Hello All!! I've been an unregistered troll for years. This is my first post because I really need some specific input from you all!
I am planning on building a green house this year. I am located in Central Siberia. I want to employ the WOFATI methods, that is PSP + PAHS.
My main questions surround the PAHS part of the WOFATI method and I REALLY would like some input from some one who has REAL experience and can give advice from the "been there done that" perspective.
First, let me give an overview of my project. I will have a 50' by 20' greenhouse. Underneath the greenhouse will be a 9' deep by 12' wide by 30' long root cellar/cold trap. Behind the greenhouse area I will have an enclosed 20' by 50' barn where I will house my two horses, 16 goats and ten sheep. Their will be another cold trap trench between the green house area and the barn area. This trench will be 7' wide and 4' deep from the floor of the green house on one side and the barn on the other.
The dirt from excavating the greenhouse (it is sunk only 3-4' into the ground) and the root cellar, the cold sink trench and the ramp to get into the green house as well as drainage ditches around it will be piled up around the structure on three sides. Above the barn will be a hay loft which will insulate the barn in winter and help ventilate in summer (no hay in summer).
I'm installing in-floor heating pipes in the greenhouse section and (hopefully) will be heating them with a Jean Paine pile for supplemental heat.
Now you have an idea of what I'm up to.
Here are my questions. In the PAHS system the author calls for insulating the ground to the tune of R20, no less than 20 ft away from the structure in all directions. How necessary is this to success? I have heard the OPINION that it is the "umbrella" effect more than anything that makes the PAHS system work so well, not so much the insulation. Is there any way of verifying this? ANd if insulation is needed R20 seems lot. Where did this number come from.
My next question, still dealing with PAHS, is the ground pipe, or breathe pipes. Great IDEA, but how much does it really add to the heating of the ground and how well will it handle ventilation when the outside temp is -40 ( a very common outside temp here in the winter, for at least 2-3 weeks). IS it worth the extra trenching, pipes, etc?
A third PAHS question. The whole concept seems based on a six month cool sixth month warm cycle. We are more like 7-8 month cool, 4-5 month warm. Think Alaska. Any experience?
This is all very relevant to me. I'm building in June, once the ground thaws out. If I can make a passive system (more or less) than can give me a growing season of 10 months as opposed to 3.5 months, it would be revolutionary, not only for me but for all of my neighbors.
HAS any one here built or seen built a greenhouse using WOFATI or PAHS systems? Was it passive, truly? what were the outside temps, vs. indoors?
MY final question is about thermal mass. I will have the barn behind the green house with no solid wall between the two. I mean fences and the trench so no animals will be in the green house but, no thermal mass collecting wall. Thats because I'll have the anymals. Will the animals give off MORE heat than a thermal wall would collect when the days are short and the outside temp is, say, -30 to -40? ANYONES gut feeling on this.
Advice, input, questions, criticism; all welcome!!!
I also live in a cold climate, although not as cold as Siberia. I work in the building industry, and while I haven't had the chance to build my own WOFATI greenhouse yet, I have quite a bit of experience with heat flows through buildings and greenhouses specifically.
With greenhouses, there are a great number of factors that effect energy use. Two of the biggest are heat losses through the glazing, and heat losses through air movement (air infiltration and the ventilation needed for your plants and animals).
With any sort of passive heating/cooling system, energy storage is based on a difference in temperature between the storage medium (earth in this case), and the energy source.
Let's take a single winter day in your greenhouse as an example:
It's a cold clear day with lots of sun. In the morning, as the sun begins to enter the greenhouse, the greenhouse warms up. When the greenhouse gets warmer than the earth surrounding it, the earth starts to absorb heat energy. The larger the difference in temperature between the earth and the greenhouse, the more energy the earth can absorb. This means that the hotter the greenhouse gets, the more energy you can store in the earth for later. Even when it is very cold out, sun can heat a greenhouse to 100F or more.
At night, the greenhouse begins to lose heat back to the outside. This can be slowed through the use of insulation and the closing of any vents, but you will lose some heat to the outside. At some point, the greenhouse temperature lowers below the temperature of the earth surrounding it. Now, the earth can give its heat back to the greenhouse. The colder the greenhouse gets, the more heat it can absorb from the earth around it. In my climate, it can get down to about -15 F. A greenhouse that is allowed to drop to freezing at night and goes up to 100 F during the day (using night-time insulation), will use very little supplementary heat. However, most plants don't like to be cycled that way.
A year is similar to my day-time example, except over a longer scale. In order to heat the earth during the summer, you need to have a heat source hotter than the earth. When it is time to use that heat, you need to have a space temperature lower than the temperature of the earth. Therefore, if you can only get the earth up to 60F during the summer, you'll only be able to use it to heat your space to some temperature less than 60F. After a while, the earth will cool down, and its ability to heat the space will diminish. The more earth you can warm, the longer your heat will last.
Bottom line - you'll need supplemental heat, but a lot less than a conventional greenhouse. The more extreme temperatures you/your plants can handle (hot during the day/summer and cold at night/winter), the less supplemental heat you'll need.
Regarding insulation - insulating the greenhouse is going to be pretty important. The north, east and west especially should be insulated. The south should be glazed (with movable night-time insulation if possible). In this case, more insulation is always better, although there are diminishing returns. Include as much thermal mass as you can inside the insulation. The coldest external temperature is the air temperature, so insulating around the south side where the glazing meets the earth is especially important so that frost doesn't push through the wall and create a cold floor.
Another alternative to the PAHS system with air tubes is, if the greenhouse is not in use in say July, and August, is to close it up and let the sun heat the greenhouse as much as possible so that the earth around the greenhouse warms. You can pre-heat the thermal mass of the earth this way, without additional infrastructure.
A Jean Paine pile is a great idea if you have a source for the raw materials and a use for the compost. You might also look into a masonry style heater for supplementary heat. They burn wood very efficiently and store and distribute heat over a longer period of time. Compared to a Jean Paine pile, you'll probably need 1/3 of the wood (but you don't get compost out of it at the end)!
Hope this helps you some.
Joined: Apr 14, 2012
Dear Karen, Thanks for the input. I'm hoping that my animals (28 head) will provide some of the supplemental heat needed to stabilize the daily fluctuation. I really want to not put in a stove for supplemental heat because once you have a system dependent on a stove you have to stoke it. I'd like to get passive. I know the JP pile takes a lot of volume but then, as you said, thats not something that just burns up. You get the compost which is stellar!
Are breath pipes worth it?
I'm planning on having a roll up canvas, felt, canvas insulation on the glazing to minimize the night time heat loss. What would your suggestion be for ventilation if I don't use a PAHS like breath pipe system?
Thanks for taking the time to write!
Joined: Apr 18, 2012
Animals will definitely help provide some of the heat needed. They will also provide CO2 for your plants (and your plants will provide O2 for the animals).
I can't really answer your question about the air tubes. They can definitely help pre-heat ventilation air, but eventually the earth around the shafts will cool off. I'm not sure if it'll be worthwhile. Another question is when will ventilation be needed? For plants, most ventilation air is needed while the plants are photosynthesizing, ie, when the sun is out. This also happens to be the time when the greenhouse is most likely to be passively heated, so bringing in cold ventilation at these times is less of a big deal. Heat loss can be minimized if there is a way to exchange heat between exhaust air and fresh ventilation air. An air to air heat exchanger could achieve this, but is a higher tech solution.
Commercial air to air heat exchangers can recover up to 80% of the heat in exhaust air and transfer it to incoming outdoor air. I can envision a lower tech solution whereby exhaust air is directed through a thermal mass chamber and warms the mass. Supply air is also directed through a thermal mass chamber, and cools the thermal mass. After a short period of time, say a day, that flow is reversed so the supply air absorbs the heat rejected by the exhaust air previously. However, I'd be surprised if the effectiveness reached 20% and you still have to deal with reversing airflow.
Another option is to provide a ventilation "vestibule". This would be a narrow section of greenhous that is glazed and insulated at night like the rest of the greenhouse. On sunny days when the plants are respirating and need fresh air, you allow outdoor air to pass into the "vestibule" where it is pre-heated by the sun and some earthen thermal mass.
You'll still need some ventilation even when the sun isn't shining to manage moisture and disease, but hopefully this gives you some ideas.
Im also looking for somebody, who did it. Where are they?
If you didnt read John Haits book, do it. Its really good, easy to read and a pleasure, because it is so practical.
I will comment on some of the above, which I think I have understood.
No water should run through your thermal mass. It would drain out your warmth.
where your room or air (tubes) have less than 6 meters (18 feet) of dry, thermal mass to all sides, it should be insulated. Else the warmth from one summer ( say 6 month and every day one inch of penetration) would travel out of your thermal mass.
The tubes are supposed to exchange the heat. Temperature out should be equal to temperature in. Which means no loss of heat.
To achieve this, the tubes have to be adequate long. According to John Hait maybe you should have them 60 meters (200 feet). They should be treated like the rest of the thermal mass. Although John Hait didnt cover and insulate 6 meters around the house in Rocky Mountains, but only 3 meters. For the tube-trenches he also used 3 meters. With 10 cold month I would consider doubling the trench with.
How it works:
Earth is actually a very bad accumulator. Add to that, that it heats and cools very slow. With PAHS John Hait supposedly created a system that make use of this "bad" qualities. Any temperature difference between mass and air will start a transfer. This transfer will travel inward in the mass like the waves on a pond, when you drop a stone. A short impulse will fade out, a hot summers day will create a heatwave travelling several meters into the mass. When winter comes, the waves will switch and the heat is drawn back. But because the warmth moves slowly in the mass, you cannot "empty" the store quickly. It takes time. Which means, that - when your thermal mass is constructed correctly - it will give you warmth back all winter long.
The tubes ensures that no heat is lost with air supply. Insulation ensures that no heat is lost through the walls. Of cause there will be a loss. Your thermal mass should compensate only that. And according to John Haits house in Rocky Mountains, it works.
The animals are heating. But they also need lots of air. A lot more than your plants can give. Here the tubes come in handy. They bring all the fresh air you need without giving any of your dear warmth away. The tubes levels all differences and gives you an even temperature, day and night, summer and winter.
From the above, I would encorage you to build a full PAHS and tell me, how well it worked
Joined: Jun 13, 2012
I would add just one thing: There is the PASH method, and there is the AGS method. I'm planning to use the AGS method, which I think is simpler and possibly more effective. (Annualized Geo Solar). You can look it up, including a comparison, and you can talk to the guy who invented it (Don?). And I'll be watching for information from anybody who has experience with either.