Hello Errica and Ernie, and thank you for sharing your RMH experiences/knowledge. I discovered this a fortnight ago so have a few questions.
1. Can you advise what space/volume can be heated by RMH. How can KiloWatt Hours be calculated? - I am wondering if organisations could benefit from this type of heating, as well as my home.
2. How are the exhaust flues cleaned out? I am wondering if ash could be vacuumed out, or whether condensed steam causes the ash to become congealed.
3. Does spitting wet wood ever occur? I used to have an open fire which needed a fireguard mesh in front, in case burning material was ejected from the fire on to the surrounding fabric (rug/carpet/upholstery).
I'm not an expert, but I see you have been waiting for a long time for an answer. I did heat completely with wood at one time and have written green HVAC curriculum.
You cannot calculate KiloWatt hours, because there are too many variables. All heating systems are exactly that: a system. Some of the variables are fuel, insulation, thermal mass and insulation. The kind of wood, its dryness and density, all impact its burn time, heat and efficiency. My green house will lose more heat than a cob house or other well insulated building. Thermal mass will be warmed by both the RMH and solar gain. I originally was going to just do sand, but now I am going to do clay with that sand to reduce the air pockets and have a more dense thermal battery.
RMH stoves are designed for gassification of the cellulose. Key areas for clean out are the feed, immediately exiting the barrel, and any turns or bends where the exhaust will slow. Rob from web4deb has included a clean out to his design feed so he can remove ash without totally letting his system cool. Exiting the barrel is another key place. I am told that the bends do not need to be cleaned as often, as little as yearly. I would only use a vacuum on a totally cold system.
Avoid wet wood in any fire. I have not seen screens, because of the drafting properties of a rocket, but hey, better save than sorry.
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.
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.
Edits after sleep:
Climate, not 'climage'
Canadian border areas (high-altitude in WA like us, upstate NY, Maine) are generally 2 to 4 times the heating loads as CA, all else being equal - California locations might need 2000-3000 HDD per year, and up here its 6000-8000. Oregon's maritime climate was more like 4000 at sea level, 5000 or more at altitude in the mountain ranges - that's what I've seen for places like Tennesee and Kentucky too. Florida was mid-1000's, with more cooling degree days than heating. Southern CA has some mountains that are back up in the 2000-4000 range.
Of course, there are LOTS of microclimates and variation year by year. A sheltered, sunny spot is going to be much easier to heat then an exposed-yet-shaded one, and so on.
The equation I used goes roughly: Heating Degree Days x (total exposed surfaces / their respective insulation R-ratings) plus factors for infiltration and time = overall heat loss per month or per year.
This calcs out based on wall and ceiling surface, not just floor square feet. So taller, or lumpier, or long skinny houses are harder to heat than the same square footage of a short, compact house (square or round would be most compact).
I like heating degree days because it's a straight multiplier on the overall design of the house. That makes it a pretty good predictor of the amount of firewood or fuel you might use in a year, with a comparable building and heater.
For maximum load capacity, it pays to look at the coldest month, not just the average, if you want the house at the same temperatures throughout a cold snap.
For 800-1000 SF, average home, Western fuels (fir/larch/pine, like cherry/pine in density)
- in Portland, we burned under 1/2 cord per year - to meet a HDD factor of 4000, R-Crappy insulation (maybe R-10 to R-15). 1-4 hrs/day
- in the North Cascades of WA we are burning about a cord per year, or 20-35 lbs/day - factor 7000, R-30 insulation. 2-6 hrs/day
- in a CA shady coastal valley, we have reports of 2 cords / 3000-5000 SF, or 2/5 to 2/3 cord per year per 1000 SF. (Other woodstoves at far ends of house, so some of the wood is used less efficiencly to balance temperature gradients in the wings of the house.) - factor 3000, R-20 standard insulation?
- in coastal Oregon, we have numerous reports of under 1/2 cord for small buildings (200-1000 SF). - factor 3-4000, varied insulation and thermal mass walls. 1-2 hrs/day
- still waiting for reports from OH, NY, ME.
- preliminary note from NY of 47 lbs/day in January for 1200 SF, factor 8000, R-50 insulation (some wood goes to kitchen cook stove)
These seem to work out to BTU amounts that are pretty similar to the rough theoretical values for the house and for the energy contents of the firewood.
We can omit the 'heat lost up the chimney' factor that is needed for woodstoves. And in practice, comparisons with neighbors suggest the actual wood used is something like 1/4 to even 1/10 of local norms. (Local norms vary; the 'type' who puts in a mass heater has usually improved their insulation and passive-solar or weatherization as well.)
If good insulation and compact, multi-functional spaces are normal in your area, the difference in heating efficiency may not be as great, but you'd still get the benefits of cleaner burn and better overnight temperature stability.
I want this post to show up if people search for 'heat loads', 'heating calculations,' or 'BTUs per sf / per square foot.'
Let me know if this has been helpful. If you want to describe a more specific public building scenario, I might have further comments.