In France, the air temperature varies, depending on the climatic zones, between – 25 ° C and + 40 ° C. This means that despite the current standards, a huge amount of cool air conditioning escapes 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 charge calories or frigories, the fresh air necessary for the ventilation of the house. Forced by a wind tunnel, the air flow 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 underground journey, 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 installation’s components, as well as the type of terrain and local climatic conditions.
Sizing of the Canadian well
The sizing of the installation aims to optimize its yield while remaining within an acceptable budget. For precise results, these complex calculations are reserved for specialized technicians. However, a basic installation can rely on empirical data largely validated by experience. Subject to special requirements, the following figures represent the average values observed for an individual pavilion of 250 to 300 m3 in volume, in mainland France.
The efficiency of exchangers
The efficiency of the Canadian well is conditioned by the length, material and the diameter interchanges, by traffic speed air in these pipes, as well as through the nature and speak humidity level from the basement. The best investment / return compromise is for the burial of 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 few meters, then amplify 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 exchangers spaced at least 1 m apart.
Diameter of the exchangers
The diameter of the pipes must allow the best flow of the ventilation flow, while limiting the pressure losses by friction. It also influences the energy consumption of the fans, so the COP of installation. In practice, the choice of the diameter of the exchangers is made according to the criteria imposed by the higher summer speed. It is generally between 175 and 200 mm.
Air circulation speed
Regulating the air circulation speed in the exchangers is an essential criterion for the good performance of the installation. The best thermal exchanges are obtained with a Maximum speed of 2 m / s in winter, and of 3.5 m / s in summer.
Characteristics of basements
Thermal conductivity increases with the water content of the basements. The best yields are obtained in silty or sandy, humid environments. Usually, the pipes are flooded, trench bottoms, in fine sand which gives them mechanical protection and stability.
The 5 components 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 aeraulic network and perfectly waterproof.
1 – The external suction terminal
First element of the device, the outside air intake is placed at least 1.40 m high. It houses a double filtration device comprising a fine grid and one or more efficient filters. The first prohibits entry to rodents or other unwanted intruders. The second retain insects, dust, pollen, certain corrosive elements, and large polluting particles. The cross section of the filters must be suitable for higher volumes planned ventilation.
2 – Buried pipes
The choice of materials is vital to ensure the safety of the air breathed by building occupants. All tubes and equipment must bear the label “Food grade“and must be able to withstand the pressure exerted by the burial depth. The tubes PVC or PTC should be avoided, as they release solvents and toxic particles. Metal tubes are not suitable, because they are difficult to install and sensitive to corrosion. Finally, sandstone tubes are brittle and not very airtight at the connections. 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 crowns 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 pain of loss of efficiency, no pipe must run under heated buildings or pass through them. less than 1.50 m.
3 – The bypass
In half-seasons, the basement 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, we shunt all or part of the air coming from the Canadian well, by having a direct air intake at the entrance to the building. It’s the bypass. A motorized valve, controlled by probes, 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 waterproof and large 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 rising water tables.
5 – The fan
The fan must allow the air in the dwelling to be renewed every 2 hrs about. It is dimensioned so as to provide the maximum flow rates required plus the pressure drops. The ventilation units “double flow“optimize the efficiency of the Canadian well, provided that all parasitic air inlets are blocked.
The maintenance of the equipment is reduced to the periodic cleaning of the filters, to the annual maintenance of the ventilations, to the possible cleaning of the exchangers and, rarely, to the pumping of the condensates.
Advantages and disadvantages of the Canadian well
The Canadian well brings many advantages:
- It is ecological, has no impact on the environment and has a very excess CO² balance.
- It guarantees the healthiness housing.
- It is naturally reversible.
- Reduced operating cost by an average input 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 disadvantages:
- implantation costly
- great need area and length available in the field
- earthworks substantial and expensive
- Ineligible tax credits
Photo credits: Mamie Coco and Michka B