I have Fault code F29 / Temperature Difference / !temp.difference fault on my heat pump (WPM or Touch screen based). What does this mean?

Fault code F29 / Temperature Difference fault means that the heat pump controller is detecting that there is a difference between the flow and return temperature and it is greater than 12k difference, or that the flow temperature is below the return temperature (in heating mode).

This requires your installer to attend.

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 air source machine, the thermal energy of the heating circuit is used to help defrost the evaporator outside. A 12k spread of greater will 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. This protection is in place to prevent water from freezing in the heat pump plate heat exchanger during a defrost, when thermal energy is taken from the water to defrost the evaporator.

Other possible causes:
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. Trying to start a stone cold underfloor system without doing so in stages may result in a large flow/return temperature spread.

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

Was this article helpful?

That’s Great!

Thank you for your feedback

Sorry! We couldn't be helpful

Thank you for your feedback

Let us know how can we improve this article!

Select atleast one of the reasons
CAPTCHA verification is required.

Feedback sent

We appreciate your effort and will try to fix the article