The not-too-rocket-science of a heated house

Posted Wednesday, September 21, 2011 in Sustainable Maine

The not-too-rocket-science of a heated house

     by Paul Kando

It is getting to be fall again. The papers are suddenly full of seasonal ads extolling the merits of the latest high efficiency widgets that promise to cut your energy bills to the bone – at terrific pre-season prices. A neighbor bought a miracle fuel additive and claims that it will make a tank of oil last longer. Not prone to fall for hype, you order an energy audit before launching into a major project of weatherization. The audit provides a blueprint of what should be done and in what order, it also helps avoid costly problems that can develop later. Still, just as you see clearly what to do, the man you hired to insulate the basement ceiling is trying to sell you on foam-coating the walls instead.    

It can get confusing, and your auditor owes you all the explanations you need to set your mind at ease. Even so, nothing beats understanding how your house works. No rocket science is involved.

Heat, air and moisture interact in any heated building in predictable ways. Therefore all three must be addressed or problems may arise. Heat is a form of energy, subject to two important laws of nature: First, it can’t be created or destroyed – a good thing to remember when someone tries to sell you more energy out of a gallon of fuel than what is in it. Second, heat flows only in one direction: from hot to cold. To make heat flow, there must be a temperature (or pressure) difference. A furnace or wood stove must be warmer than the room it has to heat.

Heat flows in three ways: by radiation (e.g. sunshine), conduction (e.g the hot handle of the cast iron pan on the stove), and convection. Heat is absorbed by a fluid (e.g. water or air). As the fluid is warmed, it expands, i.e. becomes less dense. Because it is now lighter, the warm fluid rises, taking with it the heat it has absorbed. Cooler air rushes in to fill the void left by the rising air. This is convection. Convection boilers take advantage of this and we all know that warm air rises because it is always warmer upstairs. (People say “heat rises”. This is not strictly true: heat moves from hot to cold in any direction. It is hot air that rises.)

Air absorbs not only heat but moisture as well, and the warmer the air the more moisture it can hold. We know this from weather reports that talk about Relative Humidity, the percentage of the maximum amount of moisture the air can hold at a given temperature.

Speaking of moisture, we know water has three states: solid (ice), liquid (water) and gas (water vapor). The difference between these states is the amount of heat (energy) the water has absorbed, i.e. the temperature of the water. Ice is a solid: water molecules locked into a fixed, crystal matrix. Heat ice and its molecules will vibrate faster and faster, until they absorb enough energy to overcome their crystalline cohesion and separate: ice melts into liquid water. More heat causes water molecules at the surface evaporate and it is this vapor the air absorbs. More heat, more evaporation. At the boiling point the whole body of water, not just its surface, evaporates. It boils. Ice can also sublimate: escape from the surface of ice in a direct conversion from solid to vapor. (This is why snow slowly disappears even if it is too cold for it to melt.) Vapors are different from liquids in that their molecules have absorbed so much energy that they can float free from neighboring molecules and disperse throughout the available space.

Enter that pesky second – hot to cold – law again: Sooner or later the warm, humid air encounters cooler surroundings. Cooler surfaces or particles (at the “dew point temperature” or below) cause the air to cool, so it can no longer hold all its water vapor. The vapor settles out as liquid water, i.e. it condenses. Dew, rain and snow are examples of this out in nature, fogged-up bathroom mirrors and iced up windows are visible examples in a house. What about condensation we don’t see? Moisture in a house can condense where it causes harm, often invisibly, over time.

Next time we will explore how all this plays out in a heated house, in search for the best way to control not just the expensive heat trying to escape, but also the movement of air and moisture -- reducing heating bills without compromising comfort. For now, admit it, much of the above is pretty straightforward stuff. Isn’t it?    

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