Hayward HeatPro HI/HP Error: High Pressure Cutout
Quick Summary
- HI or HP means the high pressure switch opened because discharge pressure exceeded 590 PSI. The switch auto-resets at 440 PSI.
- Restricted water flow through the heat exchanger is the single most common cause — confirm minimum 30 GPM flow, and 40 GPM when heating a spa.
- A TXV (thermostatic expansion valve) failure causes a rapid spike — high side shoots up as low side drops simultaneously.
- Refrigerant overcharge is possible but rare — only occurs if a previous technician overcharged during a repair.
What the HI/HP Error Means
HI (on HeatPro units) and HP (on Summit units) both indicate that the high pressure switch has opened because refrigerant discharge pressure exceeded 590 PSI. This is a safety cutout that protects the compressor from catastrophic failure at over-pressure. The switch is an automatic reset type — it closes again once discharge pressure drops to 440 PSI.
Normal high-side operating pressure on a HeatPro is 290–400 PSI. Higher water temperatures increase high-side pressure, so you will naturally see higher readings when the pool or spa is already warm. Anything above 590 PSI triggers the trip.
Step-by-Step Troubleshooting
Step 1: Verify Water Flow
Restricted water flow is the leading cause of HI/HP. When water cannot carry heat away from the refrigerant fast enough, high-side pressure rises until the switch trips.
- Confirm the pool pump is running. A heat pump requires a minimum of 30 GPM through the heat exchanger at all times.
- Check for any open bypass valve on the heat pump plumbing. A bypass that is partially or fully open allows the system to maintain pressure at the heat pump inlet but drastically reduces actual flow through the heat exchanger — this is a very common installation problem.
- When heating a spa at higher water temperatures, flow requirements increase to at least 40 GPM. If flow is marginal on a spa call, this is a likely culprit.
- Check and clean the filter. A high filter differential pressure reduces system flow significantly.
- Inspect the pump basket and skimmer basket for debris blockage.
Step 2: Observe Both Pressures During Startup
Connect gauge sets to both the high and low service ports before starting the unit. Watch how the pressures move in the first 30–60 seconds of operation.
- Normal behavior: High side rises gradually to 290–400 PSI range; low side stabilizes at 125–135 PSI.
- TXV failure: High side rapidly spikes toward 590 PSI while simultaneously the low side drops sharply. This opposite movement pattern points directly to TXV failure — the TXV power head has lost its charge, closing the valve and blocking refrigerant flow.
Step 3: Diagnose TXV Failure
If you observed the simultaneous high-side spike and low-side drop, the TXV is the likely culprit. The TXV is a mechanical valve with a sensing bulb clamped to the suction line at the compressor inlet. When the power head (the small bulb and capillary tube assembly) loses its charge, the valve closes, effectively blocking refrigerant circulation.
- Inspect the TXV bulb — it should be securely clamped to the suction line with good thermal contact, insulated with foam, and free from corrosion or damage.
- A failed TXV requires replacement. This is a refrigerant-side repair requiring recovery, replacement, evacuation to 500 microns, and recharge to factory spec.
Step 4: Test the HP Switch (Unit Not Running)
If HI/HP is displayed but the unit is not currently running, the refrigerant pressures in the system will have equalized (equal high and low side pressure at rest). An HP trip with the unit off and equalized pressures means the switch itself has likely failed open or there is a wiring problem.
- Disconnect the black wires from the HP switch.
- Set your VOM to the ohms/continuity setting.
- Check continuity directly at the switch terminals. With equalized (resting) refrigerant pressures well below 590 PSI, the switch should be closed.
- If the switch reads open at rest, replace the HP switch. Like the LP switch, it is a screw-on type that does not require refrigerant removal. Use thread sealant and a backup wrench.
Step 5: Check for Refrigerant Overcharge
Overcharge is possible only if previous service work was performed on the system. Hayward units leave the factory at exact charge. To diagnose overcharge:
- Recover all refrigerant from the system.
- Pull a minimum 500-micron vacuum.
- Recharge to the exact factory specification on the data plate.
- Test operation.
Frequently Asked Questions
The HI/HP error clears and the heat pump runs fine for a while, then trips again. What causes intermittent high pressure?
Intermittent trips usually point to marginal water flow — just enough to run normally under ideal conditions, but not enough when water temperature rises or filter load increases. Check for a partially open bypass valve and clean the filter. Also verify the pool pump is running the correct schedule and flow rate for the heat pump's minimum requirements.
Can high ambient air temperature cause an HI/HP error?
Indirectly, yes. Very high ambient temperatures combined with high pool water temperature can push high-side pressure toward the upper limit of the normal range. But a properly sized and flowing system should not trip HI/HP from ambient temperature alone. If it does, check that the unit has adequate clearance around it for airflow and is not recirculating its own exhaust air.
How do I tell if the bypass valve is causing the HI/HP?
Close all bypass valves fully before testing. Reinstall any valve handles that may have been removed or left in an open position. Then run the heat pump and observe whether HI/HP clears. If it does, the bypass was the problem. Educate the customer that bypass valves on heat pump plumbing must remain closed during heating operation.
Why does the HI/HP error appear immediately on startup rather than after a few minutes?
Immediate HI/HP on startup is the classic TXV failure pattern. The compressor starts but the TXV is closed, so refrigerant cannot circulate, and discharge pressure spikes almost instantly. Compare this to a flow-related trip, which takes at least a minute or two of running before pressure builds high enough to trip the switch.
The HP switch resistance tests good. Is the control board next?
Yes — if the switch is confirmed closed (continuous), wiring from switch to board is intact, yet HI/HP persists with the unit not running and equalized pressures, replace the control board. A faulty board can misread or lock in a false fault state.