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 - Hydraulics - Cylinder Anatomy - by Curtis von Fange. Let’s make one more addition to our series on hydraulics. I’ve noticed a few questions in the comment section that could pertain to hydraulic cylinders so I thought we could take a short look at this real workhorse of the circuit. Cylinders are the reason for the hydraulic circuit. They take the fluid power delivered from the pump and magically change it into mechanical power. There are many types of cylinders that one might run across on a farm scenario. Each one could take a chapter in
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