When a non-condensing boiler fires from cold, water vapor in the flue gas condenses on metal below about 130 degrees F, so the boiler appears to sweat and may drip onto the burner. A few minutes of it on a cold start is normal. Sweating that continues means return water is too cold, which leads to acidic corrosion, scaling of flue passages and rust in the burner area.
Boiler's sweating so bad it's hissing on the burners, the return's coming back at 100, we need a bypass or a mixing valve.
Also heard as
- sweat
- fireside condensation
Related terms
Cast iron sectional boiler
A boiler built from cast iron sections joined in a row, used widely for residential and commercial heating.
Thermal shock
Damaging stress from a sudden temperature change in boiler metal, such as cold water hitting hot iron.
Flue gas
The hot exhaust that leaves the boiler after combustion, made up mostly of nitrogen, carbon dioxide, water vapor and leftover oxygen.
Fireside
The surfaces of a boiler exposed to flame and combustion gases, as opposed to the waterside surfaces in contact with water.