Thermal Efficiency

(I, Soren, have no formal knowledge of this subject, but have picked up a few things along the way. Hopefully The Home Energy Diet has a few things to say on this topic.)

Insulation and thermal mass are the keys to thermal efficiency. Insulation keeps heat from moving where you don't want it to (whether that's into your house on a summer day or out of it in the winter) and thermal mass supplies a "heat buffer" to smooth out the highs and lows in temperature over time. For example, blankets provide insulation to keep your body heat inside the bed at night and a stone wall or floor absorbs the heat of the sun and continues radiating that heat long after the sun is gone. Similarly, a basement's contact with the ground will keep it cool as heat is absorbed into the ground when the air is warm and re-radiated out when the air is cool.

Even though it seems like "cool" exists, it's only a lack of heat energy. We will use "cool" as a loose term, though we will try to focus on moving heat where we want it.

Heat rises

If you want something to stay cool, put it low.

Dark objects absorb sunlight (also heat?) and radiate heat

If you want to capture the sun's heat, use dark colors. If you want to stay cool on your ride to work, wear a white shirt. Dark objects also radiate more heat at night time!

Yes, you should turn your heat down at night

On Nov 7, 2008, at 15:25, Stephanie Blandino wrote:

> http://ahok.de/en/night-set-back.html

I agree with most of his analysis, but disagree with his conclusions. In particular, he assumes that the house is well-insulated and doesn't lose heat very quickly. And he ignores the fact that the coldest part of the time is in the middle of the night{1}. He also assumes that the heat system in question is one that uses more energy when it has to work "harder." Most heating systems in America are of a forced-air type which simply runs until it is warm enough, they don't waste any more or less heat up the chimney depending on whether they are raising the temp from 66 to 68 degrees or if they're raising the temp from 58 to 68 (at least that I'm aware of).

Think about this (assuming, for the moment, constant outdoor temperature):

1. heat "runs," on average, 20 minutes out of every 60 minutes

2. turning down the heat allows the heat not to run for two hours (or, in this guy's house, much longer)

3. then heat runs, on average, 15 out of every 60 minutes to maintain the lower temperature

4. the next morning, the heat has to run for maybe 60 minutes to get back up to temperature

(these numbers are all related to insulation, etc, but increasing one decreases the others; they're based on my memory of where I lived several years ago with such a system in place)

We saved 40 minutes of run time in the first two hours (step 2) and ran for 40 extra minutes in the morning (step 4). The difference, which this guy says is small, but I say is large (particularly in Bay Area, CA houses+weather{2}) is the six hours in which we only ran the furnace 15 minutes instead of 20 ... saving a total of 30 minutes of runtime.

I will admit to not having real data on this issue, but I suspect that PG&E and others making this recommendation know more than this guy. And even his 3% savings is significant in any kind of climate.

{1} If you want to consider that it gets coldest at night, then you can assume the heat runs 20/60 minutes throughout the night *if* you turn down the heat ... but runs 30/60 minutes (increasing energy consumption at night) if you *don't* turn down the heat.

{2} Because it is not very cold here, Californian houses tend to be very poorly insulated (have you ever seen an "Eichler" (sp?) house in Palo Alto ... doesn't it make you chilly just looking at it?!) so heat is quickly lost despite the low temperature difference between inside and outside. Due to the weather, we can let the inside temperature fall significantly closer to the outside temperature ... to the point that the rate of heat transfer is cut in half. This is a significant savings, especially in a house which is already quite wasteful due to having so little insulation.