natural ventilation

If You Know, You Airflow

Ready for some more math? Well, you’re in luck! Today’s post is dedicated to calibrating the size of the ventilation openings on the Concrete Chimney Experiment.The Thermal Mass and Buoyancy Ventilation Research Project (TMBVRP) team has been researching equations for the “effective” opening.

diagram showing the exploded axon of the chimney test, with ventilation openings highlighted
exploded axon of the Concrete Chimney Experiment

The effective opening size differs from the total opening size because it accounts for friction. For example, 1’ x 1’ window has a total opening of 1 square foot, but due to friction caused by airflow around the edges of the window the effective opening may only be 0.9 square feet. With that concept in mind, we can look into why and how the TMBVRP team has been improving their experiment through trial and error.

diagrams showing changes to ventilation strategies
section through the concrete chimney showing the insulation and ventilation openings.

The original ventilation opening for Concrete Chimney Experiment was a 12″ long PVC pipe with a 3/4″ diameter. After reviewing the temperature data of both the interior space and thermal mass, the team saw that the airflow was being choked. This means the effective area of the opening was not allowing for enough ventilation. This caused kept the thermal mass from fully absorbing or offloading the heat from the air. The length to width ratio of the pipe was too high, creating unwanted friction, and slowing the airflow.

mathematical formulas explaining the change in ventilation hole size

For the next ventilation opening iteration, the team needed to reduce the friction by making the ventilation opening a “sharp opening.”  This means that the length/thickness of the opening is significantly less than the diameter of the opening.  The 1′ thick layer of GeoFoam on the top and bottom of the chimney was preventing the ability to have a “sharp opening.” So, the team carved out the top and bottom insulation in the shape of a cone to negate the friction. The bottom of the funnel was capped with a 6″ square of ½” insulation with a ¾” diameter opening. The ¾” diameter opening is the actual area of the opening, the effective area after we calculated for friction is only about ½” in diameter.

version two of ventilation hole sizing

Third times the charm when it comes to ventilation openings!  The ¾” opening in the ½” insulation had a diameter to thickness ratio of ~0.6.  After further investigation a true sharp opening needs to have a diameter to thickness ratio that is much less.  Due to this finding we replaced the ½” insulation with a 1/16 in acrylic sheet to achieve a ratio of ~0.1.  Even after all these calculations we won’t know for certain if we are achieving sufficient airflow in the chimney until we can measure the exact velocity.

version three of ventilation hole sizing

The Thermal Mass and Buoyancy Ventilation Research Project team is looking into how to install airflow sensors into the Concrete Chimney Experiment. Until then, they will keep on analyzing temperature data and designing their experiment.

At Rural Studio, students learn through construction that the design of a building goes far beyond our architectural drawings. Builders and construction workers are designers. Through the Rural Studio Research Projects students are now learning the complexities of designing experimental methods and scientific instruments. The TMBVRP team has developed a deep appreciation for this avenue of design they may not have considered before.

Another important note from this week; Copper’s brother Wolfie came for a visit! The brothers love chilling at HomeLab and keeping an eye on the Concrete Chimney Experiment. Stay tuned to see what the Thermal Mass and Buoyancy Ventilation Research Project Team learn next!

Stacks on Stacks on Stacks

The team (finally) beat the rain for a few days and finished stacking the two test buildings. The construction process is extremely quick due to the threaded rod construction method. The team organized the wood on site then spent a few days stacking each piece of timber on the threaded rods.

Next step is the metal roof! For now, here are a few aesthetically pleasing mass timber photos for your feed. 

Back in our welding gear, 

The Metal Masses

Soundtrack: Woodstock | Portugal. The Man

Temperature Swings

Now that the Concrete Chimney Experiment is built, let’s take a look at what should be going on inside! To understand if the Optimal Tuning Strategy is cooling and ventilating the space within the chimney, we compare four temperature signals. Quick reminder, the Optimal Tuning Strategy refers to the set of mathematical scaling rules that proportion thermal mass and buoyancy ventilation to act together in a natural feedback loop. The Thermal Mass and Buoyancy Ventilation Research Project team prefer nicknames, typing their project name is enough work.

Let’s take a look at these four temperature signals which identify how effectively the Optimal Tuning Strategy is operating with the Concrete Chimney Experiment. The temperature signals are; exterior air temperature, interior air temperature, thermal mass surface temperature, and thermal mass interior temperature. These temperatures are taken within the chimney using GreenTeg sensors. The exterior air temperature is the temperature of the air outside, like the temperature you read on a forecast. The interior air temperature is the temperature of the air within the chimney, like the temperature you read on your thermostat in your home. The thermal mass surface temperature measures the temperature of the surface of the concrete panel. This surface interacts with the interior air. The thermal mass interior temperature is the temperature inside the mass. We can use a melting ice cube to understand the difference in the thermal mass temperatures. When an ice cube melts, the surface melts first while the center of the cube remains frozen. So, the surface and interior temperatures of the thermal mass can differ just as the outdoor and indoor temperatures can.

Theses four temperature signals describe if the thermal mass is absorbing and offloading heat from the air which should, in turn, drive conveyance ventilation cycles. The times of day the mass is absorbing and offloading heat should be relatively consistent day-to-day due to the diurnal cycle. The diurnal cycle is the variation between a high temperature and a low temperature that occurs during the same day. In other words, for most days the temperature rises until a peak typically in the afternoon and then falls again until reaching a low before the sun begins to rise again.

Each day the cycle repeats. Though the time of day of the high and low can vary. Here, you can see the diurnal cycle for a typical summer day in Hale County. We can then normalize that temperature swing into a Sin Wave for mathematical analysis. This is the exterior air temperature.

To see how all these temperatures should compare to each other throughout the day we can look at this graph. Notice the axis of temperature and time are simplified radially, but we are still looking at a full day with a typical temperature swing. This graph represents an Optimally Tuned space where the proportions of thermal mass and buoyancy ventilation are ideally balanced. The solid black line represents the exterior air temperature. The dotted gray line represents the interior air temperature. The solid orange line represents the thermal mass interior temperature. The dotted orange line represents the thermal mass surface temperature. As you can see, the interior air temperature is never hotter or cooler than the exterior air temperature. It is dampened by the thermal mass absorbing and offloading heat from the interior space. The thermal mass surface and interior temperatures show the mass warming by the absorption of heat from the air and cooling when the heat releases back into space. When the thermal mass and buoyancy ventilation proportions are not balanced the graph looks drastically different.

On the left, you see what it would be like if there were a lot more ventilation and a lot less thermal mass. Too much ventilation causes the interior environment to act just like the exterior environment and there is not enough thermal mass to affect the space. This would be like being in a tent. On the right, you see what it would be like if there were a lot less ventilation and a lot more thermal mass. Too little ventilation does not bring in enough heat for the thermal mass to absorb. The thermal mass is also so large it takes too much heat to fill up, which means it takes longer for the mass to start offloading it into the space. This is like being in a cave.

temperature signal graph
Temperature signal data graphed to compare to ideal Optimal

Finally, here is some of the data we have pulled from our sensors so far! Although the Concrete Chimney Experiment is definitely damping the temperature within the space the thermal mass temperatures are essentially the same. This means we may not have enough ventilation, not enough heat being brought in with be absorbed and offloaded. We are working on getting airflow sensors to see if this could be the case. The team is also recalculating the size needed for the ventilation openings.

If you stuck around until the end of this one, big thanks! Here’s a picture of Cory’s kittens Rosemary and Dijon to ease your mind. As always, we will be back soon with more rural science so stay tuned…. optimally tuned.

Rain, Rain, Go Away

The Breathing Wall Mass Timber Research Project team has been quickly jumping between testing scales as the research continues. The test cell, introduced in the last blog post, is now finished- fully covered in a rigid layer of Geofoam insulation and ready for testing! The team is currently working on a large scale thermally active surface design to get these tests running. 

In the meantime, the team also built 1 of 2 test buildings. As a reminder, there will be two test buildings. One will be only mass timber construction and the other will couple mass timber construction with the Breathing Wall system. The team took advantage of every ounce of sunshine last week to build the mass timber test building in just under 3 days. Because of all the wood prep done before Christmas break and the threaded rod construction, all the team had to do was stack the 2x4s and 2x6s to form the walls. 

The ceiling was the real trick. Because the threaded rods for the walls run through the floor and the ceiling (tying the whole test building together), the team had to ensure the holes on the ceiling would line up perfectly with the vertical threaded rods. So the team built the ceiling off site, tightened it down to an exact measurement, drilled the holes, then took it apart. When the walls were up, the ceiling was installed in exactly the same order as it was assembled before to ensure the holes lines up with the threaded rods. Next up, doors! 

Stay tuned for another test building, doors, and a roof coming together very swiftly. 

Praying for sunshine, 

The Always Damp Breathers 

Soundtrack: Have You Ever Seen the Rain? | Creedence Clearwater Revival 

Chimney Cricket!

The desktop experiment mock-up, “The Chimney,” is complete and already bringing in data! Here is a quick look into the making of the Thermal Mass and Buoyancy Ventilation Research Project Team’s first dive into building a scientific instrument.

Before we get into the construction of a scientific experiment in the non-scientific environment shown above, let’s go back to the Fabrication Pavilion where all the prep work was done. The team used the twelve (recently built) 1′ x 1′ concrete panels to create the four walls of the chimney. For each wall, three concrete panels were screwed to a base of foam atop OSB through the 1/2″ pex pipe that was cast into the panels.

After the four walls were completed, the team tested how they fit together. The OSB and foam base extends past the concrete panels in order for the walls to fit into one another. This also allows for continuous insulation of the concrete chimney within. As seen in the last TMBV Research Project team post, insulation is key. Therefore, the chimney sits atop 1′ of geofoam and has another 1′ geofoam hat. The ventilation PVC pipes run through this geofoam on the top and bottom and connect to the chimney’s interior chamber. This is why the chimney is lifted off the ground by the wooden base, to let air in and out the bottom ventilation pipe.

Next up we have the sensors. The sensors must make it through a foot of insulation in order to take the temperature of the chimney interior chamber air, the surface of the concrete panel, and the backside of the concrete panel. There are also sensors outside of the chimney to measure the exterior air temperature.

The interior air temperature tells the team how the thermal mass and buoyancy ventilation proportions are effecting the interior space while the panel surface and backside temperatures tell the team how efficiently the thermal mass is working. The sensor wires are encased in gasket that runs through holes in the foam, OSB wall to the outside so sensors can be charged without disassembling the whole chimney.

The sensors the Thermal Mass and Buoyancy Ventilation Research experiment used are called Green TEG sensors. They transmit data using cell service so all your data can be downloaded from online or watch your data in real-time. This is a blessing and a curse as this makes Green TEG data very convenient, however, while the Morrisette campus has great Wifi, the town of Newbern does not have great cell service. Therefore, the experiment was transported to an undisclosed carport in Greensboro, just 15 miles down AL Highway 61 where it was assembled the rest of the way.

Next up, the Thermal Mass and Buoyancy Ventilation Research Project Team will calibrate, or modify, the experiment once they see how it is performing. After that, the team can start with a wooden chimney. Thanks for tuning in!