The Boiler That Only Fails in January
You are scoping a 96-unit, 1970s garden apartment complex with a single central gas-fired boiler and a two-pipe steam distribution system. Tenant complaints cluster in the coldest weeks: units on the top floor of the three-story buildings overheat badly, while first-floor end units stay cold. The maintenance supervisor has already raised boiler pressure twice to "push the steam out further," and now the system cycles on high-limit frequently. A pressuretrol setpoint of 5 psi is documented, and the boiler short-cycles roughly every 8 minutes on design days. Window air conditioners are installed in roughly 70% of units, many through sleeves cut into walls, and several tenants report condensation and musty odors near those sleeves. Domestic hot water is a separate atmospheric storage tank with a standing pilot.
1. Diagnose the overheating/underheating pattern using building science reasoning rather than equipment fault logic. What is the likely root cause and why did raising pressure make it worse?
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The pattern is a distribution and balancing problem, not a boiler capacity problem. In a two-pipe steam system, steam travels to radiators because of pressure differential and returns as condensate by gravity. Raising pressure to 5 psi increases the velocity of steam and pushes it past radiators before it can condense, so upper-floor and near-boiler radiators overheat while far-end units still starve. High pressure also compresses the useful condensing band and raises the chance of water hammer and condensate backup.
More importantly, pressure is the wrong variable. Steam systems are controlled by venting rate at the radiator and main vents, pipe sizing, and insulation. If main vents are plugged or undersized, air cannot leave the mains, so steam cannot enter them; raising pressure only forces steam through paths of least resistance. The correct diagnosis sequence is: verify main vent operation, check radiator vents, confirm traps and returns are clear, then lower pressure to the manufacturer's low-pressure setting (often 1-2 psi) and re-observe. Short-cycling at 8 minutes on design days is consistent with over-pressurization and air binding, not with undersized firing.
The top-floor overheating is also a stack-effect and envelope issue: warm air rises through the building, and top-floor units receive heat from below as well as from the radiators. End-unit first-floor cold complaints point to wind exposure and greater envelope area per unit. Balancing must address both the steam side and the envelope side; fixing only the boiler will not resolve the complaints.
2. The through-wall AC sleeves are causing moisture complaints. Explain the likely mechanism and what assessment steps you would specify before writing any scope.
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A sleeve cut through a wall creates a permanent hole in the pressure and thermal boundary. In heating weather, the sleeve is a cold surface and an air leak path. Warm, moist interior air reaches the cold sleeve and condenses; if the sleeve is not air-sealed or flashed, water also enters from rain and from capillary wicking in the masonry. Musty odors suggest sustained moisture and possible microbial growth on framing or drywall paper.
Assessment steps: (1) a blower-door test with zonal pressure diagnostics to quantify the leakage attributable to sleeves; (2) infrared imaging of the sleeve perimeter in heating and cooling conditions to locate missing insulation and air sealing; (3) moisture meter readings on surrounding drywall, framing, and sill plates; (4) a visual inspection of exterior flashing, caulking, and sleeve pitch, since sleeves must slope slightly to drain outward; (5) a check of whether the AC unit itself is sealed to the sleeve with a gasketed boot, since a loose fit is both an air leak and a water path.
Scope implications: specify a removable, insulated, air-sealed sleeve cover or an approved through-wall unit boot, not just caulk. Where the AC is removed seasonally, the opening needs an insulated, weatherstripped plug. Any scope that only addresses the boiler and ignores the envelope will leave the moisture complaints unresolved and may worsen them by increasing interior humidity through tighter air sealing elsewhere.