Boiler Won't Build Pressure: Troubleshooting Guide

Step 1: Check the Burner

The most obvious place to start — is the burner actually firing?

• Verify fuel supply valves are open and fuel pressure is within the burner's operating range
• Check the burner control panel for fault codes or lockout alarms
• Listen for the ignition sequence — if the burner isn't attempting to fire, the problem is in the control circuit
• If the burner fires but goes out immediately, check the flame scanner (UV or IR) — a dirty or failed scanner shuts the burner down on safety
• Check combustion air supply — blocked air inlet or faulty damper prevents proper combustion

Step 2: Check Safety Interlocks

If the burner won't fire at all, a safety interlock may be preventing startup:

• Low water level — the most common cause. Check the water gauge glass; if the level is below the low-water cutoff, the burner is locked out.
• High gas pressure switch tripped — check and reset if safe
• Low gas pressure switch tripped — check fuel supply
• High boiler pressure limit switch tripped — verify the boiler pressure is actually below the limit
• Flame scanner faulty — clean or replace the scanner
• Combustion air proving switch failed — verify the forced draft fan is running and the damper is open

Step 3: Check for Steam Leaks

If the burner is firing but pressure won't build, steam may be escaping faster than the boiler can generate it:

• Check all isolation valves — a process valve may be open, venting steam to atmosphere or to a process
• Check the blowdown valve — if it's passing (not fully closed), steam and water are being lost
• Inspect for visible steam leaks at flanges, valve packings, and pipe connections
• Check failed-open steam traps downstream — live steam blowing through traps pressurises the condensate return, not the steam system
• Check the safety relief valve — if it's weeping or lifting, steam is escaping. A weeping relief valve is a common cause of pressure not building.

Step 4: Check Boiler Efficiency

If the burner fires, all valves are closed, and there are no leaks, but pressure still won't build, the boiler may not be transferring enough heat:

• Fouled tubes — scale or soot on heat transfer surfaces reduces the heat input. Check stack temperature — if it's higher than normal, the tubes need cleaning.
• Burner not reaching high fire — check the modulating control, fuel pressure at high fire, and combustion air damper position
• Undersized burner — if the plant demand exceeds the boiler's rated capacity, pressure won't build even at full fire
• Excess combustion air — too much air dilutes the flame and wastes heat up the stack. Target 3–5% O₂ in flue gas.

Quick Reference