Pre-Set High Limits
The temperature at which a high limit switch in a boiler engages to stop the boiler from heating is pre-set by some manufacturers. AquaSmart boilers, for instance, have a high temperature limit of approximately 200 degrees Fahrenheit as do pre-set Weil-McClain boilers. Aquastat boilers, on the other hand, have a lower high temperature cutoff of approximately 180 degrees. Most boiler manufacturers pre-set the high temperature limits on their boilers within this 180- to 200-degree range.
Manual High Limits
Some boilers allow you to determine the temperature at which a high limit switch engages to stop the boiler from heating. A Honeywell boiler, for instance, can be set with a high temperature shutoff between 180 to 240 degrees Fahrenheit while manual-set Weil-McClain boilers have a maximum shutoff temperature of 220 degrees. Boilers with manual settings do generally come with a high limit setting already in place. In most instances, the manufacturer’s pre-set high temperature setting offers the best results.
High Temperature Differential
When a high temperature limit is set on a boiler, the boiler doesn’t actually stop heating at that exact temperature. Instead, it ceases to heat by adjusting to a slightly lower temperature, commonly called the high temperature differential. Depending on the make and model of boiler, the high limit differential may be either 10 or 20 degrees below the high limit setting. When the control shuts off at the differential, it has the ability to turn back on automatically if the boiler requires more heat.
On the other end of the boiler temperature spectrum is the low temperature limit. This temperature serves the same purpose for cooling that the high temperature limit does for heating. When the boiler reaches the low temperature limit or, more accurately, the low temperature differential, the unit automatically begins heating again to ensure that the water supply does not drop below the low temperature limit. As with high temperatures, the low temperature limit on a boiler may be either pre-set or manual.