this is my first post so i would like to introduce myself and say thank you for this enlightening and valuable forum!
i am the co-owner of a residential construction company in princeton. i have limited knowledge of boiler systems but have learned a bit. probably still closer to a homeowner than a professional with regards to heat systems..
i have an old farmhouse that was built in 1780. the heating system is steam radiators on an oil boiler. the domestic hot water is heated on the same boiler, controlled by an aquastat.
i keep my thermostat quite low, 59 degrees. i keep my DHW set at around 120 and my aquastat is set at 135. the probe for the aquastat is rather short, doesn't actually enter the boiler but instead terminates about 4" outside the furnace.
problem: the aquastat is calling for heat so the boiler comes on and stays on. it goes all the way to steam temperature (212, well past the 135 that it is calling for) and starts heating the house. it usually happens early in the morning, i wake up because it's 75 degrees in the house and my thermostat is set at 59. i know the problem is coming down the aquastat line because when i disconnect it the problem is solved and the heat functions normally. as a test i have disconnected it for days and had no problems (and had hot water because the boiler is hot from heating the house). as soon as i hook it back up, the boiler kicks on the the furnace rages. the circulation pump is off on the water tank, the water has reached the desired temperature, yet the aquastat calls for heat.
history: this has happened a few times in the last two years. to my furnace techs' credit, they have replaced nearly every part on the furnace (switches etc.) and the aquastat and circulation pump on the DWH side. it won't happen for ages, i don't think it has ever happened in the summer. often a 'fix' will work for a period of time, then the problem will happen again. replacing the aquastat worked for 24 hours. replacing the wire from the aquastat to the junction of the thermostat and the furnace worked for a few months (i think the wire was bare in places). replacing the DHW circulation pump worked for a few months, then presto! problem returned.
i am really baffled and so are all the technicians from my oil company.