De-Ice Rules

Hardware Heatpump
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Hi. Could you help me in understanding the De-ice rules for my heat pump? See attached.

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  1. The Y axis is the measured refrigerant temperature at the evaporator (my Samsung uses inlet temp, but other makes may use outlet temp). The X axis is the measured ambient temperature.
  2. If your controller finds that actual measurements of these two fall below the lower shaded line, it will start a defrost.
  3. If will keep the defrost going until it finds that the measurements of these two are above the upper shaded line.
  4. The controller may also apply time limits, e.g. it won’t start the defrost until you’ve been below the lower line for x minutes (so as to avoid transient conditions provoking an unnecessary defrost.)
  5. Example: Suppose it is 0degC ambient. It will start a defrost if the refrigerant temperature at the evaporator is below about -3degC, and keep it going until that temperature rises above 15degC.

Hope this makes sense…

Edit 1: I should add that you don’t necessarily know what the evaporator temperature is, unless you have Outdoor Unit monitoring. Normally it will be only a couple of degC below ambient, but when the airfin ices up it can quickly drop to or below (ambient - 10)degC because heat transfer almost disappears.

Edit 2: Thinking on, it has to be what is normally the evaporator refrigerant inlet temperature (which will be the exit temperature during a defrost due to refrigerant flow reversal). When this temp gets up to 15degC (in the above example) it will prove that there’s no ice left…

3 Likes
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Thank you for the explanation. I think I understand, but still getting to grips with this stuff.

So the defrost cycle will start when the outside temperature is 6C and continue whilst the temperature is below this. Once the evaporator coil is above +15C (the ice has melted) it will stop the de-ice cycle.

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Not quite - it also depends on the refrigerant temperature. Have another careful read of my initial reply and all should become clear.

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I have looked at the chart numerous times and think I may have it.

For example. If the outside temperature is -5C and the refrigerant is also -5C the re-ice will run. Have I got it?

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Correct :smiley:.

It is worth investing in an Outdoor Unit monitor if your ASHP controller supports one and you are interested in what’s going on out there…

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Thank you for your help. I have an Hitachi Yutaki S Combi system which is not that common. I will look into this.

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Hi @trebor12345, I just had a quick look at your Technical Manual.
On the face of it, Hitachi don’t expect their users to monitor the Outdoor Unit.
The comms link to the remote display (H-LINK) seems to convey only the I/O data that shows up on the remote display, and not Outdoor Unit internal data.
Shame really (maybe you should have gone to SpecSavers… :face_with_diagonal_mouth:).
Sarah

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Sarah

Thank you again. I can see some parameters on the inside control unit.

Screenshot 2025-01-28 at 13.54.09
Screenshot 2025-01-28 at 13.54.09826Ă—992 73.9 KB

I see there is a Te - Evaporator temperature which I believe you are referring to above.

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Oh well done! I must have been looking in the wrong place…
Those should give you everything you need, and Te (along with Ta) is indeed the refrigerant temperature you need for the defrost algorithm :slightly_smiling_face:.
Are you able to log these, or simply display them?

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Its just a display at present on the indoor unit. I will monitor these and see how the system performs to the chart. (might be back with more questions)

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Sarah
I have been monitoring the de-ice cycles and have noticed something. At 10C the heat pump runs what I call a mini-de-ice cycle. There is often no ice on the evaporator and no de-ice water dripping from the heat pump.
Does the chart above indicate that de-ice cycles can be performed at 10C (I don’t think it does, but I may not be interpriting the chart correctly.)?