Four years ago, I wrote about a few home energy-saving tips in Refrigerator Sitcoms and Lethal Toaster Ovens. I said I would need to write a series because there are so many, mostly bad, energy saving tips to write about. With four years passing, it is more like a movie sequel than a TV series.
Knowing the “why” behind anything helps it stick, like training. Here goes.
The Ceiling Fan
Famous advice for ceiling fans is to have them move air down (counterclockwise) in summer, and up in winter.
A ceiling fan can do two things only. It can generate a breeze to keep occupants cooler than without it, provided they are not wearing clothes head toe, and it mixes room air to have the same properties throughout. If there is a fire, as one example, we don’t want fans pushing smoke down to floor level.
I wouldn’t bother changing fan direction. In fact, I have a high ceiling in my great room, which is heated by a woodstove. If I took the above advice, I would be wasting fan energy and heat energy because heat rises and it stays at the ceiling level. The fan reversal to blow it upward does not generate enough velocity to get it to the floor level where I’m typing this blog.
I would only use a ceiling fan during heating season if your heat source is not coming from the floor. I.e., you have diffusers high on the wall, like in the south, or maybe ductless heat pump coils around eye level.
In summary, here are my recommendations for ceiling fans. Complicated? Do you want to save energy or not?
Open or Close Diffusers?
It seems like a good idea to shut diffusers, from central heating and cooling systems, in spaces that are unoccupied. Why condition an unoccupied space such as a bedroom or basement? Most sources say shutting diffusers does not save energy. The one source that sparked this post says “do it.”
Energy can be saved if cooling or heating that would go to the unoccupied space were instead directed to an occupied space. This will happen to some extent as the closed diffuser increases duct pressure, which means more air pushes out elsewhere.
However, residential fans generate very little pressure, regardless of how much air flow is pinched off. You might save a tiny bit of energy by shutting diffusers in a couple rooms. Slightly more heating and cooling air will be delivered to where it is needed. Beyond that, the furnace will simply waste heat via higher exhaust temperature and the AC compressor will use more energy because the suction temperature and pressure drop. In fact, if air flow is pinched off too much, an evaporator coil can turn into an ice block, and that is absolutely a waste of energy – huge, 99% waste.
I will also mention that for retro-commissioning commercial buildings with direct expansion cooling (like home central AC), engineers need to be aware of control sequences because curbing fan power can result in an ice block like this. Not good.
Blinds and Drapes
Blinds and drapes pose one of the most complex and impossible-to-predict heat transfer problems imaginable. Most energy-saving tip lists advise closing blinds and curtains to block or reflect incident solar radiation. However, it doesn’t work like the average person imagines.
Solar energy is transmitted, absorbed, or reflected, by anything, including windows and people. Johnson Window Films provides some of the easiest to understand graphics in this regard to the right.
Most readers will have windows that perform more like the one depicted in the cartoon on the bottom. Using blinds on the inside does little to stop the energy that has made it through the window. The reflective properties are a function of wave length (electromagnetic spectrum shown above). It’s hard to tell what portion of the energy reflected by blinds actually bounces back outside, but my bet is almost zero.
Most blinds and drapes (especially blinds) don’t fit tight enough to prevent convection from moving the transmitted energy into the conditioned space. However, there are two things to consider.
First, blinds block solar energy from projecting onto other things in the space, like humans. In a way, blinds are like fans. Avoiding transmitted solar, according to this guy, makes occupants feel five degrees cooler. I agree and think it is at least five degrees, depending on window properties.
Second, an indoor covering that will substantially cut transmission into (summer) and out of (winter) the space is a window quilt. These things include tracks to form a good seal to prevent convection as I described above. The downside: they remind me of Quilted Northern toilet paper. And they are pricey!
The best way to save energy involving windows is to design with solar impacts in mind. Designers are best off avoiding direct solar altogether – avoiding east/west windows and using exterior shading devices. It looks far hipper than toilet paper.