Boiler Not Heating

Common Symptoms

• Boiler does not fire at all — no ignition attempt
• Pilot light is out and won't stay lit
• Boiler fires but no heat reaches radiators or baseboards
• Pressure gauge reads below 12 PSI or above 25 PSI
• Relief valve weeping or dripping
• Circulator pump hums but no water flow
• One zone cold while other zones heat normally
• Boiler locks out on error code — Burnham red LED, Weil-McLain blink codes, Peerless fault codes
• High-efficiency boiler (90%+ AFUE) shuts off — condensate line blocked
• PVC vent blocked in winter — ice at flue termination

Most Likely Causes

1. 1

   Pilot Light Out or Thermocouple Failure — Older Standing-Pilot Boilers

   Older gas boilers (pre-2000) typically use a standing pilot with a thermocouple safety device. The thermocouple is a bi-metallic rod inserted into the pilot flame that generates 18–30mV of DC voltage when heated — this voltage holds the gas valve's safety seat open. When the thermocouple degrades below roughly 12mV output, the gas valve closes and the pilot will not stay lit even when held. To relight: turn the gas valve knob to PILOT, press and hold the red reset button, light the pilot with a long lighter, and hold the button down for 60 seconds. Release slowly — a healthy thermocouple holds the pilot. If it drops immediately, the thermocouple needs testing. Test procedure: disconnect the thermocouple lead from the gas valve (7/16-inch wrench), set a digital multimeter to DC millivolts, hold the pilot lit manually, and probe the thermocouple lead against the gas valve body as the return. A good thermocouple reads 18–30mV; below 12mV replace it. Use a #38 universal thermocouple with a 30-inch lead and 7/16-inch nut — fits most residential standing-pilot boilers, $8–$15. Also inspect the pilot flame: it should be blue and wrap around the thermocouple tip. A weak yellow flame from a partially clogged pilot orifice (#47 drill bit to clean) will not generate enough heat to hold the thermocouple. Burnham Series 2 and Weil-McLain Gold series cast-iron boilers commonly present this fault on units over 15 years old.

2. 2

   Hot Surface Igniter Fault — Electronic Ignition Boilers (80–120Ω Cold Resistance Test)

   Modern residential boilers (post-2000) use an electronic hot surface igniter (HSI) — a silicon carbide or silicon nitride rod that glows orange-hot to ignite the main burner without a standing pilot. HSIs fail in two ways: cracking from thermal stress (visible hairline crack) or burning open (the element loses continuity). Test procedure: turn off the boiler, locate the igniter at the burner assembly, disconnect the igniter leads, and set a multimeter to resistance (Ω). A good hot surface igniter reads 80–120Ω cold for silicon nitride types (Fenwal 35-501xxx, Honeywell Q3400 series) or 40–90Ω for silicon carbide types (Carlin EF series used on some Burnham and Weil-McLain Ultra units). A reading of OL (open circuit) means the igniter element has burned open — replace it. After replacing, reset the boiler lockout and verify the igniter glows orange within 30–45 seconds of the heat call (visible through the burner sight glass if present). Peerless WBV and Buderus GB142 use similar HSI assemblies; Navien NCB-E combi boilers use a spark igniter rather than a hot surface type — on Navien, a 003 error code indicates ignition failure and the electrode gap (3–4mm) should be checked. Igniter cost: $25–$80. A flame sensor rod (separate from the igniter on most boilers) can also cause lockout if coated with carbon — clean with fine steel wool. Gas valve not opening: with the boiler calling for heat, use a multimeter on AC volts to verify 24VAC at the gas valve operator terminals (the two-wire connector on the gas valve body). Correct voltage but no main burner = gas valve has failed. No voltage = check the control board output and wiring. Gas valve replacement ($80–$200) typically requires a licensed professional.

3. 3

   System Pressure Too Low — Fill Valve Procedure and PRV Check (Hot Water 12–15 PSI, Steam 0.5–1.5 PSI)

   Hot water boilers require a cold-fill pressure of 12–15 PSI — low enough to allow thermal expansion headroom but high enough to push water to the highest point of the system. Below 12 PSI, the pressure switch (low-water cutoff) may lock out the boiler or water may not reach upper-floor radiators. Fill procedure: locate the manual fill valve (a bronze ball valve on the cold water supply line feeding the boiler) or the automatic pressure-reducing valve (PRV, typically factory-set to 12 PSI fill pressure). Slowly open the manual fill valve and watch the pressure gauge — fill to 15 PSI, then close. If pressure won't build or drops immediately, suspect a leak in the system or a failed PRV. Check PRV setting: the PRV has an adjustment screw on top — the factory setting is 12 PSI. If it was cranked down, the system will never fill adequately. For steam boilers, operating pressure is much lower: 0.5–1.5 PSI for single-pipe steam, up to 2.5 PSI for two-pipe systems. Steam boilers have a pressuretrol (Honeywell PA404A or similar) that shuts the burner when steam pressure exceeds the set limit. If the pressuretrol is set too low (below 0.5 PSI), the boiler shuts off before generating useful steam. Check the pressuretrol setting and verify the sensing port tube is clear of sludge.

4. 4

   Waterlogged Expansion Tank — Bladder Test and Recharge (Pressure Too High, Relief Valve Weeping)

   The expansion tank absorbs the thermal expansion of system water as it heats from cold-fill pressure to operating temperature. A standard residential hydronic system uses a bladder-type expansion tank (Amtrol Extrol 15 or 30, Watts DET series) factory pre-charged to 12 PSI with air or nitrogen on one side of an internal rubber bladder. When the bladder ruptures, the tank fills completely with water and loses its expansion capacity — system pressure spikes rapidly when the boiler fires, causing the T&P relief valve to weep (at 30 PSI) and potentially triggering the high-limit aquastat. Bladder test procedure: shut off the boiler and allow it to cool to below 100°F. Close the manual fill valve. Connect a standard tire pressure gauge to the Schrader valve on the end or side of the expansion tank. If you get a firm air reading (expect 12 PSI on a cold, drained system), the bladder is likely intact. If water sprays out instead of air, the bladder has ruptured — the tank must be replaced. To replace: drain system pressure to 0 PSI at the boiler drain valve, disconnect the tank from its fitting (typically a 3/4-inch NPT fitting), pre-charge the new tank to 12 PSI with a bicycle pump, and reinstall. Refill system to 15 PSI. Amtrol Extrol 15 is the standard size for most residential systems ($45–$75). CRITICAL: never operate a boiler with a weeping relief valve by replacing the valve without diagnosing the root cause — a weeping T&P valve at 30 PSI is a symptom of expansion tank failure or temperature runaway, not a valve defect. Replace the T&P valve after bladder testing, but diagnose the pressure cause first.

5. 5

   Lockout Error Codes — Burnham, Weil-McLain, and Peerless

   Modern boilers with electronic control boards indicate fault conditions via LED blink codes or digital displays. Burnham Alpine and ES2 series: a single red LED on the Beckett AFG or Riello burner control board flashes a specific number of times indicating fault type — 1 flash = flame sensor fault, 2 flashes = gas valve fault, 3 flashes = ignition lockout. Burnham manual reset button is a red button on the burner control box. Weil-McLain Ultra and ECO Tec condensing boilers: display E01 (ignition lockout), E02 (sensor fault), E05 (over-temperature); reset by pressing the reset button on the control board. Weil-McLain Gold series and older cast-iron models use a red LED on the intermittent ignition control — typically 3-flash for ignition failure. Peerless WBV series: the control board (typically Hydrolevel VXT or Honeywell S8610M) shows fault codes on a small LED or code indicator — check the label inside the front panel for the specific sequence. Navien NCB-E combi boilers: errors display on the built-in LCD (003 = ignition failure, 012 = flame loss, 016 = overheating) with a reset button on the front panel. Buderus GB142: the Logamatic control board shows fault codes as numbers — E32 (flue gas sensor), E109 (gas valve), E130 (ignition). For all brands, always record the error code before pressing reset — this helps a technician diagnose if the problem recurs.

6. 6

   Circulator Pump Not Running — 120VAC Test, Bearing Failure, Zone Valve Stuck Closed

   The circulator pump moves hot water through the system. Two failure modes: (1) Pump not running — no hum, no vibration. Test: use a multimeter on AC volts (120V range) to verify 120VAC at the pump motor terminals. No voltage means the relay on the control board is not closing — check for a blown fuse on the control board and verify the boiler is calling for heat (thermostat set above room temperature). Correct voltage but no movement = pump motor failed. (2) Pump hums but no flow — motor runs but impeller is stuck from iron oxide sludge. On Grundfos UP15-42F and Taco 007 pumps, find the center brass purge plug on the pump housing. With the boiler OFF and the pump power OFF, insert a flathead screwdriver into the shaft slot and manually attempt to rotate the impeller. A free-spinning impeller with voltage but no flow indicates trapped air — open the pump bleed screw 1/4 turn to release air. A locked impeller = pump replacement needed. Zone valve stuck closed: if the pump is running but a specific zone stays cold, test the zone valve. Push the manual lever on the zone valve actuator to the OPEN position — if the zone heats, the actuator motor has failed. Verify 24VAC at the valve motor terminals with the thermostat calling. Correct voltage + won't open = dead actuator. Taco 571-2 or Honeywell V8043F actuator head replacement: $25–$45, no system drain required.

7. 7

   Flue or Condensate Blockage — High-Efficiency Condensing Boilers and Winter Ice Blockage

   High-efficiency condensing boilers (AFUE 90%+) — including Buderus GB142, Navien NCB-E, Weil-McLain Ultra, and Burnham Alpine — vent through PVC pipes rather than a metal flue. Two unique failure modes apply: (1) Condensate drain clogged — condensing boilers produce acidic condensate (pH 2.9–4.5) that must drain through a PVC condensate line to a floor drain or condensate neutralizer. When the condensate drain line clogs with algae, debris, or freezes in an unheated space, condensate backs up into the boiler heat exchanger and triggers a lockout or error code. Inspect the condensate drain line for kinks or blockages. Pour warm water into the drain cleanout to clear an algae clog. Slope the condensate line a minimum of 1/4 inch per foot toward the drain. (2) PVC vent ice blockage — in cold climates, the exhaust vent termination cap (typically through an exterior wall) can develop an ice dam from condensation. The boiler fires, exhaust freezes at the termination, back-pressure builds, and the boiler shuts on a pressure switch. Inspect the vent termination (usually within 12 inches of the exterior wall surface) for ice. Clear ice with warm water — never an open flame. Consider a vent termination guard or relocating the termination to reduce ice risk. Traditional non-condensing boilers (80% AFUE, metal flue) can block with bird nests, deteriorated liner debris, or damper failure — visually inspect the flue terminal and check for adequate draft with the boiler running.

Quick DIY Checks

1. 1Confirm gas supply and thermostat call: verify the gas shutoff valve near the boiler is fully open (handle parallel to pipe). Check that the thermostat is set to HEAT mode, 5°F above current room temperature. If your system has a smart thermostat (Nest, ecobee), verify it is calling — look for an active 'W' or 'W1' output on the thermostat's status screen. At the boiler control board, verify the call-for-heat LED is lit (present on most modern boilers). No call light with thermostat demanding heat = wiring fault between thermostat and boiler (check thermostat wire for breaks and verify 24VAC between R and W at the boiler terminals).
2. 2Read and record the lockout code: open the boiler's front access panel and locate the control board LED or digital display. Count or note the flash pattern before pressing reset. Burnham burner control: count red LED flashes, then press reset button. Weil-McLain Ultra/ECO Tec: note the E-code on the display. Peerless WBV: check the fault LED against the label on the inside panel. Navien NCB-E: read the LCD error number. Always record the code — it tells you which subsystem failed. Press reset per the manufacturer's procedure (typically hold the red button for 3–5 seconds) and observe the next ignition cycle from start to finish.
3. 3Check system pressure and fill if low: locate the pressure gauge on the boiler supply manifold or boiler jacket front. Cold system (not yet fired today) should read 12–15 PSI. If below 12 PSI, open the manual fill valve slowly (counter-clockwise) and watch the gauge rise to 15 PSI. Close the valve. If pressure won't rise or drops back immediately, there is a system leak or failed PRV. If pressure spikes above 25 PSI within a few minutes of the boiler firing, the expansion tank is waterlogged (see expansion tank check below).

1. 4Test the expansion tank bladder: with the boiler shut off and cooled below 100°F, locate the expansion tank (typically a blue or gray steel tank on the supply piping or boiler jacket top). Find the Schrader valve (looks like a tire valve) on the tank end. Press the valve core pin — air should release. If water squirts out, the bladder has failed. To replace: close the fill valve, drain system pressure to 0 at the boiler drain valve, unscrew the old tank from its 3/4-inch NPT fitting, pre-charge the new Amtrol Extrol 15 to 12 PSI with a bicycle pump before installing, thread on and tighten, reopen fill valve and bring system back to 15 PSI.
2. 5Test the hot surface igniter (electronic ignition boilers): turn off the boiler. Locate the igniter at the burner assembly — a ceramic rod or disc connected to two wires, positioned at the burner opening. Disconnect the igniter leads. Set multimeter to ohms (Ω). Probe both igniter terminals. Expected: 80–120Ω for silicon nitride (Fenwal/Honeywell Q3400 series); 40–90Ω for silicon carbide (Carlin EF series, Burnham Alpine). A reading of OL (open loop) means the igniter element has burned open — replace it ($25–$80). Also check the flame sensor rod for carbon deposits — a gray or black coating on the rod tip causes lockout. Remove and clean with fine steel wool.
3. 6Test the circulator pump for power and flow: with the boiler calling for heat, set a multimeter to 120VAC and probe the pump motor terminals. Expect 120VAC. No voltage = relay or control board issue. Correct voltage + no pump noise = motor failed. Hum + no heat = impeller seized. Find the center bleed plug on the pump body (Grundfos UP15-42F: brass screw on the side; Taco 007: plastic plug on the end face). Open 1/4 turn — if trapped air exits followed by water, close and let the system circulate. If air persists and the pump still hums without flow, turn off power and use a flathead screwdriver in the shaft slot to manually rotate the impeller. If locked solid, replace the pump ($80–$150).
4. 7Inspect condensate drain and flue termination (high-efficiency boilers only): for PVC-vented condensing boilers (Buderus GB142, Navien NCB-E, Weil-McLain Ultra, Burnham Alpine), locate the condensate drain line — a 3/4-inch PVC pipe from the boiler to a floor drain. Verify it is not kinked, blocked, or frozen. Pour warm water into the drain cleanout if flow is sluggish. Check the exterior vent cap for ice accumulation in winter — clear ice with warm water. A blocked condensate line or iced vent is a common late-winter shutdown cause that is often misdiagnosed as a gas or ignition problem.

Repair vs Replace

Recommended Tools & Parts

• Universal Thermocouple #38 (30-inch lead, 7/16-inch nut)

  Replacement thermocouple for standing-pilot gas boilers. Fits Burnham Series 2, Weil-McLain Gold, and most residential standing-pilot boilers. Test output: should read 18–30mV DC when heated in pilot flame.

  $8–$15

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• Fenwal/Honeywell Q3400 Hot Surface Igniter (Silicon Nitride)

  Silicon nitride hot surface igniter for modern electronic ignition boilers. Cold resistance: 80–120Ω. Open/OL reading means element has failed. Compatible with many Burnham, Weil-McLain, and Peerless HSI boilers.

  $25–$80

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• Amtrol Extrol 15 Bladder Expansion Tank

  Bladder-type expansion tank for residential hydronic heating systems. Pre-charge to 12 PSI before installing. Resolves pressure spikes, T&P valve weeping, and over-temperature lockouts caused by waterlogged tank.

  $45–$75

  Buy on Amazon →

• Grundfos UP15-42F or Taco 007 Circulator Pump

  Direct-swap 1/25 HP circulator pump for most residential hydronic systems. Resolves pump hum-no-flow (seized impeller) or dead pump with correct voltage. Flanged connections fit existing piping.

  $80–$150

  Buy on Amazon →

• Taco 571-2 or Honeywell V8043F Zone Valve Actuator Head

  Replacement actuator head only — no system drain required. Resolves zone stuck closed with correct 24VAC voltage at terminals. Includes end switch for boiler relay circuit.

  $25–$45

  Buy on Amazon →

• Digital Multimeter

  For testing 24VAC gas valve terminals, 120VAC pump terminals, thermocouple DC millivolts, and igniter resistance. Required for accurate boiler no-heat diagnosis.

  $18–$40

  Buy on Amazon →

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

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