Earth Air Tubes

So here’s the thinking: If the earth is a constant 50-55F at six feet underground, why not take advantage of that to heat your house? The big “eco-bling” thing to do that takes advantage of that heat is to install a geothermal heat pump. Here’s how it works: You put a huge loop of pipe underground and pump liquid through it. The liquid warms up as it travels underground and expands. When it returns to the house, the liquid runs through a compressor, which makes it even hotter. Then you run air over that hot liquid, the hot air heats the house, and the liquid, now cooled, is funneled back into the ground to complete the loop.

It’s like running a big A/C unit in reverse: instead of cooling a room to heat the outdoors, you’re cooling the earth to heat your house. As proponents like to say: With a geothermal heat pump you heat your house for free (the heat is already in the ground); you just have to pay for the shipping (moving it from ground to house).

Here’s the problem: Shipping is expensive. It takes roughly 1 unit of energy to move 4 units of energy into your house. And since the system runs on electric energy, and producing electricity is only about 30% efficient (ie you have to use 100 units of coal energy to get 30 units of electrical energy) your total net gain of energy of a ground-source heat pump is only about 30%. And the system has high up-front costs.

Then I read about a much cheaper option: the earth air tube (also known as earth-coupled tubes) is simply a big tube that you bury 6 feet underground and then use that as your house’s air intake. Since we’re using an active air handling system in the house (an ERV system, which I’ll discuss in a later post) we can suck the air either straight from the outside or run it through a buried pipe. The theory is that as the air travels through the tube it warms up (or cools in the summer). Here are a few photos of our install:

Mark Installs the section that goes under the house footer

A section of the earth air tube: 12 inch PE (polyethylene) pipe

Digging the trench for the air tube

The air tube intake in the back of the yard

Where the tube comes in the house

So how much does the air actually heat up inside the pipe? That’s a great question. Several experts in the field claim that I will gain a surprisingly large amount of heat, although I remain somewhat skeptical.

A German company, Rehau, makes a system that includes an anti-microbial lining in the pipe that sells for about $4,000. The claim is that the anti-microbial coating is important for condensation that settles in the tube. I was offended at the $4,000 price, since I did some research and I could get a 80 feet of 12″ PE tubing for about $700 (and the Rehau system is just an 8″ pipe, and it’s PVC, which I don’t think is ideal for air you plan to breathe). I looked at coating my PE pipe that I bought from the local plumbing store with that antimicrobial lining (www.bioshieldtech.com), but although the owner of BioShield was willing to give me a great deal, it was still going to cost hundreds of dollars. Even more importantly, since I plan on using the pipe for heating, air flowing through the tube will be warming and absorbing moisture. I think the anti-microbial lining (and advanced condensation drainage) is important if you’re planning on using an earth air tube for cooling your house in a humid place like Louisiana.

Some calculations: my pipe is 80 feet long and has a 12″ diameter, which means it holds about 62 cubic feet of air. My house has a volume of roughly 25,000 cubic feet (three floors of 1,000 sqf with an average ceiling height of just over 8 feet). The standard for air exchanges is one every three hours, although I think this is high. With an air change every 5 hours I would need to move 5,000 cubic feet/hour, or 83 cubic feet/minute. That means that air will spend about 45 seconds traveling through the pipe. How much will that warm the air? I’ll be happy with 5-10F, although one guy I called claimed I could heat 0F air to about 32F with my pipe. We shall see!

An interesting point is that the low thermal conductivity of the plastic pipe apparently matters little. The earth is a terrible conductor as well, so if you used a metal pipe you would transfer the heat that last little distance very efficiently, but it would still take the earth behind it a while to catch up. But apparently that slow heat migration through earth and plastic is enough to heat the air significantly.

There are two benefits to this setup: one is simply the warmed air, which means that my heating system has that much less work to do. The other is that my ERV system will have fewer frost problems. Frost happens when the incoming air is really cold and the outgoing air is warm and relatively moist. When the ERV frosts it has to shut down and go through a defrost cycle. Or you have to hook up an electrical heater to sufficiently heat the incoming air. And the whole point is to avoid heating your air with electrical heat!