Pool construction in Ankara: what are the frost and heating loads in a continental climate?

The table below gives the 1991-2020 averages for Ankara. Three columns are decisive for the pool designer: the average monthly low (how many months frost lasts), the average monthly temperature (the heating deficit) and precipitation (the months in which the pool does not fill itself but loses water to evaporation).

Three conclusions follow from the table. First, in December, January and February the average night temperature is below zero; in other words, the pool shell and joints freeze almost every night and thaw during the day for three months. Second, in July and August the day-night range is between 13.6 and 13.8 °C; the pool water warms during the day and loses heat at night to the cloudless dry air through radiation and evaporation. Third, while total precipitation for July to September is 47.3 mm, evaporation is far above this; the pool's water level is kept up by the mains, not by rain.

A one-off frost fracture is when water freezes in an undrained pipe or a water-filled overflow channel, its volume increases by about nine percent and it cracks the part; it happens in a single night and its cause is clear. Freeze-thaw fatigue, by contrast, is the flaking of the surface, the opening of joints and the hollowing out under ceramic tiles after dozens of cycles, as water in the capillary pores of the concrete and the mortar under the finish freezes every night and thaws every day. At -22 °C, ice growth can produce pressures of up to 207 MPa, and the process is active between -3 and -8 °C when water is present; most winter nights in Ankara fall exactly within this range.

The modern explanation is less about volume expansion than about ice segregation: water migrates by capillary action to the freezing front, ice grows in lens shapes and cracks the material from within. The practical consequence for a pool is this: the problem is the concrete and mortar staying saturated, as well as the water in the pool freezing. A pool shell left open, saturated and without drainage through the winter is exposed to this mechanism every night.

On the first axis, the mix, joint and drainage decisions for the reinforced concrete pool are written; on the second, the heat pump capacity and cover decision. The list below gathers clauses that can be transferred directly into the technical specification of a pool project.

The degree-day logic is clearer with an example. If the target water temperature is taken as 26 °C, the deficit with the monthly averages in the table is: in May, 26 minus 16.5 equals 9.5 K, which multiplied by 31 days gives about 295 degree days; in June, 5.4 K gives 162; in July, 1.8 K gives 56; in August, 1.7 K gives 53; in September, 6.4 K gives 192. May and September together carry a larger heating deficit than the three summer months combined. This calculation shows only the deficit from the air; evaporation, wind and night-time radiation are added separately. That is why in Ankara heat pump capacity is always chosen for the ends of the season, taking into account that the COP also falls when the air temperature is low.

The heating degree day concept comes from building engineering: the base temperature is subtracted from the daily mean temperature, and if the result is positive it is that day's degree days; a building's heating energy is estimated as the product of its specific heat loss and the degree-day total for the period. In a pool, the same logic is applied with the water temperature as the base.

The typology is largely set by the plot and the climate: plots are larger than in coastal cities, pools are rectangular and 8 to 12 m long, mostly skimmer pools or overflowing on one side, with the plant room buried below garden level and below frost depth. On the Gölbaşı side, groundwater is high on some plots, so the risk of an empty pool floating under uplift enters the calculation; this reinforces the rule that the pool is not drained in winter.

An indoor pool is a meaningful option in Ankara at villa scale as well. While the outdoor season fits into five months from May to September, an indoor pool is used for twelve months; in return, humidity control and ventilation load are added. In an indoor pool, the external freeze-thaw load is removed from the shell, but indoor condensation and the capacity of a dehumidifier with heat recovery move to the centre of the design. Ankara's dry outdoor air is an advantage for dehumidification: because the humidity difference between outdoor and indoor air is large, lowering humidity with fresh air takes less energy than in coastal cities. In an indoor pool project, the shell remains a reinforced concrete pool, but impermeability and thermal insulation come to the fore instead of air-entrained concrete.

From then on, air-entrained concrete became standard practice in cold climates. The mechanism is explained the same way today: closed bubbles 10 to 500 micrometres in diameter are pressure relief spaces that make room for the expansion of freezing water without creating internal stress.

Pool construction took up this knowledge late. Air entrainment, used for decades in road and bridge concrete, was long skipped in pool shells out of concern for water-tightness; yet closed bubbles do not form a capillary network and so do not increase permeability. In a city like Ankara, where most winter nights fall in the -3 to -8 °C band, there is no technical reason to give up this admixture in shell and overflow channel concrete.

The degree-day method grew out of fuel planning and was carried over to pool heating in the same way. For buildings, the base temperature varies from country to country between 12 °C (Switzerland) and 18.3 °C (USA); for a pool, the base is the target water temperature, which is why in Ankara May and September, considered mild for building heating, are heavy months for a pool. A reinforced concrete pool shell that gets through winter undamaged, and a heat pump chosen for the ends of the season, are the two fixed numerical inputs of a pool project in Ankara.