This control is also called a "bonnet control" in conventional house gas and oil furnaces. It has a very sensitive Bi-Metal element probe which senses differences in temperature. The bonnet control is positioned such that the probe is inside the "warm air plenum" of the JUCA. When it senses that warmed air is present there, it closes a FAN switch inside itself which turns on the JUCA's blower to distribute the warm air out into the room and/or house.
At whatever point that you let the fire go out, the probe senses that, as well, and shuts the blower off to save electricity. The ON and OFF temperatures are independently settable. This is seldom necessary with a JUCA, because they are usually factory-set at appropriate settings (ON at about 125°F; OFF at about 100°F). With some of the larger optional blowers on the JUCA, it may be desirable to adjust these settings, as the Operator's Manual and the Installation Manual describe, to ensure that the JUCA blower turns on and off at appropriate times for most efficient operation.
Bonnet controls also include another switch, called the HIGH LIMIT switch, which JUCAs do NOT use. That switch's function is to shut off the electricity to the gas valve in a gas furnace in the event of a blower failure, so that the gas furnace didn't overheat. When burning wood in a JUCA, there is no gas valve functioning to shut off. In addition, a JUCA is built very sturdily, to easily withstand a few hours or days of operation with the blower off (due to no electricity). (Under those circumstances, the unit's overall efficiency drops a lot, and the JUCA operates MUCH hotter.)
Many years ago, JUCA considered using ritzy, electronic sensing devices to control the blower. Such ideas were dismissed as unnecessary and also because we wanted to use standard, available electrical products. In the event that a bonnet control or blower failed on a JUCA in Idaho many years down the road, we didn't want the owner to have to wait for us to ship a replacement part to him; a local heating/plumbing store should have it immediately available.
On an F-9A built-in fireplace, the location of the bonnet control might require some thought. If the F-9A is embedded in masonry or an enclosed chase, it may never be possible to ever see the unit again. This probe should be put somewhere where it could be accessible, both for future adjustments and in the event that it ever needed to be replaced. The control we use has a long sensing probe (11" long). Just the end 3" or so must be in the warm air chamber. This could mean that it could be mounted into the side, back, top or even front of the warm air plenum chamber, with the adjustment part being outside the masonry or surrounding chase. Another possibility would be a small access panel near where it is. Yet another possibility is that it would be mounted somewhere in the warm air duct near the JUCA, where it would be accessible.
The JUCA Home Page is at: http://mb-soft.com/juca/index.html
E-mail to: JUCA1@mb-soft.com