Two-Pipe Steam System – Boiler Locks Out on LWCO, Requires Power Reset (Mostly Mornings)

We are working on a residential two-pipe steam system and are looking for input from experienced steam professionals.

Reported Symptom

The boiler intermittently shuts down and requires the homeowner to cycle power to restore operation. This happens most often in the morning on the first call for heat.

Findings During First Service Visit

Initial Condition

• Boiler was found shut down on the Low Water Cutoff (LWCO).
• Water in the sight glass appeared relatively clean at arrival.

Control Settings Observed

• Vaporstat set at approximately 12 oz cut-out with a 6 oz differential
• Secondary pressure control (Pressuretrol) set around 0.5 PSI

Initial Actions

• Boiler was filled to the proper operating water level and restarted.
• Noted minor surging while firing.

Drainage and Flushing

• Drained the boiler from the lowest boiler drain and from the wet return hose connection.
• Discharge water was extremely dirty, indicating heavy sediment/sludge in the boiler and/or return piping.
• Performed multiple fill-and-flush cycles to remove as much debris as possible.

Operational Behavior Observed After Flushing

While monitoring the boiler in operation:

• During firing, the water level in the sight glass slowly dropped below the normal operating range, repeatedly tripping the LWCO.
• The automatic water feeder added water multiple times during this period.
• After shutting the boiler off and allowing the system to sit, the water level rose back up, at times above the normal fill line.
• This strongly suggests condensate is slow to return while firing, then returns after burner shutdown.

Return Piping Inspection

• Visible return piping near the boiler appeared properly pitched with no obvious sags.
• Access to the wet return is extremely limited due to tight spacing between the brick wall, boiler, and domestic hot water tank, making inspection, cleaning, or replacement difficult without disassembly.

Current Assessment

The observed behavior:

• Falling waterline during firing
• LWCO activation and feeder operation
• Waterline recovery and rise after shutdown

is consistent with delayed condensate return, most commonly caused by a restricted wet return due to sludge and debris.

While failed radiator traps or crossover devices allowing steam into the returns could contribute, the large volume of debris observed during draining, combined with the fact that the system reportedly operated normally for many years and the problem started recently, points to the wet return as the primary suspect.

Next Step and Question

We are scheduled to return to open and clean the wet return piping.

If cleaning or replacing the wet return does not resolve the issue, what other causes should we be focusing on?

• Traps?
• Boiler piping issues?
• Control interaction (LWCO, feeder, vaporstat)?
• Other less obvious two-pipe steam system faults?

Any guidance from those with deep two-pipe steam experience would be appreciated.