Helper script to detect defrost status for heat pump

[teknowiz]

I am trying to create a helper template to detect when my heat pump enters defrost mode using various other sensors I have monitoring it such as power and supply/return air sensors. I was able to crate a rudimentary detection method by creating a trend sensor with below config to monitor supply air temps and this works somewhat but reaction to defrost cycle is a bit delayed.

For example based on power draw and other sensor data below shows defrost cycle ran from approximately 7:15:12 AM to 7:17:50 AM (as highlighted in yellow on the graphs) but my trend sensor triggered as on 7:17 AM to 7:18:47. I tried adjusting various parameters of the trend sensors but it either results in false triggers or delayed triggers.

Just wondering based on the data graphs below if anyone has suggestion on code for a template senor that would trigger during the pattern match during the portions highlighted in yellow. TIA.

[teknowiz]

Anyone with any ideas on how to do this?

[imadunatic]

Just started down this path myself. I suspect that a derivative sensor might be the best way since the rate of change on the line or duct temp down is much greater than when it just shuts off. Also you can have the logic only assume it’s in defrost mode if the tstat is still calling for heat during this rapid decrease in line temp.

I haven’t yet had a defrost cycle since I set up my derivative sensor but I’m thinking that I can set up a threshold sensor based on the derivative that would show when it’s in defrost and also trigger a counter to increase so I can monitor total defrost cycles for the day.

[vesar (Vesa Ruohomäki)]

Wondering the same thing. Panasonic heat pump. Also thinking of external sensors. Maybe detect the water pan heating element power or temperature. Will have to log it for a while; is it always on during winter or just when defrost cycle is running.

I need control for heating cable in condensate pipe. I don’t want to have it on all the time. So could I just connect that parallel to heat pump water pan element?

[teknowiz]

The trend sensor that I setup to monitor the supply duct temperature is able to detect defrost running but the exact duration and timing of the cycle is a bit off so was hoping to refine the detection further with a modified script. Using pan heater temp sensor or moisture sensor will still not fully reflect defrost cycle timing as moisture detection can have initial delay in detection and lag after cycle. Pan heater depending on model may run without or past defrost cycle as well.

The most accurate way to determine defrost status would be to monitor d terminal (used to let air handler or furnace know when defrost runs) on outside unit if the outside unit has it (mine does) but that would require in a relay and perhaps a ESP32 to the outside unit.

[imadunatic]

The derivative sensor makes a clear distinction between a normal shutdown - slow cool off and a rapid cool off due to reversing valve:

This is the automation I use:

alias: Defrost Cycles
description: Counts Defrost cycles today for Heat Pump
triggers:
  - trigger: state
    entity_id:
      - binary_sensor.line_temp_derivative_low
    from: "off"
    to: "on"
conditions:
  - condition: state
    entity_id: climate.main_floor
    attribute: hvac_action
    state: heating
actions:
  - action: counter.increment
    metadata: {}
    data: {}
    target:
      entity_id: counter.defrost_cycles_today
mode: single

[teknowiz]

@imadunatic

binary_sensor.line_temp_derivative_low

Thanks for the reply, could you share the template code for binary_sensor.line_temp_derivative_low?

[imadunatic]

No problem.

First is the derivative sensor itself based on my line temp sensor. I found 3 minutes to be plenty good for this use case:

Then I use a threshold sensor to drive the automation since it’s binary:

[imadunatic]

Also, make sure you have the current action of thermostat is heating enabled, otherwise I suspect a cooling cycle in the summer would also trigger this. There are probably a hundred different conditions that could verify it’s actually a defrost (outside temp is another easy one) but this has been reliable.

I did not have this set up during the cooling season here to verify it would actually trigger it but just in case.

[teknowiz]

Thanks for hints, will create sensors matching your parameters and see if I can get them to accurately monitor defrost. I already have semi accurate detection going using threshold sensor but it triggers a bit late so leads to some issues with some triggered events.