Gas Boiler Not Heating: Pilot Out, Lockout, Zone Not Heating & Circulator Pump Failure

Common Symptoms

• Boiler fires up but radiators or baseboards stay cold
• No call for heat — boiler never fires at all
• One zone has no heat while other zones work fine
• Pilot light is out on older standing-pilot boiler
• Electronic ignition lockout — error LED flashing 3 times (Weil-McLain GV90)
• Circulator pump hums but no water flow
• Gurgling or banging sounds in baseboards
• Pressure gauge reads 0 PSI or boiler shuts off on high limit

Most Likely Causes

1. 1

   Pilot Light Out — Failed Thermocouple (Standing Pilot Boilers)

   Older gas boilers (pre-1990s) use a standing pilot light with a thermocouple safety device. The thermocouple generates 18–30mV of voltage from the pilot flame heat; this voltage holds the gas valve's safety seat open. When the thermocouple wears out (output drops below ~12mV), the gas valve closes and the pilot won't stay lit even when manually held. The pilot relight procedure: turn the gas valve knob to PILOT, press and hold the red reset button, light the pilot with a lighter or spark igniter, and hold the button down for 30–60 seconds to allow the thermocouple to heat. Release slowly — if the pilot goes out, the thermocouple has failed. Replace with a #38 universal thermocouple (fits most standing-pilot boilers, 30-inch lead). Thermocouple replacement costs $8–$15 and takes 20 minutes.

2. 2

   Electronic Ignition Lockout — 3-Flash Error Codes (Weil-McLain GV90)

   Modern gas boilers use electronic ignition (hot-surface igniter or spark igniter) and go into safety lockout after multiple failed ignition attempts. On the Weil-McLain GV90+/GV90 series, 3 LED flashes indicates an ignition lockout — the control board failed to detect a flame within the trial-for-ignition period. Common causes: dirty flame sensor rod (carbon buildup — clean with fine steel wool), failed hot-surface igniter (test resistance: should read 40–90 ohms on Carlin EF series; infinite resistance = open/dead), blocked flue vent, or gas supply issue. To reset after lockout: press and hold the red reset button on the boiler control board for 5 seconds. If lockout recurs within 1 hour, the igniter or flame sensor is the likely cause. On Burnham boilers, a flashing red LED follows a similar 3-flash pattern for ignition failure; Peerless and Navien have their own LED codes — consult the label inside the front panel.

3. 3

   Zone Valve Not Opening — Taco 571-2 and Honeywell V8043F

   A hydronic heating system with multiple zones uses motorized zone valves to direct hot water to each zone. When a zone valve's motor fails or the end switch does not close, the zone gets no flow and the boiler may never receive a call for heat from that zone. To diagnose: (1) Manual test — most zone valves have a manual open lever on the actuator head; push it to the 'open' position and verify the zone warms up. If it does, the valve motor has failed. (2) Electrical test — with the thermostat calling for heat and valve powered, use a multimeter on AC volts to verify 24VAC at the valve terminals; no voltage indicates a wiring or thermostat issue. (3) End switch test — the zone valve end switch sends power to the boiler when fully open. For Taco 571-2: terminals 1 and 2 are the motor; terminal 3 is the end switch common; test continuity between the end switch terminals when the valve is manually in the open position (should read ~0 ohms). For Honeywell V8043F: the end switch contacts close at approximately 250°F equivalent travel — use the manual lever to open fully, then check continuity across the end switch terminals (labeled TT on most models). A valve that opens mechanically but has a dead end switch will warm the zone but the boiler will not fire — replace just the actuator head (about $25–$40), not the full valve body.

4. 4

   Circulator Pump Failure — Grundfos UP15-42F and Taco 007

   The circulator pump moves hot water from the boiler through the piping system and baseboards. Two failure modes: (1) Dead pump — no hum at all when the boiler is calling for heat. Verify 120VAC at pump terminals with a multimeter. No voltage = wiring or relay issue. Correct voltage + dead pump = pump motor failed. (2) Hums but no flow — the pump motor runs but the impeller is stuck (common with iron oxide sludge). On Grundfos UP15-42F and Taco 007 pumps, locate the brass vent/bleed screw on the pump body (flat screwdriver slot, on the side of the pump housing). Turn the boiler off, place a rag to catch drips, and open the bleed screw 1–2 turns to release trapped air. Then, with the boiler OFF and pump power OFF, use a flathead screwdriver on the pump shaft slot (accessible after removing the pump cap or center plug) to manually spin the impeller. If the impeller is seized and won't turn, the pump needs replacement. If it spins freely and airlock was the issue, restart the system. The Grundfos UP15-42F and Taco 007 are direct replacements for each other on most residential systems (1/25 HP, flanged connections).

5. 5

   Aquastat Settings Out of Range

   The aquastat is the boiler's temperature controller. It has three settings: High Limit (typically 180°F), Low Limit (typically 140°F), and Differential (typically 20°F). The high limit shuts the burner off when boiler water exceeds 180°F. The low limit keeps the boiler hot (above 140°F) so it's ready for heat calls without having to reheat from cold. The differential is the temperature swing — with a 20°F differential and 180°F high limit, the burner fires when water drops to 160°F. If the high limit is set too low (e.g., 140°F), the boiler shuts off before delivering enough heat to baseboards. If the low limit is set too high, the burner may short-cycle constantly. Common Honeywell aquastat models: L8148E (combination), L7224 (reset control). To adjust: remove the aquastat cover and turn the appropriate dial. Standard residential settings: High 180°F, Low 140°F, Differential 20°F.

6. 6

   Air-Bound System — Trapped Air Blocking Circulation

   Air trapped in the piping and baseboards prevents hot water from circulating, causing cold baseboards and sometimes a hissing or gurgling noise. Air enters the system when water is added (pressure loss replenishment), during initial fill, or from dissolved oxygen in the water. Baseboard units have coin vents (also called manual air vents) at the end of each loop — turn the coin-slot screw counter-clockwise 1/4 turn with a flat coin or screwdriver to release air, then close when water flows without bubbles. The boiler itself should have an automatic air vent (float-type, typically on the supply manifold or near the circulator) that purges air continuously. If the auto air vent is stuck or dripping, replace it (Watts #0009 or Webstone equivalent). After major air intrusion, a manual system purge is required: connect a hose to the boiler drain valve, open zone by zone with boiler pressure at 20 PSI, and flush until no air bubbles exit the hose.

7. 7

   Boiler Water Chemistry — pH and Inhibitor

   Untreated boiler water becomes acidic over time (pH dropping to 6.5 or below) from dissolved oxygen corrosion, causing iron oxide sludge (black magnetite) that clogs circulator impellers, zone valves, and baseboard fins. The boiler water should be maintained at pH 7–9 (slightly alkaline). Test with a pH strip — take a sample from the boiler drain valve. If pH is below 7, the system needs flushing and inhibitor treatment. Add Fernox F1 (per the label rate for system volume) or Nu-Calgon 4310-08 inhibitor concentrate to the fill water. Annual flush procedure: turn off boiler, cool system to below 100°F, connect a hose to the drain valve, open the pressure relief valve slightly as an air inlet, and drain the system fully. Refill with fresh water plus inhibitor. pH should be re-tested 2 weeks after treatment.

Quick DIY Checks

1. 1Check the thermostat and zone call: set the zone thermostat 5°F above room temperature and verify the thermostat is in HEAT mode. Listen at the zone valve for a clicking sound (motor opening). If no click, remove the thermostat cover and verify 24VAC between the R and W terminals using a multimeter — no voltage indicates a wiring break or blown furnace fuse. If 24VAC is present but no zone valve sound, the valve motor has failed.
2. 2Relight the pilot (standing pilot boilers): turn the gas valve knob to PILOT. Press and hold the red reset button. Use a long lighter or built-in spark to light the pilot flame. Hold the button down for 60 seconds. Release slowly. If the pilot stays lit, turn the knob to ON. If the pilot goes out immediately, test the thermocouple: disconnect the thermocouple lead from the gas valve and use a multimeter set to DC millivolts — hold the pilot button while measuring the thermocouple leads. A good thermocouple reads 18–30mV. Below 12mV means replacement needed: #38 universal thermocouple, 30-inch lead, 7/16-inch nut at the gas valve.
3. 3Reset electronic ignition lockout (Weil-McLain GV90 and similar): locate the red reset button on the boiler control board (usually on the front of the boiler behind the cover panel). Press and hold for 5 seconds. The LED should stop flashing. Allow the boiler 2 minutes to attempt ignition. If it relocks out (3 flashes again), remove the flame sensor rod (one screw, angled at the burner), clean the rod tip with fine steel wool until shiny, reinstall, and retry. Persistent lockout after cleaning = failed hot-surface igniter or gas supply issue.

Repair vs Replace

Recommended Tools & Parts

• Universal Thermocouple #38 (30-inch lead)

  Replacement thermocouple for standing-pilot gas boilers. 30-inch lead with 7/16-inch gas valve nut fits most residential boiler brands including Weil-McLain, Burnham, and Peerless.

  $8–$15

  Buy on Amazon →

• Taco 571-2 Zone Valve Actuator Head

  Replacement actuator head for the Taco 571 series zone valve. Replace just the actuator (not the valve body) — no draining required. Includes end switch for boiler relay.

  $25–$40

  Buy on Amazon →

• Grundfos UP15-42F Circulator Pump

  1/25 HP cast iron circulator pump for residential hot water heating systems. Drop-in replacement for Taco 007 in most applications. Flanged connections fit 3/4-inch to 1-inch sweat fittings.

  $80–$120

  Buy on Amazon →

• Taco 007-F5 Cast Iron Circulator

  Alternative to Grundfos UP15-42F. 1/25 HP residential circulator, flanged. Same bolt-pattern replacement for most Weil-McLain and Burnham systems.

  $70–$100

  Buy on Amazon →

• Fernox F1 Boiler Inhibitor (500ml)

  Central heating inhibitor that prevents corrosion and scale. Treats up to 15 gallons of system water. Maintains pH 7–9. Compatible with all metals in hydronic systems.

  $18–$25

  Buy on Amazon →

• Digital Multimeter

  For testing 24VAC zone valve wiring, 120VAC pump power, thermocouple millivolt output (DC mV mode), and flame sensor resistance.

  $18–$35

  Buy on Amazon →

• Boiler Water pH Test Strips

  Tests boiler water pH to confirm treatment level. Range 6–10 pH. Use with Fernox F1 or Nu-Calgon 4310-08 inhibitor treatment.

  $8–$12

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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