Hydronic Fan control


I'm seeking help with my automatic fan control for my hydronic, oil furnace system. At times -about once a week, the fan will not come on when the thermostat is on "Auto" mode. For example, if the thermostat is set to "heat" and "80" deg; with an indoor temp of 60, the fan still won't come on even after waiting 10 or more minutes. Here are some things that may help elimnate possibilities: I have replaced the thermostat - it has not made any difference. When the fan switch on the thermostat is manually switched ON, the fan will come normally and blow warm air out of the ducts. I also have confirmed that the water circulator motor for the heating coil is running, and the pipes are very hot going in and out of the hydronic heating coil when the fan won't come on. I have no problems with my hot water and the furnace will kick on when the hot water heater demands it. Is there a system component I can probe for voltage, or continuity when the problem is evident? How can I verify if the thermostat is requesting the fan to come on when in automatic mode? What are the interlock components of the system the would prevent the fan to come on in automatic mode, but is bypassed when the fan switch is manually set to on? Thanks.
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The thermostat does not request that the fan comes on when in auto mode. The fan is controlled by an aquastat on the heating coil. When the coil gets to a preset temperature it closes the switch and should turn on the fan. I would suspect a bad aquastat, at least would be the first place I'd start. If it has a logic board, could also be something in that, but lets hope not.
-Brian
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Thanks for the reply. Is there a terminal/wire on the aquastat that I can probe for voltage to make sure the aquastat has a request to turn on the fan? (ie. should I have something like 24 volts at a certain pin of the aquastat indicating there is a demand for the fan?) Thanks again.
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On the coil itself they are usually a molded button with leads going back to the board. It should be on the schematic of the unit.
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