First, thermostats have an "anticipator" function, which shuts off the furnace before the set temperature is reached. That's because the furnace normally runs a minute or two after the thermostat cuts off, during which time the temperature will rise a bit. Most thermostats are set at the factory to work with forced-air furnaces and need to be readjusted for boilers. Because there's so much hot water in the pipes when the boiler cuts off, the anticipator needs to shut off a hydronic system much earlier than a furnace. If your thermostat has a non-adjustable anticipator, you may need to change it out for one that does, like one of the popular round Honeywell units.
Second, thermostats on hot water systems don't usually directly control the boiler. Instead, they open and close the zone valves. The zone valve has a switch on it that starts the circulation pump and fires up the boiler once the valve is open. If you have a sticking zone valve, it won't matter what you set the thermostat to. Typically what happens is one valve will be stuck and the valve for a different zone will cycle. I'm only familiar with the Honeywell valves; on these units there's a lever that sticks out of the control box for manually opening the valves and to indicate the valve position. You can tell if a valve is working right by turning up each zone with the others turned down and making sure that only the one zone is circulating.
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Today's Featured Article - The Ferguson System Principal An implement cutting through the soil at a certain depth say eight inches requires a certain force or draft to pull it. Obviously that draft will increase if the implement runs deeper than eight inches, and decrease if it runs shallower. Why not use that draft fact to control the depth of work automatically? The draft forces are
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