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The LO code is the limit string open error code which means the limit string is open and will force the heater to shut down and go into lockout. If you correct the error and the limit string closes, the automatic restart time is 2 minutes. For each problem that brings about the Hayward pool heater LO code, there are many possible solutions as shall be explained in the sections below.
The Hayward heater LO code indicates a fault in the temperature limit switch, vent pressure switch, or water pressure switch. To fix it, ensure the pump is on, is wired and connected well, the water flow is above the required minimum pressure, and the filters are clean. Replace defective switches.
Common reasons for the Hayward heater LO code
Hayward pool heater troubleshooting should be done only when you have the proper knowledge of the pool heater. Otherwise, it’s advisable just to enlist the services of a professional since you risk damaging the pool heater.
Many different issues, including the following, cause the LO code:
| LO code fault | Main fixes |
| Faulty water temperature switch | Turn on the water pump, ensure adequate flow and connections, clean filters, replace the water temperature switch if needed. |
| Faulty temperature limit switch | Ensure adequate flow, proper connection, replace the temperature limit switch if needed. |
| Faulty vent pressure switch | Ensure adequate flow and proper connection, and replace the vent pressure switch if needed. |
Time for a new pool heater?
Faulty water pressure switch
The role of the water pressure switch is to shut off the heater when the water circulation pump shuts off. This helps keep the water heater off if there is no water to heat. Besides saving on power, it also prevents damage on some parts. Check the Hayward pool heater manual for the location and role of the water pressure before attempting to fix it.
To diagnose a faulty water pressure switch, follow these steps:
- Check whether the pump is running or not.
- Check whether the water flow is adequate for the switch to work.
- Check for any faults in the electrical connections and wiring.
- Check the status of the water pressure switch contacts to ensure they’re working.
- Make sure low pump pressure doesn’t exist due to dirty filters or poorly positioned valves.
- Check for the correct settings on the water pressure switch.
- Check if the water pressure switch is in working condition or is defective.
These checks are specific to the water pressure switch and are the most common reasons behind the LO code on your Hayward pool heater.
Fixes
For each of the diagnostics carried out above, the possible remedies are as follows:
- Turn on the pump since the LO code is expected when the pump is off.
- Ensure the water flow is above the required minimum, which is 20 GPM (H150FD & H200FD), 25 GPM (H250FD & H300FD), and 30 GPM (H350FD & H400FD). The LO error code appears when water flows below these amounts for each specific heater.
- Make sure the wire harness terminals are connected to the spade terminals on the water pressure switch.
- Start by removing the wire leads both from the water pressure switch, and the jumper leads, then operate the heater while measuring the continuity across the water pressure switch. If it’s open, the water pressure switch contacts are the cause of the LO code. If it’s closed, remove the jumper and reconnect the water pressure switch since this switch has no problem.
- For low pump pressure, clean the filter and correct the positions of the valves for maximum pressure.
- If the settings are incorrect for the water pressure switch, adjust them according to the directions of the manual.
- A defective water pressure switch will also likely have a Hayward pool heater service light alongside the LO code. In such a case, simply replace the water pressure switch.
- Water pressure switch
- Fits hayward universal h-series low nox pool heater models h150fdn, h150fdp, h200fdn, h200fdp, h250fdn, h250fdp
- Also fits hayward universal h-series low nox pool heater models h300fdn, h300fdp, h350fdn, h350fdp, h400fdn, h400fdp
- Weighs 20 pounds
- Water pressure switch assembly replacement
- Fits hayward millivolt electronic h-series ed1 and ed2 style pool heater
- Weighs 23-pound
If you can’t implement any of these remedies, you should consider calling for an expert to fix it for you. This helps prevent further damage to the pool heater which may cost you even more to fix.
Faulty temperature limit switch
Another reason why your Hayward heater may display the LO code is due to a defective temperature limit switch. As the name suggests, the temperature limit switch has the role of limiting the temperature of the pool heater to within safe limits. It’s also called the furnace plenum thermostat. If it’s faulty, the LO code (among other Hayward pool heater error codes) might be displayed.
Replacement parts:
- Item Package Weight: 4.9 lb
- Country Of Origin: United States
- Temperature limiters kit replacement
- Package Dimensions: 14.223 L x 0.762H x10.16W (centimeters)
Diagnose a faulty temperature limit switch as follows:
- Check for the wiring to ensure it’s not faulty.
- Check the state of the contacts of the temperature limit switch.
- Check for the adequacy of the water flow.
- Check if the temperature limit switch is working correctly or is defective.
Other Hayward heater codes may appear if some of these parts aren’t working correctly.
Fixes
For the above issues, the fixes are as follows:
- Securely fasten the wire harness terminals to the spade terminals of the temperature limit switch. Also, correct any other wiring and connection issues on the pool heater.
- To verify the contacts to the temperature limit switch, remove the wire leads both from the jumper leads and the limit switch then operate the heater as you measure the continuity across the limit switches. If it’s open, the LO code is caused by a fault in the temperature limit switch. If closed, try the next step. After that, remove the jumper then reconnect the leads to the temperature limit switch.
- Adequate water flow should be 20 GPM (H150FD & H200FD), 25 GPM (H250FD & H300FD), and 30 GPM (H350FD & H400FD). Any values lower than that will lead to the LO code.
- If the temperature limit switch is defective, the remedy is to replace it with a new one. Ensure you get the same switch like the one that broke down to avoid incompatibility.
These fixes to a faulty temperature limit switch may require the input of a professional. Always try to enlist a professional from Hayward to fix the pool heater for the best results.
Faulty vent pressure switch
The third major reason why your Hayward pool heater will display the LO code is a defective vent pressure switch. A vent pressure switch is a safety feature that shuts down the heater if it senses any negative pressure from the draft inducer motor. This backdrafting is made of dangerous exhaust fumes which can harm the home dwellers and is prevented by the vent pressure switch. If the pressure switch is stuck open, the exhaust fumes will poison the indoor air.
Replacement parts:
- Vent pressure switch replacement
- Fits hayward universal h-series low nox induced draft heater
- Weighs 8.3-pound
The diagnosis for a faulty vent pressure switch is as follows:
- Start by checking for any wiring and connection problems to and from the vent pressure switch.
- After that, check the state of the vent pressure contacts to determine if they’re secure or not.
- Check the flue for blockage or restrictions.
- Lastly, check for a defective vent pressure switch to determine if it needs to be replaced or not.
To locate the vent pressure switch, check the manual of your Hayward pool heater. If you can’t, call for an expert.
Fixes
To fix any of the four issues diagnosed above, follow these steps:
- Check whether the wire harness terminals have been correctly and securely connected to the spade terminals of the vent pressure switch. If not, connect them properly.
- To verify the integrity of the contacts of the vent pressure, remove the wire leads from the jumper leads and the vent pressure switch. Turn on the heater then measure the continuity across the vent pressure switch to determine if it’s closed or open. If it’s open, then LO code will be caused by a fault in the vent pressure switch. If closed, the LO code is caused by something else.
- For the flue, check the manual to your pool heater to determine its appropriate vent sizing to ensure it’s not blocked or restricted in any way.
- If the vent pressure switch is faulty beyond repair, simply replace it with a new one.
With these steps, you should be able to restore the correct workings of your Hayward pool heater by doing away with the LO code. While the LO code isn’t something to worry too much about, you should consult with an expert if the fixes above don’t work for your pool heater.
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HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.
