A manhole is an access hole typically located on the top side of the boiler.
It allows access for cleanings, washing out of debris and sediment, or repairs. The manhole should be opened annually, at a minimum, for internal inspection of the vessel.
When the cover to the manhole becomes corroded, it becomes both a safety and efficiency concern. If the manhole cover seals shut, the vessel cannot be properly inspected. If the cover seals incorrectly, with a space between the gasket and boiler, a steam leak can occur. This can happen on a steam boiler running for the purpose of creating heat. This will impact the boiler efficiency as you are losing steam while running the boiler for heat and also affect your fuel costs. While losing steam will cause you to burn more fuel, it is also an imminent safety concern as an individual who comes in contact with steam under pressure can be seriously injured. Any repair performed on a pressure vessel should be done by a professional, experienced, R-stamp certified mechanic.