In France, the air temperature varies, depending on the climatic zones, between – 25°C and +40°C. This means that despite current standards, a huge amount of conditioned air escapes at great cost from the house, 24 hours a day, to be replaced by cold outside air in winter or very hot in summer. The Canadian well makes it possible to compensate for all or part of these drawbacks.
Principle of operation of the Canadian well
The Canadian well uses the underground temperature, stable and free throughout the year, to load in calories or cold, the fresh air necessary for the ventilation of the house. Forced by a wind tunnel, the flow of air drawn from the outside passes through a buried tubular network acting as aheat exchangers. The captured air, at negative temperature in winter, heats up during its journey underground, before being blown into the home to keep it frost-free. In summer, the phenomenon is reversed, the air captured at high temperature is cooled to lower the ambient temperature. The value of these exchanges is influenced by the sizing and the nature of the components of the installation, as well as the type of terrain and the local climatic conditions.
Canadian well sizing
The purpose of the sizing of the installation is to optimize its yield while remaining within an acceptable budget. For accurate results, these complex calculations are reserved for specialized technicians. However, a basic installation can be based on empirical data largely validated by experience. Subject to specific requirements, the following figures represent the average values observed for an individual pavilion of 250 to 300 m3 in volume, in metropolitan France.
The efficiency of exchangers
The efficiency of the Canadian well is conditioned by the length, material and the diameter exchangers, by traffic speed air in these pipes, as well as by the nature and speak humidity level from the basement. The best investment/yield compromise is for burying the tubes, at a depth between 1.50 m and 2.00 m.
Length of exchangers
As the air progresses through the exchanger, heat exchange is reduced. These variations relatively stable in the first meters, then increase rapidly. We thus observe an average gain of only 10% when going from 35 to 50 meters of pipes. The exchangers therefore rarely exceed 35 meters length. On the other hand, the efficiency of the installation is increased by juxtaposing two or more interchanges spaced at least 1 m apart.
Diameter of exchangers
The diameter of the pipes must allow the best flow of the ventilation flow, by limiting pressure drops by friction. It also influences the energy consumption of the fans, therefore the COP of installation. In practice, the choice of exchanger diameter is made according to the criteria imposed by the higher summer speed. It is generally between 175 and 200 mm.
Airflow speed
Regulating the speed of air circulation in the exchangers is an essential criterion for the good performance of the installation. The best heat exchanges are obtained with a Maximum speed of 2 m/s in winterand of 3.5 m/s in summer.
Characteristics of basements
thermal conductivity increases with the water content of the subsoils. Best yields are obtained in loamy or sandy, moist environments. Usually, the pipes are flooded, in trench bottoms, in fine sand which provides them with mechanical protection and stability.
The 5 building blocks of the Canadian well
In order not to transport excessive humidity into the living space and to avoid the formation of mold, all the components of the Canadian well must constitute a continuous air network and perfectly waterproof.
1 – The external suction terminal
First element of the device, the outside air intake is placed at least 1.40m high. It houses a double filtration device comprising a fine grid and one or more high-performance filters. The first prohibits entry to rodents or other unwanted intruders. The latter retain insects, dust, pollen, certain corrosive elements, and large polluting particles. The section of the filters must be adapted to the plus forts volumes ventilation provided.
2 – Buried pipes
The choice of materials is vital to guarantee the healthiness of the air breathed by the occupants of the building. All tubes and equipment must bear the label “food grade” and must be able to withstand the pressure exerted by the depth of burial. The tubes PVC or PTC should be avoided, as they release solvents and toxic particles. Metal tubes are not suitable, as they are difficult to implement and sensitive to corrosion. The sandstone tubes, finally, prove to be brittle and not very watertight at the fittings. Prefer the use of inert and flexible materials, such as:
- The polyethylene (PE) non-recycled with a smooth, stable and sometimes antistatic internal surface. It is available in coils of different diameters and lengths.
- The polypropylene (PP) rigid, easy to assemble and sometimes bactericidal, facilitates the regular slopes necessary for the evacuation of condensates. It is available in bars of 3 and 6 meters.
Under penalty of loss of efficiency, no pipes should run under heated buildings or pass through them. less than 1.50 m.
3 – Le by-pass
In half-seasons, the subsoil still retains the temperature of the previous months, while the atmosphere is temperate. The Canadian well becomes useless or worse, counterproductive. To avoid injecting air that is too hot during mild autumns or too cool during mild springs, shunt all or part of the air coming from the Canadian well, by placing a direct air intake at the entrance to the building. It’s the by-pass. A motorized valve, controlled by sensors, regulates the value of the mixture capable of optimizing the set temperature of the accommodation.
4 – The visit stamp
This manhole, intended for the maintenance of the Canadian well, collects the condensates. It is therefore placed at the lowest point of the installation. It must be watertight and wide enough to accommodate a sump pump. Any device allowing condensates to infiltrate into the ground is prohibited because of the risk of infiltration in the event of a rise in the water table.
5 – The fan
The fan must allow the air in the accommodation to be renewed every 2 h about. It is sized in such a way as to provide the maximum flow rates required plus pressure drops. Ventilation plants”double flux” optimize the efficiency of the Canadian well, provided that all the parasitic air inlets are blocked.
Maintenance
Equipment maintenance is reduced to periodic cleaning of filters, annual maintenance of ventilation, possible cleaning of exchangers and, rarely, pumping of condensates.
Advantages and disadvantages of the Canadian well
The Canadian well brings many advantages:
- He is ecologicalwith no impact on the environment and has a very high CO² balance.
- It guarantees the healthiness housing.
- He is naturally reversible.
- Reduced operating cost by an average contribution of 3 kWh for negligible energy consumption (COP between 20 and 25).
- It improves the thermal comfort (few vertical strata) and phonic (no air intake on doors and windows).
And some cons:
- implantation expensive
- great need surface et length available in the field
- earthworks substantial and expensive
- Ineligible to tax credits
Photo credits: Mamie Coco and Michka B