Buried intake ducts: air at a steadier temperature

Why a buried run beats a wall louvre

A wall louvre takes air at the temperature of the facade it sits in. On a still winter night that is the outdoor air temperature, and on a sunny summer afternoon the louvre and the wall around it can be several kelvin above it. The intake air then carries that condition into the filter, the heat recovery exchanger and the supply ducts.

A duct buried below the frost line takes air at the temperature of the soil in contact with its wall. Soil temperature at depth lags the surface by weeks and its annual amplitude is small, so the intake air arrives within a few kelvin of the local ground temperature rather than the air temperature. The effect is not free heating: it is a reduction in the range the rest of the system has to handle. The building-physics side of this question, how a thermal barrier exterior wall separates the conditioned volume from the ground and the outside, how ground heat storage behaves over a season, and how earth tubes are laid out, is set out in the Passive Climate Journal, an independent English-language magazine on low-energy building and passive climatisation.

How does a thermal barrier work in an external wall?

A thermal barrier is the continuous layer in the wall that resists the flow of heat between the inside face and the outside face. It works by conduction: the temperature difference across the wall drives heat through the solid material, and the barrier reduces the rate at which that happens by combining a low-conductivity layer with the thickness and the area of the wall.

Three conditions decide whether it works in practice. The layer must be continuous, so no structural element, balcony slab or window frame bridges it. It must be on the correct side of the dew point for the climate, so that water vapour in the wall does not condense inside the insulation. And it must be matched to airtightness, because a barrier with air leaking through it loses most of its effect. A wall built to a passive standard therefore pairs the barrier with a sealed air layer and a controlled ventilation system, which is what supplies the fresh air the sealed envelope no longer leaks.

How do earth tubes and ground heat storage work?

An earth tube is a duct buried in soil, with air drawn through it by the ventilation fan. Heat moves between the air and the soil through the duct wall. The air leaves the tube closer to the soil temperature than it entered, and the soil immediately around the tube shifts slightly toward the air temperature. Over a long run and a steady airflow, that local shift reduces the exchange, which is why the first metres of a buried duct do most of the work and why a very long duct gives diminishing returns.

Ground heat storage is the same exchange used deliberately and at a larger scale. A volume of soil is charged in one season and drawn from in another, through ducts, pipes or a fluid circuit. The storage is slow: charge and discharge happen over months, not hours, and the useful capacity depends on the volume, the moisture content and how well the volume is insulated from the surrounding ground. A buried intake duct is a small, unintentional version of the same mechanism, and its behaviour is governed by the same soil properties.

Can a roof act as a solar collector?

Yes, in the sense that a roof surface absorbs solar radiation and can transfer that heat to air or fluid moving beneath it. A roof used this way is a collector when it has an absorber surface, a path for the transfer medium and insulation on the side away from the sun. The performance depends on the orientation, the tilt, the absorptivity of the surface and the temperature of the medium being heated: a collector asked to deliver air only slightly above ambient collects far more useful energy than one asked for a high delivery temperature.

A roof that is not designed as a collector still acts as one, badly. It heats the air in the attic or the void beneath it, and that heat reaches the building. The design question is whether that gain is wanted, and if it is, whether the roof is built to move it into the ventilation air or the heating circuit rather than into the rooms below.

Condensation, filtration and drainage in the buried run

Air cooled below its dew point deposits water. In a buried intake this happens on the inner face of the duct whenever the soil around it is colder than the incoming air, which is the normal summer condition. The water runs to the low points of the run and must be removed.

Four provisions follow. The duct is laid to a continuous fall, with no sag between supports, so water reaches a drain rather than pooling. A condensate trap and drain are fitted at the lowest point and at the entry to the air handling unit, sized for the peak rate of condensation, not the average. The duct is insulated where it passes through warmer ground or through the building fabric, so that the cold duct surface does not condense moisture from the surrounding air. And the duct material is smooth, non-porous and cleanable, because a damp organic surface inside a ventilation duct is a growth medium.

Filtration sits at the intake, before the duct where possible, so that the buried run stays clean. A coarse pre-filter at the outdoor terminal keeps leaves, insects and coarse dust out of the tube; the fine filter belongs at the air handling unit, where it protects the heat exchanger and the supply air. A filter placed only at the far end leaves the whole duct as a settling chamber.

What the fan has to overcome

A buried intake adds resistance to the ventilation system, and the fan must develop enough pressure to move the design airflow through it. The pressure loss of a straight duct rises with length and with the square of the air velocity, so a long run at high velocity costs far more than the same run at low velocity in a larger diameter. Bends, terminals, grilles, filters and the heat exchanger each add their own loss, and the total is what the fan sees.

Two consequences matter at design stage. The fan curve must be read at the actual operating point, not at the free-air delivery, because a fan selected on free air will deliver less than the design flow once the buried run and the filters are connected. And the duct diameter should be chosen for the airflow, not for the space available: undersizing the duct to fit a trench raises the velocity, the pressure loss and the noise, and it makes the condensation problem worse by increasing the cooling of the air per metre.

Sources

Passive House Institute