P4 on an Ideal Heating HP290: Compressor Discharge Temperature High
Short answer
On an Ideal Heating heat pump, P4 means: Compressor Discharge Temperature High. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Compressor Discharge Temperature High.
Part involved: discharge temperature.
Likely causes:
- Discharge thermistor Tp out of calibration (table of values e.g. 25C = 58 kOhm); flow/return thermistors not installed correctly; incorrect discharge pressure P1 (Table 24); low refrigerant charge; outdoor PCB.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
What you can check
- Note the code or fault message shown on the controller/Halo and when it appeared; check the heat pump's isolator and consumer-unit breaker are on.
What the engineer will check
- Check Tp resistance vs table; check Tw_out/Tw_in installation; compare P1 to Table 24; leak test/recharge; replace outdoor PCB or thermistor.
Who to ask for: Heat pump engineer competent with R290 (A3, highly flammable); refrigerant-circuit work requires Cat I/II F-gas plus City & Guilds 6187-21 or equivalent (I&M p.93 warning).
Certification: F-gas Cat I/II + C&G 6187-21 (R290) for refrigerant work; electrical work by competent person.
Sources
- HP290 Monobloc Heat Pump System - Installation & Maintenance (UIN 240080 A09) ↗ · UIN 240080 A09 · page 109 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026