The ideal pressure for a boiler is around 12 psi (pounds per square inch) when it is cold. Most boilers have pressure gauges that display different-colored zones, and the needle should sit within the green zone. If the needle sits much outside the green zone, particularly on the high side, there may be a problem. When the unit fires up the heat, the pressure will change, and in most cases, it should rise to about 20 psi while its heating element operates. Pressure changes of a few psi one way or the other can be brought about by atmospheric conditions. Most boilers are safe up to around 30 psi, at which point seals can begin to fail. The highest pressure the boiler should ever see is stamped on the boiler tag, usually 30 or 50 psi. The pressure the system is designed to operate at is usually 12 to 25 psi for a two-story house and 18 to 25 psi for a three-story house. If you live in a multi-story home, there are exceptions to the generally accepted pressure ranges, and a professional can determine the optimal range of your boiler by its model. If you find that your boiler pressure is dropping further than you would like, it’s likely that a leak has occurred.