Hayward HeatPro LP Error: Low Refrigerant Pressure
Quick Summary
- LP means the low pressure switch opened because refrigerant pressure dropped below 80 PSI. The switch resets automatically at 120 PSI.
- The most common cause is a refrigerant leak — if pressure at the access port is below 80 PSI, a significant charge has been lost.
- Sustained ambient temperatures below 21°F can also trigger LP; the switch closes on its own once temps rise above 40°F.
- Secondary causes include a bad fan motor or capacitor, a failed LP switch, or broken low-voltage wiring to the switch.
What the LP Error Means
The LP (Low Pressure) code appears when the low pressure switch in the refrigerant circuit opens. On HeatPro units, this switch is wired with blue wires and is common to all R-410A Hayward heat pumps. The switch opens when suction-side refrigerant pressure drops below 80 PSI and automatically resets when pressure recovers to 120 PSI.
Normal operating low-side pressure on a HeatPro is 125–135 PSI. Any reading below 80 PSI at the service port indicates a meaningful loss of refrigerant charge. The system cannot heat efficiently — and in many cases will not heat at all — with low refrigerant pressure.
EPA Compliance Required
R-410A refrigerant handling requires EPA Section 608 certification. Do not vent refrigerant. Recover all refrigerant before opening the system. Recharge only to the factory charge listed on the unit's data plate.
Step-by-Step Troubleshooting
Step 1: Check Ambient Temperature
If outdoor temperatures are below 21°F, LP is expected behavior. The refrigerant pressure will naturally be low in extreme cold, and the switch is simply protecting the compressor. The switch will close on its own once ambient temps rise above 40°F. No further action is required in this scenario.
Step 2: Check Refrigerant Pressure at the Access Port
- Connect a refrigerant manifold gauge set to the suction (low) side access port on the HeatPro.
- With the heat pump running, read the low-side pressure.
- Normal operating pressure is 125–135 PSI. If the gauge reads below 80 PSI, the system has lost charge and a refrigerant leak is present.
- If pressure is well above 120 PSI but LP is still displayed, skip to Step 4 (switch testing).
Step 3: Locate and Repair the Refrigerant Leak
A reading below 80 PSI confirms charge loss. The next step is to find the leak before adding refrigerant — recharging a leaking system is a temporary fix that delays an inevitable return call.
- Use an electronic refrigerant leak detector and methodically check all brazed joints, flare fittings, the TXV, and the compressor service ports.
- If an electronic detector cannot locate the leak, pressurize the system with dry nitrogen and apply liquid leak detector (such as "Big Blue") to all suspected joints. Look for bubbles.
- If the leak is still not found, inject UV dye into the system and recharge to factory charge. Allow the unit to operate for several days or weeks (depending on suspected leak severity), then return with a UV light to trace the leak path.
- Once the leak is located: repair the leak, pull a vacuum to a minimum of 500 microns, and recharge to the exact factory charge shown on the data plate.
Step 4: Check the Fan Motor and Capacitor
An intermittent LP code — one that clears and returns without consistent refrigerant loss — can be caused by a failing fan motor or capacitor. When the fan underperforms, the evaporator coil loses heat transfer efficiency and suction pressure drops.
- With the unit running, verify the fan is spinning at full speed and there are no unusual motor noises.
- Measure capacitor microfarad rating against the specification on the capacitor label. A capacitor reading more than 5–10% outside rated value should be replaced.
- Check fan blade rotation — verify air is being pulled up through the top of the unit (discharge), not recirculating down.
Step 5: Test the LP Switch
If refrigerant pressure at the access port is above 120 PSI but LP is still displayed, the switch itself may have failed open.
- Turn off the heat pump at the breaker.
- Disconnect the blue wires from the LP switch. Set your VOM meter to the ohms/continuity setting.
- Check continuity directly at the switch terminals — not through the wires. With normal refrigerant pressure, the switch should be closed (continuity present).
- If the switch is open with normal refrigerant pressure, replace the LP switch. The switch is a screw-on style and does not require removing refrigerant charge to replace. Always use a backup wrench to avoid stressing the refrigerant line, and apply thread sealant to the new switch.
Step 6: Check Low-Voltage Wiring
If the switch tests closed (good continuity) but LP is still displayed, inspect the blue wires running from the switch to the control board for breaks, chafing, or pinching. A broken low-voltage wire mimics an open switch. If wiring is intact and the switch is good, replace the control board.
Frequently Asked Questions
The LP error cleared on its own. Should I still investigate?
Yes. The LP switch auto-resets at 120 PSI, so a recovering system will clear the error temporarily. But if refrigerant is slowly leaking, the code will return. Attach a gauge set to verify pressure is in the 125–135 PSI normal range, not just above the 120 PSI reset threshold.
Can I add refrigerant without finding the leak first?
You can, but Hayward's own procedure is to locate and repair the leak before recharging. Adding refrigerant to a leaking system creates an environmental violation as refrigerant escapes, and guarantees a callback when the unit trips LP again. If the leak cannot be found, inject UV dye with the recharge so you can locate it on the next visit.
What charge should I put in after a repair?
Always recharge to the exact factory specification on the data plate attached to the unit. Never estimate or go by a "rule of thumb" — overcharging causes HI/HP trips, and undercharging causes LP errors. Pull the system to 500 microns before recharging.
How do I tell if the LP switch is the original type or has been replaced before?
The LP switch on HeatPro is blue-wired and is a screw-on high-side access style. If you see an LP switch with different colored wires or a different style fitting, a previous tech may have used a non-OEM part. Check that the replacement switch is rated for R-410A service and has a consistent 80/120 PSI trip/reset specification.
What if LP appears immediately when the heat pump is turned on?
An immediate LP on startup before the compressor has had a chance to build pressure usually indicates very low refrigerant charge — the system pressure is already below 80 PSI at rest. Connect a gauge set and measure static (non-running) pressure. R-410A at typical ambient temperatures should show 150–200+ PSI static. Anything significantly lower confirms a major refrigerant loss.