RTPU Defrost Control Logic - Two Circuit Rooftop Package Units
Freshdesk support guide for rooftop package unit defrost complaints, including dual-circuit heating/defrost behaviour, outdoor fan operation, HP switch trips and key defrost parameters.
Important safety note: Defrost parameter changes must only be completed by a competent technician with the correct service access level. Any change that affects compressor operation, fan operation, defrost pressure termination or HP switch risk must be recorded in the job notes.
Context: The issue identified in the email chain was that, on some two-circuit RTPU systems, when one circuit enters defrost the other circuit may continue heating. Outdoor fan operation during defrost can prevent the defrost circuit from reaching the required stop pressure, which can extend defrost time or contribute to defrost time exceeded / HP switch events.
1. Problem summary
| Area | Support summary |
|---|---|
| Observed issue | On a two-circuit unit, one circuit can enter defrost while the other circuit continues operating in heating. |
| Why this matters | If the other circuit continues heating, the shared/mixed heat exchanger effect can reduce defrost effectiveness. If condenser/outdoor fans continue operating, the defrost circuit may not reach the stop pressure quickly enough. |
| Reported pressures | Defrost start set point referenced as 4.5 bar. Defrost stop set point referenced as 26 bar, with discussion that 22 bar may be more achievable if the outdoor fan is stopped during defrost. |
| Reported result | Defrost time exceeded may occur if the pressure threshold is not reached. In some cases the manual HP switch can open before the pressure transducer reaches the stop threshold. |
| Service access path | Parameter check/change requires Level 3000 password access via Parameter > Heat Pump > Defrost > 1. |
2. Main defrost fault flow
| Start: RTPU has defrost complaint Examples: defrost time exceeded, HP switch trip during defrost, poor defrost clearing, one circuit defrosting while the other continues heating, or fan operation affecting pressure rise. |
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| Step 1 - Confirm unit configuration Confirm whether the RTPU is a two-circuit / two-stage system. Record how many compressors and how many circuits are fitted. |
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| Step 2 - Witness defrost behaviour Observe which circuit enters defrost and confirm whether the second circuit stops or continues heating. |
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| Decision - Does the non-defrosting circuit continue in heating? If yes, record this clearly. This may reduce defrost performance on the mixed heat exchanger. |
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| Step 3 - Confirm outdoor fan behaviour during defrost Check whether external heat exchanger fans are off, running as in cooling mode, or switching on after defrost for dripping time. |
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| Decision - Are outdoor fans running during defrost? If yes, the defrost circuit may struggle to reach stop pressure. Check d05 fan management setting. |
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| Step 4 - Monitor pressure and time Record defrost start pressure, stop pressure target, actual pressure achieved, defrost duration and whether HP switch opens before transducer reaches stop pressure. |
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| Step 5 - Check defrost parameters Use Parameter > Heat Pump > Defrost > 1 with Level 3000 password access. Record current values before changing anything. |
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| End: Adjust only approved parameters and retest complete defrost cycle Confirm whether defrost terminates on pressure before timeout and without HP switch opening. |
3. Parameter access and change-control flow
| Start: Technician needs to check defrost settings |
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| Enter Level 3000 password |
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| Navigate to Parameter > Heat Pump > Defrost > 1 |
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| Record existing values before changing parameters |
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| Check d05 fan management while defrosting |
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| Check d17 number of active compressors in defrost circuit |
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| Save approved changes, run full defrost test and record outcome |
4. d05 - Fan management while defrosting
| d05 option | Meaning | When it matters |
|---|---|---|
| d05 = OFF | External heat exchanger fans are always OFF during defrost. | Use this logic where fan operation is preventing the defrost circuit from reaching the defrost stop pressure. |
| d05 = EqUA | Fans are managed as in cooling mode. | This may allow fan operation during defrost, which can prevent or slow pressure rise. |
| d05 = ONdr | Fans are OFF until the stop defrost condition. After that, fans turn ON at maximum speed for d20 dripping time. When d20 expires, the unit returns to heating mode and normal fan management. | Useful where fans should remain off during defrost, then run after defrost to assist dripping/clearing before returning to normal heating. |
Support interpretation: If the complaint is that outdoor fans are running and the defrost circuit cannot reach stop pressure, d05 should be checked first. The email chain specifically raised the need to stop outdoor fan operation during defrost.
5. d17 - Number of active compressors in defrost circuit
| Parameter | Meaning from supplier response | Field interpretation |
|---|---|---|
| d17 | Number of active compressors in the defrost circuit. It allows defrosting on time without reaching the pressure threshold using a small number of compressors, to maintain constant defrost temperature for the time defined by d15. | On RTPU systems with one compressor per circuit and two separate circuits, confirm whether this setting applies per circuit rather than total unit. Record the current value and confirm with technical support before changing if unsure. |
Important: Do not assume d17 will automatically stop the second circuit from heating unless this is confirmed in the field. The email chain notes that the second system was expected to turn off, but site testing found the second system continued normal heating operation.
6. Site checks and evidence to capture
| Check | Record in job notes |
|---|---|
| Unit configuration | Model, number of circuits, number of compressors per circuit, controller software/version if available. |
| Defrost trigger | Which circuit entered defrost, pressure at start, ambient conditions and coil condition. |
| Second circuit behaviour | Whether the non-defrosting circuit stopped or continued heating. |
| Fan behaviour | Whether outdoor fans were off, running, or running after defrost during d20 dripping time. |
| Stop condition | Target stop pressure, actual pressure achieved, whether defrost ended by pressure, time, or fault. |
| HP switch event | Whether the manual HP switch opened before the pressure transducer reached stop pressure. |
| Parameters | Record d05, d17, d15, d20, defrost start pressure and defrost stop pressure before and after any change. |
7. Freshdesk ticket note template
Suggested wording:
Attended RTPU defrost complaint. Unit confirmed as [model] with [number] circuits and [number] compressors per circuit. During test, circuit [1/2] entered defrost. Non-defrosting circuit [stopped / continued heating]. Outdoor fan behaviour during defrost: [off / running / ran after defrost during d20]. Defrost start pressure recorded at [x] bar. Defrost stop target set to [x] bar. Actual pressure achieved [x] bar. Defrost ended by [pressure / time / fault]. HP switch [did / did not] open before pressure transducer reached stop pressure. Checked Parameter > Heat Pump > Defrost > 1 using Level 3000 access. Recorded existing values before changes: d05 [value], d17 [value], d15 [value], d20 [value]. Changes made: [insert]. Full defrost retest completed: [pass/fail]. Outcome/recommendation: [insert].
Attended RTPU defrost complaint. Unit confirmed as [model] with [number] circuits and [number] compressors per circuit. During test, circuit [1/2] entered defrost. Non-defrosting circuit [stopped / continued heating]. Outdoor fan behaviour during defrost: [off / running / ran after defrost during d20]. Defrost start pressure recorded at [x] bar. Defrost stop target set to [x] bar. Actual pressure achieved [x] bar. Defrost ended by [pressure / time / fault]. HP switch [did / did not] open before pressure transducer reached stop pressure. Checked Parameter > Heat Pump > Defrost > 1 using Level 3000 access. Recorded existing values before changes: d05 [value], d17 [value], d15 [value], d20 [value]. Changes made: [insert]. Full defrost retest completed: [pass/fail]. Outcome/recommendation: [insert].
Escalation trigger: Escalate if the second circuit continues heating when supplier logic says it should stop, if the HP switch opens before the pressure transducer reaches defrost stop pressure, or if d05/d17 changes do not allow the defrost cycle to terminate safely before timeout.