I'll cut to the chase – gain refers to how you generate heat for your home without solar panels. You can think of it as gaining heat by manipulating the power of the sun. There are three categories of gain: direct gain, indirect gain and isolated gain. So much for the mumbo jumbo, what are we really talking about here?
Direct gain refers to improving your house in a manner that allows the sun to…[drum roll]…directly provide power in the form of heat to your home. For instance, if you install large windows in the south facing side of your home and put down heat storing flooring such as masonry, you are pursuing direct gain. The sun will directly heat the home through the windows during the day and will also heat up the flooring. As the sun goes down, the flooring will continue to radiate heat. The advantage of direct gain is it is fairly easy to implement. The disadvantage is it only works during the day and for a few hours afterwards if you've put in a heat storing flooring material.
Indirect gain is a bit more complicated. It refers to the idea of using a structure between the exterior and interior of the house to store heat produced by exposure to sunlight. The basic idea is to get more heat production for a longer period of time. Indirect gain is typically accomplished by building a thermal wall out of masonry, known as a Trombe Wall, as the south facing wall of your home or a part of it. The wall is built out of a material that absorbs heat such as concrete or brick and then has glass placed over it. Put another way, the south wall is a window with a brick wall behind it. The wall materials suck up the power of the sun and store heat. This heat is then radiated when you need it by opening vents in the wall. The advantage of indirect gain is you get longer, more controlled heating. The disadvantage is you have the world's weirdest looking window on the south side of your home.
Isolated gain is a simple concept. Have you ever used a greenhouse to grow flowers or tomatoes or…well, something? Isolated gain works just like a greenhouse, except you are providing heat to yourself instead of plants. Essentially, you build a self-contained glass structure on the south side of your home which is also well insulated. The structure heats up in the sun during the day to very high temperatures. When you need heat, you just turn on a fan, which moves it into the house through venting you've installed.
The beauty of any of these systems is they are fairly simple concepts to understand. If solar panels don't appeal to you, just go for some gain.
