Image Credit: All photos: Malcolm Isaacs The buried ventilation ducts were routed near the septic tank so that the ducts would be warmed by heat from the tank's wastewater. Backfilling the earth tubes. The ventilation ducts were run under the slab to the far side of the house. The total buried length of the three parallel ducts was 90 feet. The three PVC ducts penetrated the slab at the location of the future mechanical room. The buried ducts were connected to the fresh air intake duct of a Zehnder HRV. When necessary, a 1-kW electric resistance heater downstream of the HRV is used to raise the temperature of the supply ventilation air to approximately 113°F, turning the ventilation system into a “micro-furnace.” The house is located on a steep, rocky site — not the ideal terrain for installing an earth tube. The blue line shows the outdoor air temperature in degrees Celsius. The red line shows the temperature of the incoming ventilation air after it is tempered by the 90-foot-long earth tube.
I saw my first “earth tube” back in 2004, on a tour of row houses in Darmstadt, Germany — a tour which had been organized by the Passivhaus Institut (PHI) to show international visitors some examples of Passivhaus construction. As a visiting Canadian engineer specializing in residential energy efficiency, this was a novel and, for me, unheard-of way to temper incoming ventilation air from extremes of heat and cold.
As Dr.Wolfgang Feist, PHI founder and our tour leader that day, explained, “The efficiencies of this approach are extremely high… These earth tubes generally work very well, but they have to be installed correctly or you can have problems.”
Condensation and mold
Bad news travels fast, and it only takes one “problem” house to undo a lot of good work. During the past ten years in Europe, more than one earth tube was incorrectly installed, leading to pooling of condensate in summer, local mold growth, and major downstream problems with the incoming air quality.
What followed was a lot of bad press. Suddenly opponents had a legitimate complaint with one of the more unusual aspects of the Passivhaus approach (even though you certainly don’t need an earth tube in a Passivhaus building). As a result, many potential clients simply refused to have one installed, despite the excellent track record of such units.
Another approach: brine loops
Instead, designers and builders turned towards low-pressure geothermal “brine loops” for preconditioning incoming ventilation air. The typical loop consisted of a 300-foot length of 1-inch plastic pipe placed around the foundation or under the slab, with a brine or glycol solution circulating through a heat exchanger upstream of the HRV.
These systems don’t need a heat pump, also work well, have a reasonable cost (typically under $2,000 installed, or far less if you build your own heat exchanger and controls), and there’s no danger of air contamination.
I built a house with earth tubes
But my 2004 visit to the Passivhaus Institut predated all these debates about earth tube viability; I returned to Canada with all the enthusiasm of the newly-converted, determined to build myself a passive house — and right away. My understanding was that I needed an earth tube, so that’s what I did. To the best of my knowledge it remains one of only a handful of such installations here in Eastern Canada. So — does it work?
We learn most from our mistakes. I’ve now taught the international Certified Passive House Designer course across Canada and the U.S. for four years, and my own house often serves as a case study of how not to build a Passive House. So, if you want some advice: pick a suitable site with good solar gain; don’t rush into things; don’t assume you know everything; get expert help when you need it and plan every detail before you begin.
I chose the wrong HRV
One of the mistakes I made back in 2006 was to install a Canadian-made VanEE HRV in my new, close-to-Passivhaus. Sure, I did my due diligence and picked a model rated by HVI at 86% efficiency. It was the best I could find and it cost $1,350 with the control unit.
I subsequently found out this unit was completely inappropriate for a Passivhaus. Far from being “high-efficiency,” the HRV produced serious noise in the house and cold drafts at every supply outlet (by cold I mean a supply air temperature of between 41°F [5°C] and 44.6°F [7°C] when the exterior temperature was 14°F [-10°C]). In a Passivhaus, where comfort is paramount, this is totally unacceptable, and when I did the calculation it corresponded to an overall ventilation system efficiency of around 50 – 55%.
The result was that I hardly used my HRV for 7 years — it got turned on only to disperse cooking smells or other odors in the house. Measurements of indoor air quality during that time showed CO2 levels which were often well above the accepted ASHRAE and international guidelines, especially in the bedrooms.
My house has a tested airtightness level which exceeds the 0.6 ach50 Passivhaus threshold, but at 1.0 ach50 it is still remarkably airtight compared to conventional construction. The situation was highly unsatisfactory, and as a result I never even bothered to hook up my earth tube — it remained capped for those 7 years, sticking up through the slab in my mechanical room, an apparent waste of time and money.
In my defense, back in 2006 there were no Passivhaus-quality HRVs available in North America, but thankfully that’s changed today. As Passive House educators and advocates we’ve been helped enormously by the efforts of Zehnder America to bring high quality Passive House-Certified units to the US and Canadian market. This winter I ordered a Zehnder Comfoair 200 HRV in an attempt to improve the sorry state of my indoor air quality. It made sense to hook up the long-neglected earth tube at the same time, and see if it all worked as theory suggests. The little-used VanEE unit was removed and replaced with a European Zehnder HRV, and I also hooked up a data logger and temperature probe on the incoming airstream.
The tubes should have been buried deeper
Good practice suggests you should install an earth tube well below the local frost line, ideally at a depth of 5 or 6 feet, and that you need 100 feet of run at that depth for an average single-family home. My own 2005 installation hadn’t achieved anything like this — for a start, my tight budget was unable to accommodate a specialized 8-inch diameter pipe, so I went with an array of three 4-inch white PVC plumbing pipes*, which could at least be found at the local hardware store.
Next, my site was a steep rocky slope, so it was obvious that nowhere could I bury these pipes much deeper than 24 inches. And lastly, even running the pipes under the slab to the far side of my house, I could barely manage 90 feet of total length. Partly to offset these issues, at the intake end I ran the ventilation pipes right up against my septic tank, which was being installed at the same time, to take advantage of residual ground heat in that area.
The system has (belatedly) been commissioned
I finally hooked the earth tubes up to my new Zehnder unit in January, 2014. I didn’t have high expectations, and I wondered whether the HRV intake fan actually had the power to draw air through three 90-foot pipe lengths, each with several bends and elbows.
Yet, as of March 2014, results have been spectacular. Despite one of the coldest winters for many years, the earth tube has consistently provided a dramatic temperature lift to the incoming ventilation air. On the coldest morning of my monitoring period, at an outdoor temperature of -17°F [-27°C], the supply air was entering the HRV at 36.7°F [2.6°C], for a delta-T of almost 54 F° [30 C°]. That’s consistent with the past month of measurements, during which I’ve seen the supply air temperature entering the HRV fluctuate mostly between 37.4°F and 39.2°F [3°C and 4°C], depending on the outdoor temperature.
The data log plotted below for the 10-day cold weather period starting February 26, 2014, provides more detail.
Results from just two winter months have convinced this skeptic that the earth tube is an absurdly simple and cheap source of limitless free energy, in much the same way as the sun shining through a window.
This past month, the houses in my town all had continuous infiltration of -4°F [-20°C] (and colder) air, while I had a flow rate of 60 cfm entering the house at an average temperature of 39.2°F [4°C]. It’s hard not to feel smug.
I can’t wait to see the earth tube performance in hot, humid weather. My guess is that air at, say, 86°F [30°C] and 80% relative humidity will be conditioned down below 68°F [20°C] and 40% RH. And in my installation, the underground condensation will flow right back down the 45° slope to a drain sump.
In a very high performance house such as this, with good summer shading, this will be all the air conditioning I’ll need. And the real surprise is that this system runs off around 50 watts of fan power. Would I do it again? Absolutely.
* Yes, I know that PVC is hardly ideal as a ventilation pipe material, and that polyethylene would be desirable, but in this case the pipes have cured for over seven years and have already seen thousands of cubic meters of airflow.
EARTH TUBE Q&A
Q. Would you generally recommend earth tubes? What are the biggest problems with installing one?
A. I’d recommend this approach to anyone who lives in an area of climate extremes and who will take the time and trouble to install it properly. There are certainly difficulties sourcing components, and the whole process would be far easier, for example, in Austria, where this technology is relatively common, and the right components can be found more easily. I’d recommend a significant slope with a good accessible drain. If you’re digging a straight trench with a backhoe then over 100 feet you will drop 2 or 3 extra feet, so be aware of that and plan accordingly.
Q. What was the cost of the earth tube itself?
A. I spent around $500 on plastic pipe and fittings, but I still need to build a proper housing for the downstream air intake. Since my site was steep and had difficult access I also used $100 or so of used concrete blocks to protect the pipes from being crushed by trucks and heavy equipment during backfill and leveling. Labor was not much more than a person-day, and landfilling cost was negligible, since I completed the piping during construction, before any backfill had commenced.
Q. Are all soil types suitable for earth tubes?
A. Clays and silts will have more geothermal heat capacity than light or sandy soils, but perhaps the biggest issue is water: it makes no sense to me to install this technology in areas with a high water table, where it will be put under hydrostatic pressure. Therefore part of the installation procedure can (and perhaps should) involve a pressure test, to check whether the joints are airtight and water-tight, which they need to be.
Q. What is the approximate COP of your system?
A. The fan draw is around 50 watts, so with an average air temperature increase (delta-T) in winter of (say) 36 F° [20 C°], and a flow rate of 60 cfm, and assuming a heat capacity for air of 0.0182 Btu/cf/°F, then the geothermal “heating” power of the earth tube system in winter works out to around 660 watts. That corresponds to a not-too-shabby COP of 13.
Q. How can you be sure of avoiding contamination?
A. These days its cheap and easy to check underground pipes through a ground camera — I’m told a contractor can get one of these for his tool kit for $1,000 or less. It’s the same camera used to check for roots and leaks in septic system pipes.
Q. Won’t the ground freeze up over a full winter for a shallow system like yours?
A. After six weeks of running this system in very cold winter weather there was no sign of this happening — the lowest supply air temperature entering the HRV that I measured was 36.7°F [2.6°C] at -17°F [-27°C] outside. My guess is that the bedrock which lies just below the pipes and under the house acts as a high-conductivity heat sink, and this mitigates the relatively shallow pipes. I have 9 inch of EPS insulation under my slab, so there is minimal heat flow downwards from the house, and it must be said there have been over 2 feet of snow on the ground around here since early December. In addition, I like to think that all our warm drainwater going down into the septic tank provides a significant extra temperature lift for the incoming air, as the ventilation pipes lie right against it.
Q. Can you clean the earth tube?
A. Sure — I can easily mix up a bucket of warm water with bleach and just pour it in. The steep slope down to the condensate drain will handle that — no problem.
Q. Is radon infiltration a potential problem?
A. This is another good reason to seal the joints properly, and if there were a rupture in the pipe then yes, there could be a potential issue. But I was pretty careful with sealing every connection and protecting the pipes well during backfilling. Nevertheless, occasional radon testing at very low flow rates might well be a good idea.
Malcolm Isaacs is a civil engineer with over 20 years’ experience as a consultant in Canadian low-energy construction techniques. In 2005, he designed and built the first house in Canada to Passivhaus specifications. He now works full-time on developing Passivhaus construction solutions for clients in the Ottawa area, and in teaching Passive House techniques as a member of CanPHI’s training team.