By installing both the freshwater line and the outgoing sewer line into the same, sand-filled Finnfoam casing, an inexpensive heat exchanger is created. Part of the wastewater's heat is collected, freezing of the freshwater line is prevented and installation depth can be reduced.
To avoid damage caused by freezing, water and sewer lines are traditionally installed in depths where ground frost no longer occurs. Deep plumbing channels that extend into the groundwater table may cause groundwater levels to fall and, in areas with soft soil, cause streets, plumbing and buildings to sink and cause changes in vegetation and potentially have other negative environmental impacts.
The depth of the lowest floors of the buildings for which sewer lines are built and the grading and elevation of the streets and/or the yard areas permitting, pipelines installed closer to the surface (within ground frost depth) may be protected from damage caused by freezing by insulating the pipes.
Good insulation prevents ground frost damage
Thermal insulation and ground frost protection of water and sewer lines is based on utilising the geothermal heat from the ground and the heat released by the pipelines. Insulation is used to reduce heat loss during the winter and to prevent pipelines from freezing by keeping the lowest ground frost depth above the top of the pipes. This also prevents ground frost and the damage caused to the pipes by soil movements caused by ground frost (frost heave).
Planning time considerations
When designing ground frost protection, climate conditions, soil properties and the heat released by the pipelines must be taken into account. Insulation of plumbing lines must be planned on the basis of the harshest winter in the location, i.e. the lowest temperature (F50) expected to occur once within a period of 50 years.
The impact of soil type
In addition to climate conditions, the depth of ground frost depends considerably on soil type. In soil consisting mostly of sand and gravel and in coarse moraine soil, ground frost reaches deeper than it does in clay or silt soil. This is caused by the higher thermal conduction of the coarse soil types and their poor heat retention capacity. Similarly, pipelines installed near the surface in coarse non-cohesive soil require more insulation than plumbing installed in cohesion soil with a high water content. Insulating pipelines installed into shallow bedrock channels using geothermal heat is also often difficult due to the high thermal conduction properties of bedrock and its small heat reserves.
In areas with clay, silt or fine moraine soil prone to ground frost, pipelines can be insulated using horizontal layered insulation (see illustration). A transition structure can be used to level out steep frost heave in areas where ground frost is especially heavy.
In sand and gravel soils unaffected by ground frost, it is often best to use a convex, U-shaped insulation structure that extends over the sides of the pipe. This allows more efficient utilisation of the heat released from the pipeline (see illustration).
Pipelines installed in shallow bedrock channels can be insulated efficiently using an insulation structure that extends around the pipe (see illustration). The flow in the pipeline and the water temperature must remain sufficiently high to keep the pipeline from freezing. When necessary, electric heating cables may be used as an additional source of heat to protect the pipes.
Finnfoam casing 130 x 160
Casing (sewer 160 + water line)
Finnfoam casing 500 x 285
Finnfoam casing 1200 x 600
Wastewater heat capture
Depending on the type of pipe material used, the heat released by the pipelines may be utilised when determining the amount of insulation required. A rainwater drain may have a cooling effect on adjacent pipes due to the flow of cold surface water and ventilation occurring through the drain. The heat released by a sewer line in the same channel with a water line, on the other hand, may often be utilised in the dimensioning of the insulation layer. If the temperature of the liquid travelling in the pipe is allowed to fall in the insulated section of the pipe, and if this additional heat is continuous during the seasons with sub-zero temperatures, this can be utilised when planning the amount of insulation required.
Installation depth affects cost
The installation depth of the pipelines significantly affects their construction cost, as usually over 50% of this arises from digging, excavation, installation and filling work. Installation of pipelines near the surface in soft soil also reduces the need for bottom reinforcements as the load caused by channel filler materials is reduced. This may also reduce the time required for construction work and potentially avoids the need for erecting construction-time support structures for the excavation channel. Significant cost savings may be achieved in these cases.
Ground frost insulation of pipelines must always be dimensioned on a case-by-case basis following the graphs in the illustrations.
Horizontal insulation thickness (mm, FI-300) and width as a function of additional heat in different base soil conditions.
U-shaped insulation thickness (mm, FI-300) and width as a function of additional heat at maximum ground frost depth z = 1.2 metres.
U-shaped insulation thickness (mm, FI-300) and width as a functional of additional heat at maximum ground frost depth z = 1.6 metres.
Thermal resistance and width of case-shaped insulation as a function of additional heat in bedrock channels.
Ground frost protection of wells
Wells of sub-surface drain systems can easily act as cold bridges. It is easy, however, to cut a hole shaped like a land drain in a Finnfoam board. The hole-shaped piece can then easily be used as an interior top cover for the well. The cover of the well should be as airtight as possible. The strong, sturdy, waterproof Finnfoam insulation will last well as a top cover insulation. Screws are screwed to the side of the drain at the same height as the surrounding ground frost insulation. It is a good idea to also attach a screw on the cover of the drain so that the cover can be lifted easily.
The same principle can also be applied to other wells to protect them from freezing. The sturdy Finnfoam insulation can also carry its own weight when longer spans are used.