Control method for defrosting the outdoor coil of a heat pump machine

1. A control method for defrosting an outdoor coil of a heat pump machine configured to exchange heat in a first cycle between an external cold source comprising ambient outdoor air, and a hot source, and, on command, in a second reverse cycle between the hot source and the external cold source, the heat pump machine comprising a compressor, the method comprising:

a) defining, in a plane of Outdoor Air Temperature (Taria_ext) against evaporation temperature (T_evap), a first and a second line of separation, said first and second lines of separation defining three zones (A,B,C) within said plane;

b) measuring an outdoor air temperature (Taria_ext);

c) measuring an evaporation temperature (T_evap);

d) identifying in which of the three zones (A,B,C) of the plane of Taria_ext against T_evap a point identified by the Taria_ext and T_evap measurements falls;

e) operating the heat pump machine in the second reverse cycle when the point falls in zone (C) and is below said second line of separation;

f) decreasing a rotation speed of the compressor of the heat pump machine when the point falls in zone (B) and is between said first and second lines of separation;

g) measuring Taria_ext and T_evap after a predetermined time interval;

h) repeating steps d) to g) until the point identified by the Taria_ext and T_evap measurements falls in zone (A) and is above said first line of separation.

2. The control method for defrosting the outdoor coil of a heat pump machine according to claim 1 , wherein said first and second lines of separation are defined in the plane of Taria_ext against T_evap as a function of the value DT=Taria_ext−T_evap.

3. The control method for defrosting the outdoor coil of a heat pump machine according to claim 2 , wherein said first and second lines of separation defining the three zones (A,B,C) in which the value of DT is defined according to values in the following table:

T Aria EXT

A

B

C

−13

<5

5 < DT < 9

>9

0

<7

7 < DT < 9

>9

7

<7

7 < DT < 9

>9

15

<13

13 < DT < 16

>16

>15

<13

13 < DT < 16

>16.

4. The control method for defrosting the outdoor coil of a heat pump machine according to claim further comprising the steps of:

i) storing in a datalogger a duration of one or more occurrences of the second reverse cycle and the outdoor air temperature (Taria_ext) in which they have occurred;

j) computing an average duration of second reverse cycles carried out; and

k) increasing the value of DT when the average duration of second reverse cycles is less than a threshold value.

5. The control method for defrosting the outdoor coil of a heat pump machine according to claim 4 , wherein said threshold value is 2 minutes.

6. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 5 , wherein the increase in the value of DT is 10% of the DT value.

7. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 6 , wherein step k) occurs after a step of dividing an outside air temperature (Taria_ext) range into a plurality of sub-intervals, and wherein an average duration of second reverse cycles for each sub-interval is recorded in the datalogger.

8. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 7 , wherein the number of sub-intervals is three or more.

9. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 5 , wherein step k) occurs after a step of dividing an outside air temperature (Taria_ext) range into a plurality of sub-intervals, and wherein an average duration of second reverse cycles for each sub-interval is recorded in the datalogger.

10. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 9 , wherein the number of sub-intervals is three or more.

11. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 4 , wherein the increase in the value of DT is 10% of the DT value.

12. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 11 , wherein step k) occurs after a step of dividing an outside air temperature (Taria_ext) range into a plurality of sub-intervals, and wherein an average duration of second reverse cycles for each sub-interval is recorded in the datalogger.

13. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 12 , wherein the number of sub-intervals is three or more.

14. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 4 , wherein step k) occurs after a step of dividing an outside air temperature (Taria_ext) range into a plurality of sub-intervals, and wherein an average duration of second reverse cycles for each sub-interval is recorded in the datalogger.

15. The control method for defrosting the outdoor coil of a heat pump machine according to according to claim 14 , wherein the number of sub-intervals is three or more.