A thermosyphon system with large vents distributed across the width of the back (house) wall of the sunspace—high for the hot air vents and low for the return vents—might be able to provide the required airflow, but a fan-forced system will be the right choice for most situations.
The LTMS should be equipped with a fan or blower to move the heat to the house as quickly as it is produced. The testing suggests that an actual fan flow rate of about 3 cubic feet per minute per square foot of sunspace glazing is sufficient to remove the heat under sunny conditions. A variable-speed fan or multiple fans that can be turned on one at a time would allow the flow rate to be adjusted for less-sunny or early- or late-day conditions. Select the right fan capacity, but also be sure to consider fan type and placement to avoid annoying noise.
My test sunspace, a stand-alone structure that eventually was converted into a high-mass greenhouse, uses a Ranco ETC controller—a simple heating controller that switches a fan on when the temperature exceeds a setpoint temperature. (A snap-disk switch would have served the same function.) This controller activates two Dayton 10-inch-diameter fans (27 W each), one mounted at the peak of the west wall and the other at the peak of the east wall. While the fans have free air delivery of 600 cfm each, with duct losses, they actually produce 325 cfm each. This setup was close to the specification of 3 cfm per square foot of glazing and worked well. This could easily be made into a two-stage system by hooking up each fan to its own controller and setting one controller to a lower temperature so that the first one comes on at 80°F, and the second one comes on at 90°F.
The Sikora sunspace control scheme (see “Inspiring Sunspaces” sidebar) uses a line-voltage cooling thermostat in the sunspace and a line-voltage heating thermostat inside the house to control a blower that distributes heated, filtered air from the sunspace into the loft of the home. Inside the home, ceiling fans help distribute the heat. When both thermostats are closed, the 260 cfm blower starts. Cool air enters the sunspace from the house through a one-way damper, which prevents backflow when the blower is off.
The duct into the house must include a passive, spring-return damper that opens only when the fan is on to prevent cold air from flowing into the house at night. That said, some of the spring vents do not seal very well—the ones I have are just fair. Fabric dampers might work better. The poly-film dampers I used on my shop thermosyphon collector could work well inside a sheltered sunspace, with inlet and outlet dampers facing opposite directions. Motorized damper actuators (such as those made by Honeywell) also are available that could provide a positive seal.
Keeping the ducts inside the sunspace, rather than outside, is also helpful for mitigating heat loss. This shelters them from the wind, and eliminates the need for insulating them. The sunspace also reduces heat loss from the house to the outdoors since it provides extra insulating and sheltering where it is attached.