There are three cycles for a heat pump: heating cycle, cooling cycle, and defrost cycle. This image depicts the defrost cycle. InterNACHI® members can download this for free here:
[REVISED IMAGE COMING SOON]
Additional commercial inspection images are available for download from CCPIA’s Commercial Inspection Image Gallery.
Nice graphic Maggie !!
This is not a Defrost Cycle.
I don’t like calling it that (a third cycle) because it is actually in the cooling cycle with the outdoor fan turned off!
1/ the hot gas goes from the compressor too the outdoor coil.
2/ the outdoor coil is not pulling in heat (or anything for that matter). The fan is off.
3/ the indoor coil is cold (pulling heat out of the house. The coil heat (which I assume your talking about the electric/gas auxiliary heat), is on to keep the house from getting cold while it is essentially in air conditioning while in defrost.
What you are showing is a heat cycle with aux heat turned on. Which happens when the heat pump can’t make enough heat when it is too cold outdoors. 2nd stage heat then comes on. Your “heat in” at the OD coil is cold air, but actually is heat absorption.
Thank you David. I will have that label adjusted as soon as possible.
Looked like a heating cycle from here.
This is a similar image that they posted a couple weeks ago. The thread was immediately removed without further discussion when I pointed this out. Thank you for verifying what I has already pointed out.
Don’t just change labels. Use a cooling mode pic with the OD Fan Off, and the 2nd stage heat coil “On”.
You may want to put a defrost initiation / defrost termination thermostat, a circuit board with connection to the OD fan and the Aux heat. That is the complete “defrost cycle” picture.
You are correct and the modern styled units operate like that but generically the defrost mode is, as I understand them to be, the cooling ing mode being activated during the heating mode. Some units will turn the fan off and others will keep the fan running. I certainly understand your comments and suggestions. This image is showing the flow of the system without the operations of the fan and the initiation of a supplemental heating method to temper the air. This is only a image without the context of the article that circles around it. Thank you again David for the clarifications and suggestions.
Absolutely NOT!
The only way for the ice to melt (rapidly) is to shut off the fan.
The state of the refrigerant indicated is the reverse of what it should be. Color and state indicated is the heating mode.
I always liked the end of the defrost cycle. Especially when we get nervous clients asking why their AC unit is doing this. Be sure to turn up the volume a bit when watching the attached.
A few recommendations to prevent confusion for new readers.
That “Coil Heat” is awfully close to the coil to show temperature/flow from the indoor coil.
In the defrost mode, it is important to know that hot air comes in, goes out cold, and is reheated before it enters the house.
In the heat mode, it is important to know that hot air comes in warm, goes out warmer, and may get even hotter if the coil heat (2nd stage) comes on. May consider expanding and mark up the indoor coil, like I did in the defrost mode.
You have low pressure liquid (color) coming out of the outdoor coil back to the compressor. It is low pressure vapor. If liquid, it will not collect heat from the outdoor air properly. It will also damage the compressor, which can only pump vapor.
Thanks, David.
