HVAC Heat Pump Basics



In this video, HVAC technician Jesse covers some heat pump basics for beginners. The basics cover how a heat pump works and the operation of all components of a typical heat pump.

In the cooling cycle, the liquid line leads into the TXV where the refrigerant pressure rapidly drops and some flash gas is produced. From there, there’s a short expansion phase into the evaporator, where the refrigerant absorbs heat from the space and boils. The refrigerant travels to the compressor via the suction line as a superheated vapor. The compressor rapidly raises the temperature and pressure of the vapor and sends the refrigerant to the condensing coil through the discharge line. The condenser rejects heat from the refrigerant and turns it back into a liquid. From there, the cycle repeats itself.

In a heat pump, you have all of the same basic components. However, the evaporator coil in cooling mode becomes the condenser in heating mode, and the condenser in cooling mode becomes the evaporator in heating mode. In the air handler, you will also have heat strips, which provide electrical backup heat. When the system goes into defrost, the heat strips also turn on to prevent the space from cooling too much. Heat pumps also have two metering devices: one for cooling mode and one for heating mode. TXV systems have check valves that allow the refrigerant to bypass the metering device that is not in use.

At the outdoor equipment, a heat pump has a defrost board, reversing valve, a common suction port. The defrost board is located in the same place as the low and high-voltage wiring underneath a panel, and it has a defrost sensor connected to the outdoor coil. The common suction port is located between the suction and liquid line coming out of the outdoor unit. A 4-way reversing valve has a solenoid that can slide the reversing valve and swap the suction and discharge lines when energized. That process reverses the flow direction and allows the heat pump to switch between heating and cooling mode. There will always be a common suction line and a common discharge line leading into the reversing valve.

There may also be an accumulator on the system, which prevents liquid refrigerant from getting into the compressor. In cooling mode, the refrigerant flows through the suction line and then passes through the reversing valve before the compressor. In heating mode, the refrigerant passes through the compressor before it reaches the reversing valve.

The defrost board depends on a defrost sensor to tell the unit when to defrost. (In this case, it’s a pink wire.) In defrost mode, the defrost board will send a 24v call on the white wire to energize the backup heat inside (heat strips).

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41 Comments

  1. Roger F

    Thank you! Great info. Looking forward to more.

  2. H Bo

    in the indoor unit we have two coils and a directional valve? One for cooling and one for heating? Or Is there one and the same element that performs two functions?

  3. Anthony Losego

    So like, what's the point of one line being big and the other being small if you literally run it in reverse???

  4. Luis Marquez

    Just graduated HVAC trade school and have a buddy that's having an issue with his outdoor heat pump unit. Says he hears his outdoor unit run randomly in cool (lives in las vegas about 70 degree weather currently). He noticed it because his unit is right next to his bedroom. Thermostat is set to off the times that this has happened and the said he didn't feel any air coming out of his Vents. Told him to check for any schedules on the thermostat and there was none aside from it already not calling for it because the tstat is off. What are some possible things that can cause this. When I went I didn't see anything out of the ordinary and it happens randomly.

  5. Chance Cooper

    This is THE best HVAC channel out there. This dudes like a surgeon but only in the HVAC field….

  6. Javeed Azad

    Amazingly clear and precise video. Incredibly thorough video. Thank you so much!

  7. Garcia Choz

    What happens when it's very cold outside how take heat on temperature colder ..? It's normally head pump running and doesn't stop on this colder days

  8. igorhump

    I see. The flibjabber encapsulates the mobibblybob at the same time the jigglydiggly sucks out the whammyjamjibblybop. Got it!

  9. Mr Green

    Do these also have a reheat coil?

  10. Motion in Mind

    I think having it an air conditioner is misleading. Yes the same components are there and doing the same thing backwards. Refrigerators do the same thing, etc But a heat pump is optimized for the heat cycle, do you can't just run an air conditioner backwards and say it's a heat pump.

    The biggest factor in the efficiency of a heat pump is the heat load of the building being heated. Air to air heat pumps are not efficient at all and probably waste more energy than a modern gas condensing boiler. Heat pumps are best combined with radiant underfloor or in wall heating systems, which are virtually non-existent in the US

  11. VFL Willie

    As a HVAC student I love your videos

  12. Peter Hicks

    If you disconnect the emersion hidden inside do the bills go down or does it just go into overdrive ?

  13. Spartan 510

    If its warm outside, shouldn't be even warmer inside? Why the need for a heat pump then?..

  14. Rob Law

    Very helpful video – thanks for posting! 😀

  15. Brenda Martinez

    Thank you so much for these videos. I struggle to pay attention, my mind will wander constantly as the instructor is teaching. These videos help me focus. Watching, listening and taking notes helps me retain the information better. Plus, I get to rewind when I become distracted. Thank for taking the time to educate me!

  16. Michael Costello

    What is the transformer on outside unit for. Does this supply the 24V to the inside of the house as well.

  17. unknown5

    Wow this is a lot. Thank you for the information very much needed

  18. luigi 74

    Who in the hell thought this up

  19. DJ Kaiser

    Moved into a new-to-us house that had this system, from a home with a gas furnace, and had no clue how it all worked… now I do! thank you!

  20. P J

    I have an 8-year-old Bryant 213B heat pump & Bryant gas furnace. This morning (at 35 degrees outdoors) I upped my thermostat from 64 to 69 degrees. The heat pump and furnace fan came on and ran for just a couple of minutes. Then, due to the call for 5 degrees of heat all at once, the Aux gas heat was initiated which makes sense. I heard the gas burner fire up and the vent pipe got hot. My concern is this: I could hear that my heat pump was still running. I learned that the 2 units (HP & gas furnace) should never run at the same time. I suppose it could be that the HP is possibly running in a different mode, such as defrost or possibly in some way that would not be harmful, but I don't know. So, I immediately dialed back the thermostat to shut it down. I don’t know how to confirm if this is a ‘dangerous’ situation or not. I don’t know if the heat pump is supposed to ‘run on’ in some otherwise safe mode temporarily. I am afraid that if I let it run on just to see if it eventually shuts down I might do damage in the meantime. The fact is that it may have been doing this all along but since I installed a new thermostat recently I am being hyper-vigilant now. I am very confident that my thermostat wiring and setup is correct. I would feel better if I knew that the HP is supposed to behave like this under these conditions. Can anyone comment on this if you happen to know?

  21. Alberto Reyes

    Really help I use to always wonder how did the heat pump work, but it’s amazing how the coils make that switch. 3 main things that he spoke was reversing valve defrosting board the common suction line how they rotate depending if it heat or cold air

  22. Tim USA

    Thank you guys for keeping it super simple so anyone can learn HVAC 🇺🇸

  23. Joshua Smith

    I like how he just starts off saying most of us know this then glossing over it like we already know lol my dude we're here watching this video because we know almost nothing.

  24. LaosGPSmap Midnitemapper

    nice description of the systems and operation, can you clean up those wires with some wire ties so it is easier to see what the components are

  25. leslie Lucci

    Can this be used with an existing heating/cooling system i.e. existing furnace/AC kicks on in extreme cold/heat otherwise heat pump is on? Also does system have a backup so it works when power goes out?

  26. tribus4

    Sounds easy enough.

  27. Dean Burke

    We need a troubleshooting sequence to figure out what went wrong with heat pumps, besides typical operation sequence. Like with furnace operation. Trace what's supposed to happen, and find the defects. Like stuck or worn out reversing valve. What would you see, how would you know? What would your pressures be like?

  28. Troy Dobson

    Love these videos. Excellent description of the system and the process.

  29. William Copeland

    In my experience, heat pumps do not do the job they are advertised to do. Long run times and very low heat generated. The heat strips are very inefficient, much more so than an electric furnace.
    Many complaints that the discharge air is not heated enough except when heat strips are energized.

  30. grigaale

    good info

  31. jose jaime

    Hi my name is Jose I am from Los Angeles ca I watching your videos and I loved but you never told about water source heat pump please if you have any time to talk about water source

  32. Anthony McMillan

    Thanks you for info. Do you have Books I can buy to learn a Heat Pump system better. I maintain a 12 story 151 room Hotel has the VTAC contained as one unit.
    I am having 5 to 10 units crashing a year, and the Hotel is going on it's 2nd year I need to learn more about a heat pump function. I think the refrigerator tech is over charging the Hotel on service calls, with the cost of a new VTAC over $2500. I need to find ways to cut opperating costs.

  33. Cam Strong

    I'm so confused.

  34. Kelvin G.

    Do the fans blow the same direction or do they change in respect to the coils absorbing and dissipating heat?

  35. knockitofff

    Jessie, lose that wedding ring before you lose a finger, no woman is worth that, LOL!

Comments are closed.