Boiler troubleshooting guides

26 guides. No heat, noise, pressure and pump problems on hydronic boilers. Each one lists the likely causes and what to check, in order.

All brands (9)

• Condensing/corrosion of conventional boiler
  Possible causes and checks for: condensing/corrosion of conventional boiler.
• No heat or some zones cold
  The boiler fires but one or more zones stay cold, or the boiler does not fire.
• Noise
  Possible causes and checks for: noise.
• Poor heat / circulation
  Air-bound sections, cavitating pump, failed circulator, closed zone valve, waterlogged tank.
• Relief valve weeps every cycle / pressure climbs
  Possible causes and checks for: relief valve weeps every cycle / pressure climbs.
• Short cycling / wasted fuel
  Possible causes and checks for: short cycling / wasted fuel.
• Symptoms of air in system
  Pump motor failure / leak at packing, noisy wet-rotor pump (no lube), cavitation and turbulence noise, poor terminal-unit heat, accelerated corrosion.
• Uneven heating
  Possible causes and checks for: uneven heating.
• Won't fire
  No power / polarity, blocked vent or intake, vent damper not rotating, tripped rollout or blocked-vent switch, open limit/operating control, thermosta

Glow Brand (5)

• Boiler pressure keeps dropping / FILL shows
  The system loses pressure and the display asks to fill.
• Cleaning the combustion side of the heat exchanger
  Annual service or when combustion readings drift, flue temperature climbs or Loc 16 appears.
• Display is blank
  Nothing on the LCD at all.
• Display normal but no domestic hot water (indirect tank)
  The boiler runs for heating but the indirect tank does not get hot.
• Display normal but no heat to the space
  The boiler looks fine but the heating loop stays cold.

Navien (3)

• Annual service checklist - Navien NHB-H
  Annual start-up and service items for the boiler from the User's Information Manual maintenance schedule.
• No space heat - Navien NHB-H / NFB-H
  Thermostat calls for heat but the boiler does not heat the zones and shows no error code.
• Relief valve discharging - Navien NHB-H / NFB-H
  The pressure relief valve drips or discharges, or pressure climbs high when the boiler heats.

Rinnai (6)

• Central heating works, no domestic hot water
  Heating runs but no DHW.
• DHW temperature drops
  Hot water temperature falls during use.
• Heating system warms without a call
  Radiators get hot without a demand.
• Hot water works, no central heating
  DHW is available but there is no space heating.
• Insufficient hot water quantity
  Low DHW flow.
• Radiators not hot enough or slow to heat
  Heating performance is poor.

Viessmann (3)

• Acknowledged fault comes back
  An acknowledged fault that is not fixed is shown again the next day at 07:00 and external fault signalling restarts.
• Condensate not draining, burner shuts down
  If condensate cannot leave the boiler it collects in the lower part and the burner stops with a fault message (F.540 / A.18).
• Lag boilers lose contact with lead boiler (cascade)
  After about 60 s of lost communication each boiler switches to emergency operation with its own flow setpoint.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.