Heat Exchanger Key Cut

That is a rather interesting cut of a concrete block. The reason for the odd cut is that allows us to pour a beam that will unify everything from the back wall at the north of the tiny cottage, across the marine aquarium tank above the bookshelf, over Holly’s closet and into the heat exchanger tubes that sit behind the masonry stove. Getting this couple thousand odd pounds of stone and concrete all locked together is rather important. I don’t want the chimney ever coming down in my lap and the resulting beam will support the attic floor above the bathroom and bedroom.

That concrete block is one of the blocks that we’re mortaring up as a tall chimney behind the wood burning masonry stove who’s firebox is the Vogelzang wood stove. Heat from the baking oven, stove and chimney will be transfered to the incoming fresh air that arrives via the earth air pipes buried up the side of the hill. In the winter, our critical heat season, the very cold (-45°F) air will be warmed to above freezing (>+35°F) by the passive geothermal heat. The wood stove will warm it and pump it even more adding both heat and chimney effect. This creates a ventilation system with no moving parts or electrical consumption for fans. During the summer, if there is no fire in the wood stove, then the system will still run but more slowly, bringing us cool air.

I say, with no moving parts. In theory it will pump air without any motors, without any electricity or other outside energy. But, just incase, I’ve built in spaces where fans can be placed if I find a need a boost in the air flow. We’ll see how the theory pans out in the test bed of reality.

But back to that interesting key cut. The block is the section of the chimney just before the attic floor. After cutting and splitting out the extra piece of concrete from the middle I stuffed pink foam into the core of the block. Now when we pour the bond beam, which will also act as the bottom of my office bookshelf, the concrete will fill that section I cut out of the concrete block thus locking the whole structure solidly together. That block is now mortared in place and curing while I build the form work for the unifying beam – today’s afternoon project.

Outdoors: 85°F/56°F Partially Sunny, Two hard rains of about 3″ each, a few lighter rains
Farm House: 77°F/66°F
Tiny Cottage: 74°F/68°F Windows closed

About Walter Jeffries

Tinker, Tailor...
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8 Responses to Heat Exchanger Key Cut

  1. karl says:

    walter, a while ago i did a bit of research regarding this very topic. i had several links bookmarked but all were broken–too much time had passed. this link provided a search engine and i put in my old criteria. as i recall this was the most negative article regarding this topic. it might provide you with some interesting perspectives.

  2. Karl, There was a problem with the link. Here’s the right link. What specifically were you referring to in the article?

  3. David says:

    Walter can you tell more about yourrr heat exchanger? I’m not picturing it well.

  4. Ah, yes, the use of earth tubes for cooling the air, especially down south, can be risky if one isn’t careful about the mold and such.

    We’re mostly using the earth tubes to warm the air rather than cool it. Here in the mountains of northern Vermont the problem is more of keeping warm enough than cooling.

    For the past seven years we’ve been using earth air tubes to warm our house. We have four parallel 4″ diameter 70′ long earth tubes that we put in for the old farm house back in 2001. They have performed wonderfully making our home much more comfortable and the air fresher.

    In the winter our outdoor air is often -45°F in the worst of the winter. Running the air through the earth tubes warms it to +35°F even on those coldest of winter days. That’s a 80°F temperature rise that we’re getting essentially for free. When the outdoor air temperature is more reasonable the incoming air from the earth air pipes is around 45°F. This gives us lots of fresh air in the winter when the house is otherwise closed up tight as I can get it.

    The cost of installation in our case was nominal because I have backhoe and I was already digging a trench to lay other utilities and grounding for fencing. I just used the rigid white drain pipes under 4″ of foam insulation under about 12″ of dirt(?). The reason for the insulation is we can’t dig down all that deeply here – ledge is a big issue. Earth = R0.1 per inch and the foam is R5 per inch so that works well as an umbrella. With our deep snows we typically only get about 3″ of frost penetration so the pipes are well below the frost line.

    We have never had any mold in our air tubes. The whole system drains outward and I angled the last 20′ or so of the tubes down significantly. In that last 20′ I used the perfed tubes in that section to maximize drainage. This also helps to create a chimney effect so we use no fan on the ‘system’. I left the tubes accessible so I can check them and the pipes have always been dry inside – no condensation so no mold. If we were in a hotter, more humid area I suspect that this would be an issue.

    Do prevent rodents and insects from getting in – use a screen and hardware cloth.

    A more important issue for us here in Vermont is radon. ventilating helps get rid of radon. The other way we deal with radon is we are simply outdoors a lot in the fresh air – probably the best fix.

    Granite, which we’re using in construction of the tiny cottage, is a source of radon. Like with the farm house, we’re planning on using dilution as the solution to pollution – e.g., lots of incoming fresh air.

    The incoming air passes up through masonry ducts behind the massive masonry wood stove. In the winter the earth air tubes will warm the air to around +35°F to +45°F. Then the heat from the fire is transfered to the incoming air warming it further to a comfortable home temperature.

    The chimney will include additional flues for venting the house in addition to the wood stove’s flue. By changing flaps within the duct work we will be able to use the extra flues to remove the hottest air from the house during the summer thus drawing in cooler air from the earth air tubes during the day if we want and from the windows at night when the temperature drops.

    We also aren’t building with plywood or such that out-gases lots of fumes. One more thing that helps with indoor air quality.



  5. karl says:

    this link is the one that i was talking about.

    it describes using earth tubes as cooling for dwellings. it addresses several caveats regarding sizing and usefulness. the most important issue that i consider paramount is proper drainage to prevent mold issues–which lead to health concerns.

    i hope this helps.


  6. Harry says:

    Great blog. Can you do more about your house and how you manage the heating and cooling?

  7. Steven Green says:

    Another innovative use of heat exchangers. And yes, we don’t want rodents in, so better put in some kind of screen especially on the vents.

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