Low Thermal Mass Sunpaces: Page 3 of 4

The Little-Known Gem of Solar Heating
Intermediate

Inside this Article

Low Thermal Mass Sunpaces
Low Thermal Mass Sunpaces
The author’s “low-mass” sunspace
The author’s “low-mass” sunspace is actually a high-mass greenhouse that was tested for thermal performance—before the mass and plants were moved in.
The greenhouse used for sunspace testing
The greenhouse used for sunspace testing. The long ducting here was used to measure airflow. Make sure to size the fan to deliver 3 cfm per square foot of glazing with the duct system that you have chosen.
10 mm double-wall polycarbonate
The glazing used in the test LTMS is 10 mm double-wall polycarbonate with an R-value of 1.8. At about $2 per square foot, it is less expensive and easier to work with than two panes of glass, although its lifespan is shorter.
Ranco ETC thermostat
The Ranco ETC thermostat turns the fans on and off based on the sunspace’s temperature. This is a temporary installation for testing; permanent installation would be made more cleanly.
10-inch Dayton fan and R-21 fiberglass insulation
The west end of the sunspace shows the 10-inch Dayton fan and R-21 fiberglass insulation (to be covered with Rboard). The Rboard on the north wall is painted black for improved solar energy absorbance.
The east-end ducting and blower fan
The east-end ducting and blower fan—a Dayton 10-inch, which provides 665 cfm of free air delivery. The shade cloth was an experiment at preventing overheating in the space.
Low Thermal Mass Sunpaces
The author’s “low-mass” sunspace
The greenhouse used for sunspace testing
10 mm double-wall polycarbonate
Ranco ETC thermostat
10-inch Dayton fan and R-21 fiberglass insulation
The east-end ducting and blower fan

How Much Heat?

The test 200-square-foot sunspace in southwestern Montana has 200 square feet of south-facing twin-wall polycarbonate glazing tilted at 60° for good winter solar collection. The walls and roof are insulated to R-27 and the floor to R-8. The low-mass walls are Atlas Rboard rigid insulation painted with black latex paint—they heat quickly and pass that heat into the air. The Rboard has a fiber face sheet that takes paint well and is durable, but is not approved for direct exposure to a living space for flammability reasons. The floor consists of EPS rigid foam panels laid over compacted sand, with plywood laid over the trafficked areas. For the test, black weed barrier cloth, which makes a solar-absorbent surface, was placed over the floor. The structure was sealed using spray foam and caulking to reduce infiltration.

You don’t have to be this particular in specifying surfaces that are dark in color and low in mass. For the walls, any dark-colored surface will work, and wall coverings like wood paneling or 1/4-inch plasterboard will not significantly sacrifice performance. For the floor, a surface like cork or carpet or even garden bark will work as long as it is insulated underneath.

The solar radiation was measured using an Apogee pyranometer mounted in the plane of the glazing. The heat output to the house was calculated from the temperature rise from inlets at the lower southeast and southwest corners to the fan outlet ducts and the flow rate out of the two 10-inch ducts. To measure airflow, a flow velocity survey was taken in the long, straight section using a calibrated Kestrel-turbine-style anemometer. The inlet air is ambient air that averaged 27°F.

The “System Temperatures” graph shows the inlet, outlet, and ambient temperatures and solar radiation for a sunny-day test on January 4, 2013. Under steady collector conditions a little after 1 p.m., the heat output of the collector is 42,200 Btu per hour, and the solar input is 69,900 Btu per hour—which calculates to an efficiency of 60.3%. This is comparable to high-quality commercial solar collectors operating under the same conditions.

If the heat output is calculated for each hour, the total heat output for the day adds up to 232,000 Btu. These heat-output numbers are higher than typical because the test was done with outside ambient inlet air (at 27°F) instead of room-temperature air (65°F, for this calculation). The cooler the inlet air, the more efficient the collector. The heat output adjusted for room-temperature inlet air instead of 27°F inlet air would be about 19% lower, or about 188,000 Btu per day. This is equivalent to 2.9 gallons of propane burned in a 70% efficient furnace. So, even this modest-sized sunspace produced a lot of useful heat—even on a day of the year that has close to the fewest number of sun-hours. Near midday, the sunspace is producing 10,000 watts of heating power—and the only energy being used to “produce” this is the two 27-watt fans.

At about 2 p.m., the fans that push hot air out of the sunspace were turned off. Without fans or vents to remove heat, the temperature in the peak of the sunspace quickly climbed to more than 150°F. This is an indication of how fast an LTMS responds when ventilation is stopped and how effective the insulation and double-glazing are in reducing sunspace heat loss.

I wondered how sunspace output would be affected by not following the design rules, so I tested it before the insulation was added, and with a bare dirt floor. The difference was dramatic. Under similar sun and ambient temperature conditions, the heat output of the unfinished sunspace was about 33% of the finished sunspace. Following the design guidelines significantly improves the heat output.

Comments (1)

Dan Rhodes's picture

Very good article on Solar rooms. I have a 1250 sf off-grid home near Flagstaff, AZ. My home also has a 270 sf solar room with insulated slab that helps provide about 70% of my heating needs. Highly recommend this type of heating.

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