Temperature spike on boiler

Toad

I was hoping to get group’s input on the following:

(I’m not at all technical, so apologies if some of my descriptions are lacking.)

Older but well cared for Weil McLean boiler in basement of 3 story home. Three zones each with own thermostat. Zone 1 - hot water radiators through majority of house with a B&G 100 (I think) low head circulator. Zone 2 - hot water baseboard radiators in basement with B&G NRF-22. Zone 3 - kitchen in-floor heating (some sort of hot water tubes but we didn’t put it in so not really sure of details) with NRF-22. 

Typically don’t even run basement or kitchen loops unless temps outside really drop.

Temp gauge on boiler typically stays at or below 180 degrees, but I recently noticed on two occasions that temp spiked to 220 degrees. Pressure remained in normal range. 

Both times, I isolated the fact pattern to the main loop and basement loop were not running (temps high enough in house so thermostats for those zones didn’t need to be calling for heat). Both times, I had forgotten to turn the kitchen floor thermostat down so it continued to call for heat for that radiant floor heat zone. Is it possible that given the low volume of water in that one loop and its proximity to the boiler, it just kept heating up beyond the normal system limits?

Once I lowered thermostat for that kitchen floor zone, temp gauge on boiler reverted to 180 or below even if I turned main thermostat up again for rest of house. (I guess enough water on a long enough main loop to keep temps down as it returned to boiler?)

Any suggestions beyond “just don’t ever run the kitchen loop on its own/don’t ever set the radiant floor thermostat above the main radiator loop thermostat? (Since temp gauge has only spiked when kitchen loop running on its own.)

Any other major concerns to think about here? System has otherwise been operating smoothly and got a clean bill of health during annual preventative maintenance not too long ago.  

Thanks!

delta T

I'd bet on a bad circulator for the kitchen zone. If its the only zone calling, and the circulator isn't moving water, the boiler will heat up very quickly and hit its high limit and shut off. Those wet rotor circulators don't like to just sit for long periods of time, they tend to seize up.

Toad

Thanks deltaT. Any easy way to test/confirm that with or without reaching out to my HVAC guys (who seem to be pretty good)? I can hear/feel a steady something happening in the circulator when it’s zone is calling for heat (and stops when it’s not), but I guess that could just be it trying (but failing) to function properly.