My boiler seems to randomily turn on and burn for a few minutes often during the day. This occurs when the room temperature is above the call temperature setting. The circulator pump does not start, only the boiler lighting and burning for a few minutes. I will come home from work and see my room is a good 4 or 5 degrees warmer than the call temp and the baseboard is warm(but not hot like when the pump is circulating), so i know its been burning. And the room temp would not have gotten low enough to call for heat, so it's not like the heat was being run and I came home after it stopped and the heat i felt was the residual. It is a gas boiler, not at home, so i cant recall the make or model, with one zone. Recently had the aquastat replaced, also a new thermostat unit. Im thinking it might be something to do with the low or high setting on the a-stat Any ideas would be appreciated, Sorry I can't provide more details off the top of my head.

A neighbor has a boiler with forced hot water and radiant heating in the floor of a single story house. I would guess the house and boiler was new 10-15 years ago. Each zone has a circulator pump and there is also one for the hot water tank. It seems the circulator pumps need to be replaced every 4 years or so. The reason they have been replaced is because they were beginning to make noise. I looked at the latest pump that was replaced after the technician left and found the bearings were just fine and it appears the noise was the result of a buildup inside the pump that was beginning to contact the vanes as they spun. I scraped some of the buildup onto a piece of paper and it appears to ferromagnetic. The residue was easily picked up with a magnet. She does have iron in her well water but it is treated with a greensand/permanganate treatment before reaching the domestic water and the latest water test done less than a month ago after the treatment indicates the free iron in the water is < .05 mg/L. There is also no evidence of insoluble iron oxides being left in the water after the treatment. Normally I might expect there to be a leak in the system so that a continuous supply of oxygenated water is being introduced into the boiler water but there is no sign of a leak. We even turned off the makeup water supply and there was no sign of a pressure drop during a thirty minute period when the boiler was not being fired. Any idea why there would be so much buildup in a closed system?