Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Error Code: Weil-McLain Boiler E13
What it means: Error E13 on Weil-McLain boilers indicates the boiler sensed insufficient water. The low water cutoff (LWCO) or pressure switch detected that water level or pressure dropped below the minimum safe operating threshold. The control board responds by shutting down the burner and displaying E13 to prevent dry-fire damage to the heat exchanger.
E13 appears across multiple Weil-McLain boiler platforms: Ultra series, Evergreen, GV90+, CGs, LGB, and WGO models. The diagnostic path differs slightly depending on whether your boiler uses a mechanical LWCO probe or a pressure transducer, but the underlying principle is the same. The boiler thinks it does not have enough water to operate safely.
Normal System Pressure
A properly filled hot water heating system should maintain these pressure ranges:
| Condition | Pressure (PSI) | Notes |
|---|---|---|
| Cold system (60-70°F) | 12-15 PSI | Normal fill pressure for most residential systems |
| Hot operating (160-200°F) | 15-25 PSI | Pressure rises with temperature |
| Below 12 PSI (cold) | E13 likely | System needs water |
| Above 30 PSI (hot) | Pressure relief valve opens | Expansion tank or fill valve problem |
Check the mechanical pressure gauge mounted on the boiler supply piping. If the gauge reads below 12 PSI when the system is cold, start with the fill valve procedure below. If the gauge shows normal pressure but E13 still appears, the issue is likely sensor-related.
Common Causes
- Low system water pressure. The most direct cause. Normal water loss through micro-leaks, purge valves, or automatic air vents slowly reduces system pressure over months.
- Failed automatic fill valve (auto-feed). The auto-fill valve did not open to replenish normal losses. Valves fail closed due to mineral deposits, diaphragm failure, or electrical failure on solenoid-operated types.
- Waterlogged expansion tank. When the bladder-type expansion tank loses its air pre-charge, system pressure swings wildly. Pressure spikes high when hot and drops below the E13 threshold when cold.
- System leak. Any leak in the piping, circulator shaft seals, zone valves, or radiator valves will gradually or rapidly reduce pressure.
- LWCO probe scale buildup. On Weil-McLain boilers with a conductance-type low water cutoff probe (common on Ultra and Evergreen), mineral scale or sediment on the probe surface can cause a false low-water reading even when water is present.
- Pressure transducer or switch failure. The sensor that reads system pressure can fail, sending an incorrect low-pressure signal to the control board.
- Airlock in the system. Trapped air prevents water from circulating, which causes localized boiling and pressure fluctuations that trigger E13.
Step-by-Step Diagnosis
Step 1: Verify the Pressure Gauge Reading
Look at the boiler pressure gauge. If it reads 0-10 PSI when cold, the system needs water. If it reads 12-15 PSI or higher but E13 is still displayed, skip to Step 4.
Step 2: Add Water to the System
Locate the manual fill valve. It is typically a lever or gate valve on the cold water supply line entering the boiler, often near the auto-fill valve. Open it slowly while watching the pressure gauge. Fill to 15-18 PSI for a typical two-story home. Close the valve completely when pressure reaches the target.
For boilers with an automatic fill valve (Watts 1156F, Sparco, or equivalent), you may hear the valve open automatically when pressure drops. If the auto-fill does not open and you have no manual bypass, the valve has failed and needs replacement.
Step 3: Bleed Air from the System
After adding water, purge any trapped air. Open the highest radiator or baseboard bleed valve in the house until a steady stream of water flows. Work down to lower floors, bleeding each zone. Air trapped in the system mimics low-water conditions by preventing proper circulation.
Step 4: Inspect for Leaks
Walk the entire system while it is cold and pressurized. Check every fitting, circulator pump shaft seal, zone valve body, and radiator valve. Look for drips, rust trails, water stains on floors, or green oxidation on copper fittings. A leak that drips once every 10 seconds can drop system pressure by several PSI per week.
Step 5: Test the Expansion Tank
With the system cold and the boiler off, press the Schrader valve core on the expansion tank’s air fitting. If water sprays out, the bladder has ruptured. Replace the tank. If air comes out, check the pre-charge pressure with a tire gauge. It should match the system’s cold fill pressure (typically 12 PSI for ground floor). Add air with a compressor if low.
Step 6: Clean the LWCO Probe
On Ultra, Evergreen, and GV90+ models with a conductance probe in the boiler return header, scale buildup causes false low-water readings. Drain the boiler below the probe level, remove the probe (typically 1/2” NPT), and wipe it clean with a fine abrasive pad or emery cloth. Reinstall and refill.
Step 7: Check the Pressure Sensor
If the gauge reads normal pressure, the expansion tank is fine, the probe is clean, and E13 still appears, the pressure transducer or pressure switch may have failed. Test by measuring resistance or voltage across the sensor according to the Weil-McLain manual for your specific model. A failed sensor reads open or shorted.
Code Reference Table
| Code | Model Series | Meaning | Common Fix |
|---|---|---|---|
| E13 | Ultra, Evergreen | Low water cutoff tripped / insufficient water | Fill system, clean LWCO probe |
| E13 | GV90+ | Pressure switch detected low water | Fill system, check pressure switch |
| E13 | CGs, LGB, WGO | LWCO open / low water condition | Fill system, inspect for leaks |
| E10 | All models | Low water pressure warning (below 10 PSI) | Add water, check fill valve |
| E14 | Ultra | External limit device open | Check rollout switch, blocked vent switch |
When to Call a Professional
E13 is a safety shutdown. Never attempt to bypass the low water cutoff or operate the boiler with confirmed low water. Dry-firing a cast iron or aluminum heat exchanger will destroy it within seconds.
Call a licensed technician if:
- Pressure drops repeatedly after filling (hidden leak or failed expansion tank)
- The auto-fill valve needs replacement
- The expansion tank bladder has ruptured
- You need to clean or replace the LWCO probe inside the boiler
- The burner side of the system needs service (gas pressure, combustion setup)
- Pressure readings do not make sense and you suspect a failed control board
For Weil-McLain Ultra and Evergreen boilers, the control board records a fault history. A technician can pull the logs to see exactly how many times E13 appeared and at what system pressures. This helps distinguish a real low-water condition from a sensor fault.
Need Replacement Parts?
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