowing cold air into the home lowering the inside temperature 2 – 3 degrees while defrosting. Doing a little research here online it seems that the air handler should not be running during the defrost cycle. What are your thoughts and is this a normal operating condition?
It seems to me something is not wired correctly. Best to call an authorized Lennox dealer and have them send a heat pump tech to check it out.
This site was great, I have learned a lot. I read in regular heating operation the thermostat controls when the secondary or back up heat comes on. It looks like there is a heat pump outdoor thermostat that could also be bad if the secondary heat will not come on. How does this work with the secondary heat. Does it send a signal to the indoor thermostat to tell it to turn on the secondary heat?
“Does it send a signal to the indoor thermostat to tell it to turn on the secondary heat”? Yes – “How does this work with the secondary heat” – this depends on the type of secondary heat you have. The outdoor thermostat will likely turn the heat pump condenser off since below a certain outdoor ambient temperature it is not efficient. That depends on the unit and every unit is different so engineering data needs to be reviewed for determining that temperature.
we have a yr old lennox unit,should we set the temperature and let the unit run or should we set unit on the follow me mode? we are looking for the best efficiency for the unit.
According to Lennox here are the specifics for “Follow Me” mode: Follow Me button. Press to activate
the wireless remote’s air temperature
sensor. This will also transfer the
temperature sensing function from
the indoor unit to the remote. The
indoor unit’s air temperature sensor
will be disabled. The indoor unit
will regulate the room temperature
based on the temperature sensor
in the remote controller, rather
than the sensor in the indoor unit.
The remote controller will send the
indoor unit a signal every three
minutes. If the indoor unit doesn’t
receive the signal for seven minutes,
or if the button is pressed again, the
Follow Me function will terminate.
The remote controller must remain
pointed toward the indoor unit and
must be within 26 feet (8 meters) of
the unit. Do not remove the controller
from the room or obstruct the signal
of the remote controller during Follow
I would say use this feature if you are not comfortable. It will/should maintain temperature setting of where the remote is located which is closer to where you should be than where the temperature sensor is in the indoor unit.
Tks for the detailed description of the Heat pump Sequence of operation. I’ve learned lots.
A question: During an unrelated service call, an A/c tech reminded me that I may not have wired properly to take advantage of my heat pump when I installed a smart thermostat. Apparently i’ve been running in direct heat strip mode all this time. We then talked about how efficient heat pump runs vs. straight heat strip. However, one thing he mentioned that had me thinking is that he mentioned if running in heat pump mode for heating, it’ll subject the indoor evaporator tubing to a much higher pressure (~300 psi) and “may” cause it to fail prematurely, just like during summer time, the outdoor condenser unit is subject to the same pressure (~300psi) to dump heat outside.
And obviously, replacing the indoor evaporator costs ~$2500, so now I’m confused if I should continue running in straight heat-strip mode (lowest efficiency) or in heat pump mode?
I often pressure test the entire system with ~500 psi nitrogen and have never had a problem. I do this when looking for refrigerant leaks. Depending on the manufacturer the higher pressures are by design. Hook up the thermostat to give maximum efficiency versus worry about the coils living up to pressure issues. Most leaks I find are from vibration issues and rubbing against a screw, tube sheet, or rubbing against themselves. You should be able to find the maximum rating for pressure for evaporator coils and condenser coils in the installation paperwork. If not, call the tech line and ask them what the max rating is for pressure. It should far exceed the 300 psi you mentioned here.
I have a Trane Heat Pump and a Lenox Air Handler, each of different age (this was the arrangement when I bought the house 5 years ago). The system worked very well until the fan burned out last year. The new fan, set on automatic, with the system on heat runs continuously when the outside temperature is below 45.
Sounds like something isn’t wired right or you have some bad wiring/control issues.
I have a 2014 Trane XL14 dual fuel heat pump package system. The unit works OK in heat mode but recently (80 degree day) when I placed the thermostat in cooling mode, the outside compressor stayed on for only 45 seconds. The compressor and condenser fan quit, but the blower continued to operate. After about 2 minutes, the unit tried to start again, but tripped in about 10 seconds. I powered everything down and then connected a multi-meter (in AC volt mode) across the low coolant pressure switch. I restarted the unit and observed 0 volts across the switch until the compressor electrical contactor de-energized, at which time I observed 24 volts across the pressure switch. I also noted 24 Volts to the reversing solenoid (coil resistance read good). Could the reversing solenoid be stuck causing the Low coolant pressure switch to open and shutting down the unit or would a low freon charge cause the problem in cooling mode only as the unit still works in heating mode?
Sorry, without looking at it myself I cannot answer your question. One problem can cause several things not to work. And yes, a heat pump will heat in heating mode but not cool in cooling mode if you have a low-pressure problem with the refrigerant.
Hello. Thanks for the reply. It looks like the expansion valve is bad according to a technician that looked at the problem. He put the unit in heat mode and said the gauge pressures “looked text book” and that the expansion valve is a main component when the unit is in AC mode. The tech also said that if it gets below 30 degrees, I should not use the heat pump because in the defrost mode, the unit would switch into AC and would probably trip off. Again, thanks for your reply!
Why does anyone think that it’s better for a reversing valve to fail so you still have heat? If your system is anyway done right you would still have your backup/aux heat, but in warm weather if your r/v fails, your cooling is lost and there is no backup for that. I finally got it right when I switched to a Rheem. Seems that the manufacturers could offer a choice of which type r/v you would like depending on where you live.
Because heat is far more important to survival than air conditioning in most cases. It is how they also do it for commercial systems. If anything fails it fails to heat.
I have a RHEEM system and I had the blower motor replaced in July 2017. My question is should the system be running cool air when in Heat mode? Last winter the system would come on when the temp in house reached the temp the thermostat was set to but now it just runs and runs and doesn’t shut off until I manually lower it by 2 degrees. I have also programmed my thermostat to keep electric bill down but the settings are within reason -69-72 degrees. Thank you
Sounds as if you could have another issue that needs to be looked at by a technician. It should not just run and run unless the temperatures are extremely low and there is a high load on the system.
At the end of a heating cycle of my Lennox heat pump system the compressor shuts off first and then the variable speed air handler spins up to full speed for about 20 seconds before spinning down to off. This is extremely annoying as the system has run practically noiselessly for the cycle and then revs up at the end also creating a draft of lower temperature air. I understand the principle of other systems I have experienced that run the air handler at the “same” speed at the end to expel all the warm air in the supply ducts before turning off but I can’t get anyone to tell me if the Lennox system’s sequence of operation is supposed to behave the way it does or not. All the technician says is “everything seems to be operating normally”. How can I be sure?
you will have to find a certified Lennox dealer and ask for their lead tech. Sometimes they will answer your question on the phone but sometimes they will want to visit which will likely result in a service call. You could also call Lennox and ask for someone in the tech department and ask them the same question.
When in “heat” mode, my 2001 RUUD heat pump outdoor unit fan will not come on when the condenser comes on. I get heat okay but I’m just worried about the fan not coming on. The outdoor temp was 66 degrees and the inside thermostat was set to 73 degrees. I also do not have the blue auxiliary light on at the thermostat so I assume that the unit is functioning properly. Should the outdoor unit fan come on when the condenser operates?
Maybe you are catching the heat pump in defrost mode as many manufacturers shut the condenser fan motor off when the systems goes into defrost mode. With a heat pump you could still have heat even if the condenser is not functioning. The heat will come from the secondary source usually electric heat strips or a gas furnace if you have dual fuel. However, it is far cheaper, in certain conditions, to have the condenser provide heat for you. It would not hurt to have a specialist look at it to make sure you don’t have a bad condenser fan motor or bad control board. However, I would check to make sure that the condenser fan motor issue you are describing is happening all the time or is only happening when it is in defrost. Most systems including Ruud used a timed method for calling for defrost. It depends on what the time interval was set to by the installer or start up guy when the system was installed. Most settings on defrost control boards are can be set to 30 minutes, 60 minutes, 90 minutes, or 120 minutes.
When my Heat Pump goes into “Defrost Cycle”, the “Air Handler” keeps running and blows out cold air at high speed, it lowers the inside house by 4 plus degrees. When Defrost Cycle down it does go back to heating.
My question, should the Heat Handler be putting out COLD air at a high speed.
The answer is no but that is relative to what you have for cooling tonnage and the back up heat source in the air handler. Also, there could be a problem with the heating inside the air handler. Call a professional to check it out. Remember, there is high voltage inside the air handler and I never recommend making a repair yourself unless you have been trained in it.
Yes, defrost means your unit is running in air conditioning mode to take the heat from your house to melt the ice on the outdoor unit. However, your auxiliary heat is supposed to come on to counteract the cooling of the defrost cycle. What kind of auxiliary heat do you have?