drgrieve
Member
Hi,
Long time lurker - I'm building a house at the moment was wondering how long the pipes should be. For example if the inside temperature is 30C and the soil is 20C, how much 90mm pipe would you need to drop the temp 5c to 25c pumping 1000L of air a minute?
Not sure if 1000L is too much or not .. but interested in how this would be calculated.
Thanks.
matas
Member
"How Long?
The longer you make the earth tube, the closer the air flowing through the tube will get to the earth temperature.
But, there is a diminishing returns effect as the tube gets longer and longer.
Somewhere around 75 to 100 ft might be pretty close to the sweet spot.
The temperature change in the tube air after 150 ft appears to be small.
As the tubes get longer, the cost to install them goes up, and the fan energy to drive air through them also goes up."
http://www.builditsolar.com/Projects/Cooling/EarthtubeNotes.htm
"Tube length
There is no simple formula for determining the proper tube length in relation to the amount of cooling desired. Local soil conditions, soil moisture, tube depth, and other site-specific factors should be considered to determine the proper length. "
http://www.daviddarling.info/encyclopedia/E/AE_earth_cooling_tube.html
MKK
Member
Just out of curiosity. Has anyone had experience doing this? I may look at incorporating tubing under the slab which will support a large concrete rainwater tank that I plan to install in the basement area under the house. It will be an above ground tank but in the order of 70,000 lt capacity. It seems that the volume of water above the slab, which would be insulated,would provide most of the same conditions as digging trenches 3m deep outside. It seems intuitive but I'm not sure. I haven't worked out what I would do with the water in the tubing nor what kind of tubing would be best but it did seem like a good idea. I am building a new house just north of Sydney. Any comments would be much appreciated.
drgrieve
Member
I put some pipes in. Around 20m 1.5m underground in a semi-shady spot. There is another 10m or so under the garage slab, but not back filled.
Builder forgot to put in ag-line, so I had to dig up a section and drill some holes in.
That was hard work.
Anyway next hot day next spring or summer I'll be able to see how it works (or not) - I'm intending to integrate with the air-con as a fresh air option that is hopefully precooled.
Cheers.
underground
Member
This is a copy of an email I sent to Dave.
Hi Dave,
I live in Canowindra in the central west of NSW. I am an electrician.
I bored a hole and took measurements at different depths.
At 2 metres it was constant at 19.2 degrees C and at 2.5 metres it is constant at 19.6 degrees C.
I think I'll have a look at 3 metres and see what that is and then decide what depth I'll use.
I don't have the digging equipment so I'll have to hire one of the contractors I use all the time for underground installations of electrical cables.
I'm going to install 6 x 90mm stormwater pipes 70 metres long and about 600mm to 700mm apart and bring them into a manifold above ground. There will be a slope in the underground section to collect any condensation and then remove it by pump or by going downhill on the land in a 25mm pipe to a lower level where it will emerge at ground level.
I believe that more pipes are better while the ground is dug up rather than only putting in a minimum amount of pipes. More is better for temperature exchange rate and efficiency.
At the same time while the ground is dug up I will install electrical conductors and rods to provide power from an earth battery and also from telluric currents in the earth.
Keep in touch mate,
George.
nyreedunn
Member
Schnark
I know this is an old thread but I am interested in whether you managed to get your earth tubes to work? I am looking at doing the same in the Riverina of NSW. Initially I wanted to couple earth tubes with a solar chimney but have since found a solar extraction vent that may do the trick.
Thanks
schnark
Member
Hi,
We're still in the progress of building and will start installing the earth tubes upstairs in the coming weeks.
We won't be digging out the trench for the tubes themselves for several months.
Will report back how it goes in the future.
Cheers
Indali
Member
Hi,
Thanks everyone for the various informative posts. I was considering the installation of earth tubes as part of my extension I am planning in Perth. The links provided here were useful in the research.
I thought I would share some of the information and the resulting conclusion I reached in deciding not to proceed.
One of the most important aspects in making a reasonable assessment of the likely effectiveness of the concept is reliable soil temperature data at a suitable depth. Although not perfect we appear to be lucky here in Perth with Murdoch University making 10 minute data available to download for 125, 250, 500 and 1000mm depths (http://wwwmet.murdoch.edu.au/).
Based on a quick analysis of the data for 2010 shows the annual average temperature of the soil at 1 metre deep was 21.1 degrees which compared to 18.1 degrees for the annual average ambient temperature. So the average soil temperature at one metre was 3 degrees above average ambient. Even greater than Catopsilia 2 degrees observed in Manilla.
The one metre depth temperatures ranged from 14.4 to 27.9 degrees (fairly similar to Catopsilia’s data, which isn’t too surprising given similar ranges in ambient temperature). For the shallower 125mm depth there were 6 days where the soil temperature exceeded 40 degrees.
I then extrapolated this data for 2 meters depths which is about the maximum depth that earth tubes would be installed.
My first method was to double the difference between the 500 and 1000 mm depth to guestimate the 2m temperatures. The graphed results are here:
http://www.scribd.com/doc/78724444/Soil-temperature-recorded-at-Murdoch-University-2x-factor-for-2000mm.
After a more detailed review some of the depth temperature tends with depth at various times of the year the 2x factor looked too optimistic and better data fit was achieved using a 1.3 factor:
http://www.scribd.com/doc/78724442/Soil-temperature-recorded-at-Murdoch-University-1-3x-factor-for-2000mm
The resulting estimates suggest that the January/February soil temperature at 2 metres will be 24-26 degrees. So allowing for some loss in heat transfer efficiency I doubt air from an earth tube system would be below 26 degrees in summer, which would barely be below the maximum house temperature. At such a high temperature I couldn't see the benefits for my circumstances and therefore have decided not to progress any further.
Are there any fundamental flaws in this logic?
A couple of cautions with respect to the above are:
1) Murdoch is situated on the Swan coastal plain, and although I don't know the exact location of their weather station it is almost certainly in an area of partially cleared bush with dark coloured sand and grassy weeds.
2) Only a single year (2010) of data was analysed.
3) I suspect sub-soil temperature has a lot to do with the soils ability to absorb solar radiation (and our dark sandy soils are good at absorbing heat).
4) My 2 metre depth estimation method is ‘quick and dirty’ (but I believe sufficient for my purposes)
The main assumptions/methodology used to generate the graphs:
1) The 125mm, 250mm and 500mm temperatures are a 7 day rolling average (this removes daily and weekly temperature change - does introduce some time lag). Perth’s weather typical repeats on a 7 day cycle.
2) The 1000mm depth reading is measured data.
3) The 2000mm is a rough approximation by subtraction of either twice or 1.3 times the difference between the measured 1000mm data and the 500mm rolling average. The result was then averaged over a further 14 day period (rolling average) to account for attenuation of the temperature changes. This second computational time lag was removed by moving the data a week earlier (although that doesn't really have a material effect given the purpose of the data analysis).
cava
Member
Great information, which I found interesting as we are going the earth tunnel route with our new house. Thank you. However, on a different side of the coin, have you thought of using the earth tunnels as a form of supplementary heating during winter?
Indali
Member
Hi Cava,
I'm glad you found the information useful.
I just did a cross check of the data by measuring the temperature of my scheme water. That was around 29/30 degrees. Based on my analysis, that temperature would correspond to a soil depth of ~500mm. I don't know the exact depth of the water mains, but it would be around 500-1000mm deep so that seem to support the analysis.
I just ran my bore for a while and measured the water temp out of a tap near the pump. As I only ran for a minute and the pump is a submersible it should be a reasonable indication of temperature at depth. The water table is 4-5 metres down at my house and although I don't know the pump depth it would be several metres lower. The bore water temp was 23 degrees. If I extrapolate my prediction method I used for 2 metres, I would predict 23 degrees at around 4-5 metres. As water is far more thermally conductive, temperature changes with depth within the water table would be much less than dry soil accounting for a higher than expected temperature from the pump intake (plus the cone of depression from the pump will favour taking water from near the water table).
Although none of it is conclusive (which would require either installation of temperature sensors or the earth tubes), it all seems to align with the same conclusion.
I did also consider using the tubes for winter. My extrapolation of the Murdoch data suggest a soil temperature of around 16 degrees for July and August. The air from the tubes would be colder than that. My house rarely gets below 17 degrees (and that was prior to sealing the large gaps in my floorboards and insulating). So I think a strategy of house insulation (and passive solar principles in my extension) will be better value.
Recycling the bore water for heating/cooling is about the only alternative, but that feels like to much complexity and cost for minimal gain.
A bit disappointing really.
cava
Member
cava said:
Can someone advise the ideal air change rate, and suggested airflow, for a house using earth tunnel cooling. I have the information somewhere, but can't find it, and thought it easier to ask here.TIA
Any help guys/girls? A legacy of moving house, with everything packed away, no credible information available at my fingertips.
Ok -just to keep the subject going and for those interested. BOM has daily 'soil temperature' records, todays is at http://www.bom.gov.au/products/IDN65176.shtml . Canberra is showing a sioil temp of 14 at 9.00 am. You will see there is different temperatures at different cities but some have a high and greater variance which makes me think their is another variable that would assist us greatly, which is not available -possible building shade, soil type, proximity to road surfaces,etc. I have measured soil temps in Gosford at between 16 and 22º, subfloor soil- for my own testing over different times of the year. Some here have looked at yearly averages, this is not useful as similar to your house the temperature is cooler in the morning than it is at evening and different again at night!! The most important criteria for your earth cooling consideration is 'Is the air coming in going to maintain a cool comfortable temperature throughout the day for your particular building'. Other houses can only be used for 'similarity'. Wall/roof insulation, heat rise, high/low ceilings, open doorways/windows, passive shading, fridge/cooker venting, all these variables have a massive impact on the overall temperature achieved. You can have the best and biggest air con in a house with no insulation and a cathedral ceiling - you will never have a cool house as the heat radiance is greater than the cooling capacity of the air con. Remove the heat (exhaust cooling) and then draw in your cool air on a continuous basis. This way you maintain the 'core' temperature of your building through its structure, as it was over night with minimal temperature rise (by exhausting/venting the heat).
One other note not mentioned, do not use 'ag' line or ribbed duct as this will allow condensate to sit in the ribs and not flow to the exit. It then in turn will contaminate the air very quickly causing major illness to occupants of the house. When installing your duct, maintain a constant slope -at least 1/30 to allow the condensate run out to the end of the pipe where your collector pot will be. This will also allow for 'flushing' if necessary at a later date.
Keep up the good work..
Hi all,
We're currently building a house in Perth, and have installed earth tubes. We're yet to complete the house and test the system, but I have a blog that I'm doing with info on it regarding design etc. I hope to monitor the various temperatures and effectiveness of the system for a year and log the data on the blog so that it can hopefully help other folk looking to attempt similar projects...
WAYNE NEYLAN
Member
Hi Clintbergsma
How did your system work out, did it work ?
Regards
Wayne Neylan - Deluxair
Hi Wayne,
Thanks for the interest. We had it running over last summer, and it worked ok for the first month or so, it was able to pull the air temp down from 32 to around 25, and keep it there, but then that slowly diminished. Eventually it was only pulling the air down a degree or two, while really wasn't worth it in terms of cost of running the fan.
I had a look to see what went wrong, and it looks like the ground temp around the pipes heated up. Initially the ground temp was around 21/22, but it got up to around 28 and pretty much stayed there. I think what has also compounded it is that we built the house over top of the pipes, and so when it rains there is no water that seeps past it to move the stored heat; the ground around the pipes is really dry, and now that it's heated up, it's pretty well insulated.
So long story short, it worked initially, but then failed. I'm now capping them off, and hooking the system to an external vent to pull in fresh night air on those cooler evenings.
I hope that makes sense,
Cheers,
Clint.
Hi Clint, if I could follow up on yours and cave's post. The closed loop will bring the heat from the house and if higher than the ground will raise ground temps. With an open loop, draw from the cool shaded South (avoid damp mouldy areas). You will always attain cooler air than ambient. With this you can draw night cooled air into the house while cooling the ground at the same time, in preparation for the next days ground cooling. The main key factor is that you are using thermal energy with 100% fresh air, this is key to a healthier house. Yes, if your soil was wetter and your pipework more apart from each other, you would attain better readings, but you can't change that now. One key are, I don't know if you have taken this into account is that the pipework should have an area where the condensate accumulates and evacuates into the ground without contaminating the air quality. The fact the house is over the pipes will give you shading, a bonus as the sun will not have an increase effect. Your footings should isolate the external garden area. With the Solarventi system we have designed, it is the key factors I described that make or break an efficient ground cooling system. Basic 101 - if external temp is 37C, the are in the shade (south) should be 10-13C lower. Use this intake via the ground to vent the internals consistently. Running consistently, this will keep your internal structures cooler. However, here lies some failing, the air should be drawn into the breathing environment and not the attic. The attic (oven) should not be cooled, it should always be evacuated at a rate greater than the suns ability to heat. If the attic is causing a radiant heat transfer via the ceiling to the internal environment, then no natural cooling system will work. Even some air cons can't cope. If you need advice, we are always happy to help ATA members - just PM me.
Hi Cava and Eamon,
While an open loop system might be beneficial in some instances, I don't believe it would be worth my while for two reasons. The first is that my house is very well insulated/north shading of windows and has double glazing, and holds it's temp very well; if there's a hot day followed by a cool night, it remains very comfortable during the day, and then by opening the upstairs/downstairs windows on a cool evening, the house quickly cools down and is able to hold that temp the following warm day. My problem arises when there is a number of hot days in a row with evenings/nights that don't get cool (e.g. still 28+). Running warm night air through my pipes isn't going to help me much the next day; and if there is cool night air, I may as well open my windows all night, let the house cool itself down without the cost of running the earth pipe fan - and it'll be good to go for the following day. So I don't need stored coolth if a cool night is available between hot days. The second issue is that I have a 3Kw solar system of which I only use about half the output, the other half being sold back to the grid at 7 cents/Kwh. If I'm running my earth pipe fan all night to store the coolth (at 24 cents/Kwh) if/when coolth is available (remembering that my house does the same job by itself without the pipes/fan), I'm better off installing a small aircon that I run while the sun is shining and store that coolth in my building's thermal mass. That way if there's a hot day (always sunny) followed by a hot night, my house will be cool for both. The difference between the two scenarios on my power bill would be insignificant (if at all), and I'd have the assurance of a cool house on those night/day heat benders.
I'm not against earthpipes, and I know they are suitable in some situations. But in this instance, a closed loop system didn't work, and changing it to an open loop system doesn't seem to add up (I may be wrong). I think that at times the theory of earthpipes sounds better than the reality, and when earthpipes are being considered, factors like night temp, the inclusion of good insulation/double-glazing and a PV array (for e.g.), might change the best way of cooling a house. Aircon is less sexy though, but my wife says she still loves me 
