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need to calculate the amount of heat needed to keep my house warm  RSS feed

 
yannick Wenger Srodawa
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i'll be moving in a new place in the beginning of 2016. well, as new i mean different: it's a little house with some work to be done (earth floor, compost toilet, windows, RMH etc...)
i need to find out how to calculate how much heat do i need to keep the place warm during winter...that would help me decide the kind of RMH i will be working on.
i've had a look on the net, but i ain't to sure about what i read so a little help could do me no wrong...

 
r ranson
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Location: Left Coast Canada
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What an exciting project. Welcome to Permies.com, by the way.

I'm still learning about this subject, but I think I can anticipate some of the questions that will come up as you discover your answer.

For example...
  • How big is the space?
  • Will you be heating all the rooms at once, or just some of them as needed?
  • How much draft?
  • What's the insulation like?
  • What's the insulation going to be like when you finish the upgrades? I've noticed at least one home (ahem, mine) where we installed the heating system first, then found it too hot once the other upgrades were complete.
  • Is there sufficient airflow without being drafty?
  • Will you use some sort of fan to move the heat arround?
  • How damp is your winter? Damp makes things feel colder than they are. Here, zero C feels SUPER COLD, but back east, -20 C is merely cool
  • What fuel will you have access to? Hard wood? Soft? Something else?
  • If you feel comfortable, can you give us an idea of what part of the world you are moving to? Different places have very different ideas of winter.
  • Will you be cooking with the same system you heat with?



  • What little I know about RMH and other heating systems is ... your needs seem dependent on your individual situation.

    I'm looking forward to reading what others write.

    Also, I hope you keep us up to date with your endeavour.
     
    yannick Wenger Srodawa
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    hi R Ranson!
    sorry about so little information. i am no good for long mails but i'll try to be a little more precise about this one.
    i live in senegal right now where i'm running with my wife a little permaculture project (gorom-garden.over-blog.com)and i will stay here till begining of december then move to normandy (france) where i originally come. from. so for the details concerning the house i can't give all the answers but i'll do my best...for more details i'll have to wait till i get there in december.



  • How big is the space?
  • approximately 75/80 m2
  • Will you be heating all the rooms at once, or just some of them as needed?
  • i'm hoping to heat the all place with the RMH and i still need to research about air flow/ natural draft
  • How much draft?
  • no ideas yet
  • What's the insulation like?
  • the wall is builded with local stones, i don't know about the roof insulation. the house is oriented west/south west and we'll be puting a big window for natural sun light/heat
  • What's the insulation going to be like when you finish the upgrades? there we'll be an earth floor that'll help a lot and for the roof i still have to see before i get into speculation...
    I've noticed at least one home (ahem, mine) where we installed the heating system first, then found it too hot once the other upgrades were complete.
  • this is exactly the reason why i've posted in the first place. i was thinking of using the RMH for heat and adding a cast iron stove for cooking but i'm pretty sure the heat will be too much... this is why i need some kind of maths formula to work with
  • Is there sufficient airflow without being drafty?
  • sorry but i'm not natural english speaking and i don't understand the question
  • Will you use some sort of fan to move the heat arround?
  • nope
  • How damp is your winter? Damp makes things feel colder than they are. Here, zero C feels SUPER COLD, but back east, -20 C is merely cool
  • the house is about 2/3 high up a little hill facing south (the hill not the house) with a little river approx 100m down the bottom of the valley. the winter is very wet and doesn't often go below zero c° but the wind can be pretty strong. we are approx 70 km from the sea. being a farmer/landscape desinger for quite few years i can tell you for sure that it feels very cold and wet in winter...but the common normand house do well in wet climate...
  • What fuel will you have access to? Hard wood? Soft? Something else?
  • hard and soft woods. (i aint' to sure what you mean? do you mean wet or dry?)
  • If you feel comfortable, can you give us an idea of what part of the world you are moving to? Different places have very different ideas of winter.
  • koppen climate zone Cfb altitude: approx 100m distance from the sea: approx 70 km
  • Will you be cooking with the same system you heat with?
  • nope

    hope you get the detailed you were looking for... i'll definitley let you know about the progress i'll be making as for most of the work i'll be doing it'll be the first time to do it for me, so i'll need technical advices over more or less every work i'll get involved in...

    thank you for your help


     
    r ranson
    master steward
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    Location: Left Coast Canada
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    Hardwood is from trees with leaves that drop in winter. Softwood is from trees with needles.

    That's an extremely inaccurate definition, but it is a good one for practical purposes.

    Softwood tends to have more sap and burns quickly, but more likely to leave a residue. Hardwood burns hotter and slower - generally. Each type of tree is different.



    I don't know enough on the topic, yet, to give any specific suggestions. However, you've come to the right place. I am confident that someone will join in the conversation and offer ideas.
     
    yannick Wenger Srodawa
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    ok i'll be using only hardwood...
    not only do i get help but i learn english in the process
     
    Glenn Herbert
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    75-80 sq. meters is around 750-800 sq. feet, which is a small house in American terms. Thus, I think a 6" RMH system would probably work for you. The good thing about RMH capacity is that if it is too big, you just run it less often, and your house stays comfortable with less work. If your climate seldom gets much below freezing, the constant warmth ought to do a fine job.

    "Sufficient airflow without being drafty" means that there is enough fresh air coming in when the house is closed up for cold weather, but not so much that you feel cold winds while sitting in the room. Since an RMH works largely by conduction and radiation, heating mass and surfaces more than air, you don't need to worry as much about sealing all the cracks as you would with a forced air furnace.
     
    Glenn Herbert
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    If you are thinking of a wood-fired cast iron stove for cooking, you might consider a separate rocket stove built onto the exterior wall. Stone walls outside of a Mediterranean climate are most always cool, and you would not be as likely to overheat your house in warm weather as with a cast iron stove.

    A cooking rocket can be optimized for top surface heat with little mass to radiate into the room. In cold weather, you will learn a good schedule for firing the RMH so it is not giving maximum heat while you are cooking.
     
    Dan Huisjen
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    Location: Acadia Region, Maine.
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    I'm an Energy Auditor. This is the kind of stuff I do, calculating like this.

    What country are you in? I'm in the USA, and I'm used to working in feet, °F, and BTUs. I don't know if that will just add to the confusion or not. One other thing I'm used to using is Heating Degree Days. This is a rough measure of your climate, and is calculated from comparing the indoor temperature to the average daily outdoor temperature, and then adding all those daily numbers up for the year. In my area, with a 65°F indoor temperature, we have about a 7200 HDD climate. Looking up the climate for Missoula Montana, I see that with the same 65° base temperature, they had a 7259 HDD season last year, which I understand is warmer than normal. Washington, DC had 4117 HDD in the last year.

    There are two ways a building loses heat. It can go out through a solid surface, or it can go out with the air that filters in and out of your building. Let's look at air transported heat loss first.

    To figure out air transported heat loss, you need to know how much air moves through the building. I use a special fan and pressure gauge system. I find the air flow at 50 pascals of negative pressure, then do a bunch of math to turn that into air changes per hour. The goal for natural air exchange is .35 air changes per hour. Tighter than that is good, but will require mechanical ventilation to make sure you have enough air flow for your health and comfort. There is no good way to find your air flow without this tool. Anything else is a guess. With experience, I've gotten fair at guessing, but I'm often wrong.

    So, say you have one air change per hour, which is very leaky. Take the volume of your house, and multiply that by 24 hours. That's how much air goes through your house each day. Multiply that by .0182 BTUs per cubic foot of air. Then multiply that by your heating degree days. For instance, let's say my house is 6000 cubic feet. 6000 x 24 X .0182 x 7000= 18,345,600 BTUs. That's heat lost by air movement. That was the easy calculation.

    Now let's look at solid surface heat loss. This is heat lost through walls and ceilings and closed doors and windows, and downward through the ground. For each surface, you have to compute it separately. Find the area of the surface, the R-value, the HDDs, and factor in the 24 hour day. So, lets say I have a wall that's 20' long and 8' high. It has two windows, which are 10 square feet each. That means you have to compute the 140 square feet of the wall separately from the 20 square feet of the window.

    Let's say the wall has an R-value of 15, which is nothing special. 30 would be twice as good. 30 would lose only half as much heat. But let's say R-15. 140 square feet x 24 hours x 7000 HDD =23,520,000 BTUs. Divide that by the R-value, and you get how much heat is going through the wall: 1,568,000 BTUs.

    But here's the thing that isn't in the books: Heat moves by convection, conduction, and radiation. Air transported heat loss is convection. But in solid surface heat loss, we're dealing with conduction through the wall, and mostly radiation to take the heat away on the other side. Because of this, the radiant temperature of the background is very important. When I look out my windows, I see trees, which are mostly the same temperature as the air. But the clear sky is very cold. This is why we put more insulation in attics than walls. We find that you can multiply the BTUs going through the wall by 0.7 and get a fair number. Multiply the result from the same calculation for the roof by 0.8. Multiply the foundation number by 0.4, and when doing the foundation, count the wall area to 2 feet below the exterior soil line. Do each surface separately, and then add up all the BTUs for each surface. Use a spreadsheet program and be careful. The math is not hard, but there's lots of it, so it's easy to make small mistakes.

    You'll need a reference table to pick an appropriate R-value for each wall and door and window and ceiling. If the insulation is fiberglass in good condition, assume the value is 75% of what the table says. If it's in bad condition, assume it's 20% of what the table says. Stone has an R-value of R-1 per foot of thickness. Concrete is about the same. So is a single pane of glass. Two panes of glass is R-2. You can see why movable insulation over windows at night is such a good thing. In my region, the recommended values for new construction are R-60 ceilings, R-35 walls, and R-15 basements. Windows should be at least R-2, but I've seen triple pane windows with special coatings and xenon gas between the layers that claimed R-8 performance. I'm not sure I'd spend that kind of money though. Windows are expensive, and cover only a small area. Basic windows and good insulation elsewhere is probably better use of your money.

    The best return on investment is air sealing. Small holes are everywhere. The material to fill them in is usually cheap.

    If you burn wood, you need combustion air for the fire. It's best if you have a closed stove and a tube that brings outside air directly to the fire, rather than pulling cold air through the heated room. A dedicated combustion air supply is safer too.

    One other thing: Thermal mass means nothing. It will keep your building cold as well as warm. It will reduce temperature swings, but it will not reduce the amount of fuel you need. If you live in an all stone house in a cold climate, be prepared to burn huge amounts of fuel.

    Good Luck.




     
    Dan Huisjen
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    Location: Acadia Region, Maine.
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    There's always something else.

    Wood! Wood has a fuel value of about 6400 BTU per pound. What kind of wood doesn't matter much. If it's heavy oak, more of it will fit in the stove at once than if it's balsa wood, and the balsa will probably burn much faster, but the heat that comes out is the same, pound for pound. Make sure it's dry. To boil water takes a lot of heat, and if that water is in your wood, it has to boil away for the wood to burn.

    Generally speaking, a simple barrel type wood stove has an efficiency of about 55%. A good modern stove has an efficiency of maybe 70 or 75%. When you get up above 90%, you get into condensing territory. If you have complete combustion, the two main products are CO2 and water. A stove that is that efficient will give up enough heat from the combustion gasses that water condenses back to a liquid. I've heard of rocket stoves doing this when built right. But be sure there's a place for the water to drip out.

    So, to put this in perspective, the average house in my state needs 97 million BTUs per year to heat it. There are about 25 million BTUs in a cord of good hardwood. (A cord is a stack 4' tall, 4' wide, and 8' long.) So that would be 5 1/2 cords of wood, burned in a 70% efficient furnace or stove.

    If I want to know how efficient a house is, as a quick first estimate, I calculate BTUs per square foot of living space per HDD. If the number comes out at 5 BTU/ft2/HDD or below, that's great. If it's around 8 or 10, that's normal, and there are things my company can do to help, often bringing them back down to the 6 range. If the house has a number of 12 or higher, then it's probably a very leaky and poorly insulated old house. I've heard of houses with numbers as high as 20, where it's probably best to burn it down and start over or walk away.
     
    David Livingston
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    I think this is one of those how long is a bit of string questions as the better insulated more self heating it is the less you need to heat it . If you have the ability and cash they front load your improvements to the house and your long term needs will be less .

    David
     
    I agree. Here's the link: http://stoves2.com
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