Thanks for your comments, Beth. I don't disagree with any of them.
I've been putting in long nights this week, and this is the tail-end of another one. So these comments will not be as coherent as usual. Hope they're useful anyway. Love to hear from other engineers about the extent to which calculations are useful.
I've been looking into heat-load calculations, like BTUs, and the closest estimate I've been able to do for our house (which I know quite well) ends up with a range where the high estimate is about double the low estimate. Pretty rough numbers. But rough guidelines are still useful; if you look at woodstoves and the estimated square footage or BTU outputs, they are also a wide range with about a factor of 1.5 or 2 between the ends of the range.
There are some zone map calculators that make it super-simple (multiply your square feet by your zone on this map, and give a factor of 1 or 2 for good vs poor insulation). I've seen those for air-conditioner advertising sites.
And then there are the 'complete' equations here:
http://www.engineeringtoolbox.com/heat-loss-buildings-d_113.html
The info I've been taking into consideration, as a matter of practical discussion, when trying to estimate how well a heater will perform for someone includes:
- Square feet and cubic feet to be heated
- Climage, and particularly Heating degree days per month / per year - here's a calculator for US locations:
http://www.huduser.org/portal/resources/UtilityModel/hdd.html. Canadian border areas may need 2-times as much heat as foggy parts of California.
- Layout and era of the house: compact or sprawling? Leaky, average, or over-sealed? Convection working for you or against you?
- How many stories (chimney draft is a huge factor; and with the lower-temperature exhausts there's probably an upper limit on chimney height too)
- Insulation and Exposed wall surface areas (poor, average, or honkin herky can be good
enough as far as insulation. For exposed wall, I'm mostly looking at is the building tucked in around itself, or smeared out like a 'ranch' home from hot climates. Weird angles can increase surface area and drafty gaps.)
- Windows and passive
solar orientation: again, crappy, average, or it's getting there. A huge fishbowl picture-window-wall on the shady side can make that room effectively an outdoor space.
- Planned location of the heater - how central is it to the areas needing heat? Will it be sat on and tended and loved, or will someone have to open the cellar doors to give it a kick?
- Planned size, surface area, orientation, and chimney location of the heater. (The amount of wood you can burn, and the balance between thermal mass storage and warm air circulation, affect how it heats various areas of a house.)
- Lifestyle, skill, and time availability of the operator (cold starts are a lot harder than daily use); are any family members or housemates on board? Special needs?
- sometimes availability of
local fuels, and building materials, is a design factor too.
In general if you can calculate the heat loss from a building within an order of magnitude, you have some idea what theoretical amount of wood it would take to produce that much heat. In a mass heater of the rocket type, this roughly translates into hours of burning required.
I compare the factors above with previous examples, and now that we're into dozens of examples in a variety of climates, I can usually tell someone whether a heater will be worth installing given how much time they are willing to spend burning it.
In Portland OR we might heat with 20 lbs of wood, a 3-4 hour burn in a 6" heater or shorter in an 8".
In the Okanogan we heat with 35-40 lbs of wood and that's about 6 hours in our 8" heater. A similar 8" heater in NY in a slightly larger house uses about 45-50 lbs of wood per day.
In California, a house 4-5 times bigger yet might only use the same amount of wood: 50 lbs/day.
The main differences from a woodstove is that the heat is stored. Once the wood's burned for the day, there's no need to run the stove again later. No burning while people sleep, let alone choking the fire back to moderate or lengthen the heating cycle.
With enough insulation, most compact buildings can be heated just on cooking, appliance, and body heat.
I don't worry so much about thermal mass because the
heaters embody that.
I tend to shy away from basement, outdoor boiler, or over 2-story buildings (we've helped with one building where the heater did provide warmth to 3-story tower rooms, but the section the heater was in had a cathedral ceiling only about 2 stories tall.)
I also tend to encourage people to consider what they actually need to heat: themselves, and their
water pipes, really. Locating the heater on a central interior wall between the sitting room and bathroom is ideal. Home offices or TV rooms are also good areas for first-hand heat, with bedrooms not more than 1 wall away.
Kitchens generate their own heat if you cook at home, and even the refrigerator and computers generate a fair amount of heat. I think of the house as kind of an ellipse or egg shape, with two heat sources: the kitchen as the narrow end, and the main heater centered in the fat end. If all your major appliances are in the basement (
water heater, furnace), a lot of the waste heat is just going to areas that aren't being used. Still, a centered furnace with insulated ducts out-performs one that got stuck in a corner or scabbed on to the side.
Storage rooms, screen porches, activity rooms (ping-pong tables, treadmills, carpentry workshops) can be cooler. Provide on-demand heat (radiators, Rumford fireplaces) for occasional-use rooms like a parlor or guest bedroom.
The best homes for heating have a relatively open plan for the essential living spaces, and the unused rooms and closets form a kind of 'airlock' buffer around the central, warm zone. Windows are mostly on the sunny side; and if there is an upstairs, look for bedrooms there.
Getting some nice dense thermal mass in this warm zone, and insulation and windbreaks around that, makes for stable, comfortable temperatures that can maintain in the presence of healthy fresh-air ventilation.
The principle of thermal mass can certainly be used in larger institutional buildings, but I don't know if we currently have the skillset or mindset to use
wood heat in institutional culture these days. It used to be done routinely; in fact, charcoal brasiers or warming pans might even be brought into church or office by individuals for their own comfort. (So were lapdogs.)
Look for pictures of the novelty tile stoves used as room heaters in Versailles, for example. Engineering and insurance requirements are going to do a lot of the dictation for a custom institutional solution. You might end up with something like the hypocaust floors of Chinese or Roman administrative buildings, or a refractory furnace in the basement that feeds radiant-heated floors. We have been moving away from 'hidden servants tending hidden fires' toward 'hidden machines with refined fuels respond to thermostats', and I don't know if we're ready to go back. But there are some edifying examples out there from antiquity; I like David Lyle's 'Book of Masonry Heaters' for great examples from all over the world.
I've sat in on a massive institutional building's HVAC and hot-water-system troubleshooting, and the problems that have to be solved in order to deliver a good-enough system in a building with that much physical space inside are definitely complex. Tall buildings can generate enough updraft in stairwells to power a small wind generator, for example. They get it almost balanced, and then everybody opens windows and tapes things over the thermostat anyway.
-Erica W