So after the massive amount of information Erica shared with us yesterday, she sends me this massive emails saying "oh and here are some other thoughts ..."
And so I says I says to her ... "this is great stuff, how come you don't put this where everybody can see it?" and she says she says to me "Well, do that if you want"
So ...
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I'm posting a reply, but I also drafted a whole tangle of thoughts about the whole "hot air" question.
Since that was in response to your doubts, I'm including it as a private message.
Common Heat Transfer Media, and their drawbacks: Central Heating is often done by air.
I suspect the reason we naturally turn to air is that it's easy to move. When we feel a "cold draft," it's easy to imagine we
should warm up that air, instead of recognizing that moving air around tends to cool everything (through evaporation).
It's easy to push air through ducting via fans; it doesn't deliver heat fast
enough to burn people under most conditions; it causes few problems if it leaks, unlike other heat-transfer fluids like
water, oil, ammonia, or liquid sodium.
Forced-air allows the heating equipment to be located tidily in an out-of-sight place where only the [paid] repairman will ever need to interact with it. (This can lead to people forgetting to change the filters, and blowing mold through their houses, but hardly anybody notices these things. And everybody notices if the house is too cold or too hot.)
All kinds of nifty commercial gizmos are marketed to improve heating controls, including thermostats, timers, insulation for the ducts, etc.
Drawbacks to the forced-air method:
1) Air is a lousy heat conductor.
Its heat capacity and conductivity are so close to zero that it's the primary component in most insulation (as foam, as "fluff" space in 'glass or
newspaper insulation, as a gap between walls of a cheap Thermos, etc.)
This means you have to push a lot of hot air around a house to get it to warm appreciably, and conversely, the heat dissipates very easily through more conductive materials such as the walls of the ducts.
Hot air can't compete with cold mass in the house:
concrete or masonry in the floor, radiant heat escaping from the windows, cold damp feet.
I think this may be part of why hot air is used in the first place: it won't ever carry enough heat to burn you. Like a blow-dryer can dry your hair without setting it on fire, even though the element inside is red-hot. But this "safety" comes with a price: the
energy it takes to heat and move air is substantial.
2) Hot air isn't all that comfortable to breath. It dries out the house as it escapes through leaks or open doors. Heated air can carry more moisture: it dries out the lungs, chaps the lips, and can contribute to condensation and mold problems as it comes in contact with cold exterior surfaces (windows, doors, the interior lining of exterior walls, etc.)
3) This method requires enormous investment in insulation; effectively, you need an insulator that's even better than the insulator you are forcing to stay warm. In thin-walled wood-framed and metal/glass buildings, the high-tech insulation involved is often quite costly in money, energy, and environmental impact.
4) Enormous amounts of heat are lost in the inefficient transfer from heating device to ducted air. Not to mention in creating and sending energy to your home in the first place. (Electricity loses something like 30% of its power in transmission. I'm not sure about natural gas, but it takes a lot of infrastructure to transmit it safely too.)
Radiant heat has also been used in older American homes and apartments. An electrical or hot-water powered radiator located along the side of a room, can pump out quite a bit of radiant heat.
Problems with this approach:
- A lot of energy is lost in creating the heat
- Radiation follows the inverse square law, and is also proportional to temperature of the radiator. This makes it difficult to get radiant heat evenly distributed throughout a non-spherical house.
- The heater is often too hot to touch, making its corner of the room less usable. Some radiators are hot enough to cause serious burns or start fires if combustibles come in contact with them. And it's always a temptation to put things by/on the heater to dry or warm up.
- The edges of the room where electric heaters are placed are often exterior walls, meaning that much of the heat is lost to the outside.
- The old steam heaters were notoriously noisy, sending "pinging" through the pipes of apartment buildings.
- Radiant-heated slabs, like floors and walls, avoid some of these drawbacks by spreading the heat out evenly at a temperature that's safe to touch. But these systems are somewhat complicated, and the transfer fluids can leak.
Wood-fired heat:
Woodstoves generally put out radiant heat; occasionally it's used as conductive heat through heated water, and occasionally a forced-air element is added to a fireplace or woodstove.
Problems with wood-fired heat:
- Smoke. Massive air pollution, and wasted fuel.
- Inefficiency: Smoke and heat are discharged vertically; when your design combines "heat rises" and "get the smoke out," most of your heat goes out through the chimney.
- Fire inside: Actual combustion in the home, which is fed by amateurs, presents an obvious fire hazard. I'd like to think we learn from practice and can avoid burning the house down. But in my
experience, a campfire or fireplace will teach you more about fire than the average woodstove. And sometime the best learners also push the boundaries of
common sense when they get cocky.
- Externalizing the Fire: Attempts to move the smoke out of the house can further increase the above problems: more outdoor smoke, more inefficient outdoor fireplaces or chimneys. More-polluting devices, which are more of a hassle to tend even though they don't give you immediate feedback when poorly tended. This includes personal exterior
wood furnaces, and social utilities such as massive coal-burning plants or hog-fuel incinerators.
- Wood piles: Obtaining, processing, storing, and transporting
firewood are real-world problems. Some Americans are not healthy enough to chop their own wood, or don't know enough about wood to select and properly cure their fuel. Wood can bring pests into the house. In some areas, there is not enough wood to support the heating and cooking needs of the population, so wood is not a
sustainable resource.
- Fire danger. Surfaces for radiant heat are usually too hot to touch, and often hot enough to ignite any objects placed too close to the stove. Creosote buildup in chimney creates risk of chimney fire. Attempts to run the chimney through the house and tap its heat can result in catastrophic chimney fires.
- Heat timing: wood-burning devices in general burn best when attended and fully-fired. Most homes need heat at night, when people are asleep. This can result in increased pollution and danger from fires "banked" to be left unattended.
- Design placement: Properly-built masonry chimneys can reduce fire danger and increase the amount of heat retained in a house. But the best place for radiant heat is in the middle of the house, and requires clever roofing and framing solutions.
Chimneys are often placed on an exterior wall, or surrounded by invisible "voids" within interior walls if the owner does not like the appearance of exposed masonry. Exterior chimneys release most of the heat outdoors; interior chimneys end up radiating much of their heat into dry, trapped-air spaces of interior framing, instead of freely into habitable spaces.
A woodstove raises the same concerns: put it in the middle of the room, "in the way" of everything, or off in a corner where it won't do as much good?
Many of these problems are also present with other forms of energy: burning fossil fuels creates a lot of pollution; electricity generation often relies on polluting or inefficient sources. But these problems are externalized for the homeowner, and tacked onto the utility bill. Or in most cases, left for future generations.
Most of our heating options contribute to global atmospheric
carbon by using fossil or bio-fuels. The only options that don't contribute significantly:
1) Geothermal (uses existing heat of earth's crust; "heat pumps" move this heat from one place to another),
2) passive
solar (uses existing heat of incoming sun), or
3) locally sustainable bio-fuels (carbon discharged by burning plant matter is re-absorbed by growing more plants; in some cases this practice can become a net carbon sink by increasing standing biomass; in others, it is a net loss as biomass is cleared to make way for efficient fuel production.)
I'm not sure about
4) nuclear - I think it releases atomic heat "early," and it certainly involves a lot of processing of its materials at both ends. Probably a slight, net contributor to global warming. The threat of nuclear pollution to increase genetic mutation and cancer makes me leery of this option.
And I've just thought of another option that we used to use, but don't much anymore:
5) Domestic animals.
Some medieval homes were built in two stories, with the animals housed beneath and the people above. Others took it a step further, and brought sheep or
cattle into the house itself in very cold weather. A "three-dog night" is one cold enough that you invite your working dogs to sleep with you.
Advantages: Cosy, companionable, and stacking functions if you already have the animals around. Keeps them warm too.
Disadvantages: Communicable diseases, indoor air quality, allergies, hygeine and health hazards, smelly, noisy, annoying, intrusive, and takes a toll on personal belongings.
Some of the advantages of domestic animals can also be obtained by building a smaller home, and sleeping with multiple family members in the same room or bed. Or by sleeping in an attic or loft, which collects the heat from the household's day.
Passive
Solar design:
This is a marvelous field. Too much to go into here. But if you're designing a house, this is well worth learning before you dig. It can make the difference between a house that is comfortable in every season at minimal cost, and one which requires constant struggle to maintain at livable temperatures.
- Research traditional house designs for similar climate zones; visit at different times of year, and ask the owners what they do to maintain the home's temperature.
- Research
local climate patterns: learn your sun angles for winter, summer, and equinoxes; prevailing winds; rainy and dry seasons.
- Think about heat flows through your home:
Do you have the option of closing and opening doors or shutters to regulate heat flow?
Do you have a way to keep out summer heat, while welcoming winter sunshine?
Are you using the core of your house effectively, either for controllable draft circulation or for radiation?
Are you orienting your home, and the largest windows, appropriately for local sun and weather?
Can your house stay livable using only the energy from your personal
land footprint (sun + biofuels)?
Finally, a reminder courtesy of my grandmother:
"There's no such thing as a perfect house. Every one of them has something wrong with it." (And 'most every one has something to appreciate.)
-Erica
