My Weil McLain Ultra 105 boiler has been kicking code 18 for several years. No tech has been able to find the problem. All known sensors have been changed. The circulator pumps have been changed. The boiler temperature has been adjusted to 180 degrees. It continues to lock out on code 18

What is a code 18? Different manufactuers may use the same code for different problems.

What is a code 18? Different manufactuers may use the same code for different problems.

Sorry, a code 18 on this particular unit is: "High limit operation-burner off until outlet temp drops below 200 degrees F.

Has anyone actually measured the boiler temperature with other than devices installed on the boiler?

The actual temperature has been measured. The sweep gauge is off; but, the digital gauge temperature is within a couple of degrees. The temperature at the sensor is also within a couple of degrees.

Sounds like you just can't get rid of the heat as fast as the boiler makes it. Not being familiar with that particular boiler, is it a modulating & if so, can you actually watch or hear it modulate?

Most common problem with this code is incorrect piping of the boiler. Most likely the boiler loop didn't get piped, or piping is incorrectly sized. The ultras are very clear on piping in the install manual. Anything other than what is required will cause this issue. Basically you don't have enough flow or volume through the boiler and the temp rise is happening to fast causing it to overshoot the temp setting, hence the lockout. Was this boiler professionally installed? These are not like the old time boilers where you have an in and out and life is good. These are extremely efficient boilers (when installed correctly) and very temper mental when not installed correctly. Lots of things could be causing this. Are your zones controlled by pumps, or zone valves? Most zone valves are very restrictive, but will work as long as the boiler piping is done correctly. Can you post some pics of the piping directly above and near the boiler?

The boiler was professionally installed; but, perhaps they did not pipe it exactly as it should have been installed. Everyone makes a mistake from time to time. Your answer makes complete sense. And you're right. When it works correctly, it is a great boiler. Here are some pictures of the piping. Thanks