Part V: Passive Solar Homes and SunspacesChapter 9. Sunspaces, Greenhouses, and Solar Rooms
Moving Sunspace Heat into the House
Fans, high and low vents, and doors: how to get daytime heat inside without night-time losses.
- Difficulty
- Intermediate
- Cost
- $30–150
Heat leaves a sunspace only while you give it a path. The useful path is a loop: warm air in high, cooler house air back low, and both openings able to close when the sunspace is colder than the house. A single high hole with no return, left open at night, is how a porch becomes a chimney that runs backward and empties the house.
A door you open at noon and shut at dusk is a complete design for a small low-mass porch. A fan is for when the door is awkward or the porch is warmer than you want to walk through. The fan does not get to run at night. A backdraft damper, or a damper you close by hand, is part of the fan, not an accessory.
If the sunspace is a greenhouse with barrels, move only surplus afternoon heat. If you empty it at sunset, the crop freezes.
Difficulty
- Level
- Intermediate
- Time
- A day for a damper and a fan, after the porch exists
- Skills
- Keeping a return path, and knowing when not to cut a rated wall
Cost estimate
$30–150
A small fan, a damper, and two grilles can land in this range. A listed fan, a thermostat, and a proper sleeve through a thick wall land at the high end or above it. Cutting a fire-rated wall is a different budget and often a different person.
| Piece | Typical cost |
|---|---|
| Fan in the high tens of cubic feet per minute | $20–60 |
| Damper or a grille you can close | $10–40 |
| Thermostat that compares the two rooms | Optional, and worth it if you will forget |
Materials
| Item | Qty | Notes |
|---|---|---|
| A high opening into the house | 1 | Warm air. Damper closes when the fan stops or when the porch is cooler. |
| A low return | 1 | So the fan circulates instead of pressurizing one room and starving the other. |
| Fan sized to the porch, not to the house | 1 | Volume of the sunspace, times about 4 to 8 air changes an hour, divided by 60, is a planning band in cubic feet per minute. |
| A way to know which room is hotter | Two thermometers or a differential control | Run the fan only when the sunspace leads. A few degrees is not worth a draft. |
The loop
Warm air collects at the top of the sunspace. Take it off there and deliver it high enough on the house side that it can cross the room you mean to heat. Bring house air back low, into the bottom of the sunspace, so the glass can heat it again. Without the return, the fan still spins. The sunspace goes to a lower pressure, cracks and a damp floor become the return, and the house side is a blast with nowhere for the old air to go.
Size the fan from the sunspace. An 8 by 12 foot porch, 8 feet high, is about 770 cubic feet. Six changes an hour is about 80 cubic feet per minute. Eight changes is about 100. A fan rated at several hundred cubic feet per minute will short-cycle, roar, and still not help if the openings are the size of a postcard. The grille has to pass the air. The duct page is the same lesson in a collector: free-air fan ratings are not installed flow.
Control it on temperature difference, not on a clock you set in October and forget in March. When the sunspace is cooler, the fan is off and the damper is shut. Test that on a cold evening with a tissue at the high opening. The tissue should not stream into the porch.
What the fan must not do
Do not blow greenhouse humidity into a cold bedroom. Moisture will find the first chilly surface. A growing room vents surplus heat outdoors first.
Do not use this fan as a way to depressurize the house. A fan that pulls from the house and dumps outdoors, or into a porch that then leaks outdoors, can pull combustion products from an atmospheric water heater or furnace. The sunspace fan should only move air around the loop between porch and house. If you are unsure whether a fan in your house fights a flue, stop and read the caution on the night-flush page, and get a combustion-safety check before you leave a fan on a thermostat.
A hole in a fire-rated wall or a garage separation is not a grille from a hardware store. Listed dampers and the wall rating are a code question. Ask before you cut. Electrical for a hard-wired fan is an electrician's job if it is not yours. A cord across a doorway is a trip hazard and a pinch point.
Prefer the door if it already works
Open at noon, shut at dusk, weatherstripped. Add a fan only if that habit cannot move enough air.
Cut a high supply and a low return, if the wall allows it
Confirm the wall is not a fire separation you are about to ruin. Damper the high opening.
Size the fan from the porch volume
About 4 to 8 air changes an hour, divided into cubic feet per minute. Provide grille area the fan can actually use.
Run it only when the porch is hotter
Thermometers on both sides. Tissue test at night. The tissue stays still.
Leave greenhouse mass alone at sunset
Surplus afternoon heat only. The barrels need the rest to carry the plants through the night.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Sunspace/sunspaces.htm
- https://www.builditsolar.com/Projects/Sunspace/LowThermalMassSunspace.htm
- https://www.builditsolar.com/Projects/Sunspace/LTMSSGuide/LTMSSGuide.htm
- https://www.builditsolar.com/Projects/Sunspace/SolarRooms/SolarRooms.htm
- https://www.builditsolar.com/Experimental/GlazingTransmittance/GlazingTransmittance.htm
- https://www.builditsolar.com/Projects/SpaceHeating/Glazing.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.