Architect William Sikora designed both stories of this 80-square-foot LTMS in Minneapolis, Minnesota, to be accessible. The upper floor can be used for sunbathing, clothes drying, wood drying, or for storing items not affected by heat. The first floor is used for storing firewood and garden equipment. Twin-wall, 8 mm polycarbonate was used for exterior glazing.
The sunspace has some thermal mass storage as evidenced by the gradual temperature drop after sunset (see “Sikora Sunspace Temperatures” table). Interior stagnation temperatures in the sunspace reach 120°F with the blower off, but interior drywall and the concrete floor provide enough thermal mass to prevent overheating in the house. The blower will run as long as the home requires heat.
Heated, filtered air from the space is conveyed into the loft of the main home via a blower fan and pushed down with ceiling fans. Simple thermostats, one in the sunspace and one inside the home, control the blower. One thermostat is located near the highest point in the sunspace, and is set to close when the sunspace approaches a usable temperature for space heating: for example, 90°F. The living space thermostat is set to close when the temperature drops below room temperature, at about 70°F. The two thermostats are wired in series with the blower, so the blower only comes on when the temperature in the sunspace is between 80°F and 90°F and the temperature in the living space is below 70°F. When both thermostats are closed, the 260 cfm blower starts. (The small amount of thermal mass in the sunspace allows for use of a smaller-than-normal blower.) In the spring and fall, the air temperature delivered to the house can exceed 100°F. During the cold Minnesota winters when the sun is out, the air temperature from the sunspace averages about 80°F. “The fan kicks in every day when the sun is out, no matter what the temperature is outside,” says Sikora.
Cool air enters the sunspace from the house through a one-way damper. Summer overheating is reduced by opening the upper and lower access doors in the sunspace.
Sikora says that their entire heating bill for 2011–2012, which includes water heating, was $350—very low considering their climate’s 7,981 heating degree days.
An inline fan equipped with a backdraft damper and air filter conveys heated, filtered air to the home’s loft via a wall-mounted grille.
On the lower level, one duct serves as the supply for the air-to-air heat exchanger. The sunspace helps to preheat the air going into ductwork that serves as the supply for the air-to-air heat exchanger. The other duct is the dryer vent, which gives up some of its heat into the sunspace.