I have Fault code F20 / Defrost fault on my heat pump (WPM or Touch screen based). What does this mean?

Fault code F20 / Defrost fault means that the heat pump controller is detecting a fault with the defrosting process on an air source heat pump. Either defrosting could not be initiated, or could not be completed properly. There are multiple possible causes for this error.

This will require a service visit. See additional help link below on raising an in guarantee or out of guarantee service request.

The installer or service engineer should:

• Check the flow rate of water from the heating circuit to the heat pump is high enough compared to the published nominal flow rate required for that model of heat pump. Check any inline strainer or mag cleaner for debris which could be restricting flow.

• Check the heating circuit water pressure is sufficient (typically between 1.0-1.5 bar) and that there is no air in the heating circuit that could impede water flow. 

• Check what the flow & return temperatures are reading when the system attempts to defrost. If the temperature readings are correct but the flow temperature is low, what has caused this? Has the system been installed to the correct hydraulic layout and is there sufficient volume? Has a buffer been installed correctly?

On a "reverse-cycle" defrosting machine, the thermal energy of the heating circuit is used to help defrost the evaporator outside. If the flow temperature of the water falls below 18C then the system will halt defrosting and try again in 15 minutes. If the flow temperature is below 18C on the second attempt, the defrost error is active, the compressor is blocked and the system will bring on the 2nd heat generator until it can thermally recover. 

Also look at the temperature difference between flow & return whilst on defrost. A 12k spread of greater will also trigger a fault as that is an indication of poor flow, as will a negative temperature difference (ie flow lower temperature than return) which could be due to incorrect flow direction.

Other possible causes:

When setting a system to work for the first time, commissioning mode should be used (Special Function). For a 1 hour period defrosting is suspended. The heating circulation pump operates continiously during the commissioning period and Hot water / swimming pool demand requests are ignored.

Also, at low heating water temperatures, when first setting a system to work, heat the buffer tank first before gradually opening individual heating circuits. A large cold underfloor slab could take several hours to warm up and if the system attempts defrost whilst the flow temperatures are still too cold and there is no thermal energy in the heating circuits, the defrost will fail. Bringing a heating circuit up to temperature bit-by-bit ensures sufficient thermal energy is available for the defrost whilst gradually warming up each circuit in turn.

Hot Gas defrosting:

If the heat pump is a "hot gas defrosting" model rather than a reverse-cycle model, (check the spec sheet of the heat pump to see its defrost method), then defrosting errors will need an f-gas refrigerant engineer to investigate them further.

Require additional help? Please follow link below


Applicable to models:  

System S, System S6, System S10, LIA 0608HWCF MS , LIA 0911HWCF MS,  LI 2M, LI 8AS, LI 8ASL, LI 8AS SAL, LI 8MS, LI 8MSR, LI 9AS, LI 9ASH, LI 9AS SAL, LI 9TE, LI 9TEL, LI 9TES, LI 9TU, LI 11AS, LI 11ASH, LI 11ASR, LI 11AS SAL, LI 11ME, LI 11MER, LI 11MS, LI 11MSR, LI 11TE, LI 11TEL, LI 11TER+, LI 11TES, LI 12TU, LI 15TE, LI 16AS, LI 16ASH, LI 16ASR, LI 16AS SAL, LI 16TE, LI 16TEL, LI 16TER+, LI 16TES, LI 20AS, LI 20AS SAL, LI 20TE, LI 20TEL, LI 20TES, LI 22HS, LI 24AS, LI 24AS SAL, LI 24TE, LI 24TEL, LI 24TES, LI 26HS, LI 28AS, LI 28AS SAL, LI 28TE, LI 28TEL, LI 28TES, LI 40AS, LIH 10TE, LIH 22TE, LIH 26TE, LIK 8ME, LIK 8MER, LIK 8TE, LIK 8TEL, LIK 8TES, LIK 12TU, LIKI 14TE, LA 6MR, LA 6S-TU, LA 6S-TUR, LA 6TU, LA 8AS, LA 8MR, LA 8PMS, LA 9K, LA 9PS, LA 9S-TU, LA 9S-TUR, LA 9TU, LA 10MR, LA 11AS, LA 11ASR, LA 11K, LA 11MAS, LA 11MS, LA 11MSR, LA 11PS, LA 11TAS, LA 12PS, LA 12S-TU, LA 12S-TUR, LA 12TR, LA 12TU, LA 13K, LA 14PMS, LA 15K, LA 16AS, LA 16ASR, LA 16MAS, LA 16MS, LA 16TAS, LA 16TR, LA 17PS, LA 17TU, LA 18I-MU, LA 18PS, LA 18S-TU, LA 18S-TUR, LA 20AS, LA 22HS, LA 22PS, LA 22TBS, LA 24AS, LA 25TU, LA 25TU-2, LA 26HS, LA 26PS, LA 28AS, LA 28TBS, LA 33TBS, LA 35TUR+, LA 40AS, LA 40TU, LA 40TU-2, LA 60TU, LA 60TU-2, LA 60TUR+, LA60STU, LA60STUR, LAC 12TR, LAC 16TR, LAJ 11MUR, LAJ 16MUR, LAJ 60TUR+, LAK 10M, LAK 10MR, System M, LAK 3IMR, LAK 6IMR, LAK 9IMR, LAK 14IMR, LAK 14ITR, LAW 6IMR, LAW 9IMR, LAW 14IMR, LAW 14ITR, LIA 7IM, LIA 9IM, LIA 12IM, LIA 14IM, LIA 16IM,LSA60TUR,LSA60TPR,LSA60TPR2,LSA100TUR, WPM 2006plus (H), WPM 2006plus (L), WPM 2006R, WPM ACU, WPM Basetec, WPM Econ5, WPM Econ5Plus, WPM Econ5S, WPM EconPlus, WPM EconPlus-E, WPM Econ SOL, WPMJ ACU, WPMJ EconPlus, WPMJ EconPlus-MD, WPM PK, RTCFM, RTCSM

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