Winter is approaching here in Boston and we've already had a few days where we've had to turn on the heat. We have an forced hot water heating system. It's a bit more complicated than the steam heating system I had before. A steam system only has two states, "on" and "off" (controlled by the thermostat). A forced hot water system is a bit more complicated, with "HI", "LO" and "DIFF" temperature settings in addition to the thermostat control.
The main control for the forced hot water system is called an Aquastat. It controls "HI", "LO" and "DIFF" settings. Inspectapedia's Guide to Heating System Boiler Aquastats, their Settings & Wiring provides an excellent explanation of these settings and how they interact with the thermostat and boiler. Also see What are the Best High, Low, & DIFF Settings on a Heating Boiler Aquastat? which provides additional details.
Forced hot water heating systems often use a tankless system to supply domestic hot water. This is what I had until recently. After watching the system's behavior for about a year, I determined that it was grossly inefficient. A home energy evaluation by Conservation Services Group confirmed my suspicion (the tankless is only ~33% efficient). I recently had a propane-fired, standalone, tank water heater installed (which is 62% efficient).
Technically, I no longer need to keep the boiler water warm and should be able to set the "LO" aquastat value as small as possible. But, some corrosion has built-up around the tankless plate and my heating technician says this will only get worse if I let the boiler water cool off completely. Inspectapedia notes that some cast iron boilers require an active low setting during summer to prevent leakage.
So, I'd like to use an aquastat setting of about 180/100/10 (HI/LO/DIFF), but something went wrong around the time of the propane tank installation and now my boiler does not respond to the "HI" setting. So, I'm currently stuck having to set my LO setting fairly high to ensure the water is warm enough to heat the house. Fortunately, it looks like these aquastats are simple enough that I will be able to fix the problem. Currently, I'm checking with my plumber to see if he might have changed anything during the propane tank install.
FWIW, it looks like my aquastat is very similar to Honeywell model L4081A1023. Note that the L4081B1047 is nearly identical except that it also controls the circulator(s). Inspectapedia notes that some models will shut off the circulators when the boiler water temperature is in the "LO" range.
Here are links to manual for the aquastat I have:
Update (10/13/10): Turns out I have an 8124A Honeywell Aquastat (model number was on back of cover!).
Update (2/23/11): Forgot to note that my aquastat was busted. My heating company replaced it free-of-charge since I'm on an oil delivery plan.