On Feb 6, 10:43 am, "Brian V" wrote:
wrote in message

ups.com...





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.

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- Hide quoted text -

- Show quoted text -

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.