Södra Bancohuset

History

In the middle of the 1600s, the townsmen and council decided that a Bank of Sweden should be constructed adjacent to Järntorget in Gamla Stan (Old Town) in Stockholm. Architect, Nicodemus Tessin the elder was given the assignment to be responsible for the construction of northern Europe´s first banking building. The building was nearly completed in 1680 and the Bank of Sweden moved in.

After that, construction continued and the building was completed in 1712, first by Tessin the elder and then his son, Tessin the younger. In 1730, architect Carl Hårleman, was commissioned to carry out an extension to the building, which was completed in 1737. The property then stretched from Järntorget down to Skeppsbron and has been used as offices ever since it was first built.

Foundation reinforcement and renovation

In 2003-2004, Södra Bancohuset underwent extensive foundation reinforcement. The tenant, SFV, had to move out during this time. A major reinforcement operation of the foundation and masonry work from the 17th and 18th centuries was required to stabilise the building. In 2004- 2005, an extensive renovation was carried out and the ventilation system was also adapted to today´s requirements for good indoor climate.

Ventilation before the renovation

Södra Bancohuset was previously stack ventilated, where air with the help of temperature differences, rises out via vents or tiled stoves, the stack effect. Replacement air, (outdoor air), was drawn in through leakage between window frames and window casements.This could create draughts in winter and no ventilation at all in summer. In addition, there were many rooms that lacked exhaust air vents or tiled stoves. Rooms facing south were very hot in the summer. The majority of windows could be opened, which was utilized in the summer, but not everywhere in the building due to noise disturbance from traffic.

The stack ventilation was insufficient since the existing flues were not tight due to the settlement damage that the building had suffered. They were also partially filled with soot and mortar. The wet areas were fitted with exhaust air fans. Unfortunately these fans drew air from flues in nearby rooms, whose exhaust air thus stopped functioning.

Conditions for a renovation of the ventilation system

The building was constructed using that period´s construction techniques with thick stone and brick external walls and with a layout designed in accordance with that time´s architectural ideal with rooms opening into each other -enfilade- and with generous room areas and ceiling heights. Therefore the air exchange rate does not have to be as high as in newly constructed buildings with smaller room volumes.

Conservation principles

With the installation of the new ventilation system the greatest possible consideration was given to the building´s protection orders and the requirement not to touch the frame of the building. By restoring the original flues, the building can be ventilated without visible installations.

Authorization was given to drill holes in the chimneys in order to install propeller fans for exhaust air. Installation of new ventilation grilles in existing, previously sealed windows facing the inner courtyards (for intake air), and the disassembly of existing internal windows in the main stairway in order to access air flow up in the stairway, was also permitted. Certain rooms had clogged exhaust air vents in the brick flues and these were restored.

By placing the exhaust air fans in the chimneys in the attic, the need for walkways and ladders to the roof was minimized and through that, additions to the building´s protected exteriors have been avoided.

Fitting the facades facing the south and west with awnings was possible since the building had been previously fitted with such. During the 1800s, awnings were commonly used to help lower the indoor temperature during the summer. Therefore, the installation of a cooling unit for the entire building was not necessary.

Ventilation after the renovation

All floors and half of the fitted attic floor have a ventilation system with controlled supply air. During summer, the exhaust air is mechanically controlled and during the winter rises thermally via the stack effect.

The part of the attic floor that faces south, and can be very hot during the summer, now has a separate mechanical supply and exhaust air system with cooling and heat recovery. The unit is located in a plant room in the upper attic.

Supply air

Replacement air is no longer drawn in via the windows, and due to this there are no unpleasant draughts, polluted air or disturbance from traffic.Naturally, the windows can be opened if you want extra ventilation during the summer.

In the basement, which is on the same level as the inner courtyards, two supply air units have been installed. These units are located next to the building´s two stairways. The units, which contain filters, heating batteries and fans with silencers, get their air from the courtyards and blow it into the stairway. The air from the courtyards is less polluted than that on the street side.

Since the building has a somewhat open design without closed doors, every room can be given the necessary volume of air via stairways and corridors. Those rooms that often must have closed doors have been provided with transfer air slots in the door frames, or silenced transfer air devices. The flow of the supply air fans is controlled by a pressure sensor on the 4th floor to ensure negative pressure in the building.

Exhaust air

After the survey of the building´s 17 clay brick chimneys (with a total of 66 flues) it was noticed that there were flues enough to equip every room with vents for exhaust air. In some rooms new holes were made in previously unutilized flues. All flues were then cleaned and sealed.

The original exhaust air grilles have been retained. They are cleaned and re-painted. The existing plastic grilles were replaced with new grilles similar to the originals. Some grilles come from a previous renovation of the Manilla school.

Since the existing exhaust air flues have regained their original function, the ventilation in the winter can function with the stack effect. To ensure the flow of air in the summer, every chimney has been fitted with a rotational frequency controlled fan (propeller fan), in the attic.

The fans shall only be in operation during the warmer parts of the year when stack ventilation ceases and will be regulated so that the right flow is obtained. The fans are not in use in winter. They provide virtually no resistance when turned off and therefore do not counteract the stack effect. Since the fans in the chimneys are jointly used for all flues in each chimney, all exhaust air vents in the rooms have been fitted with (hidden) control valves to adjust the air flow.

Wet areas are connected to exhaust air fans which are in operation year-round during the daytime. The assembly hall (the Session Hall) is fitted with an exhaust air fan whose flow is controlled by the carbon dioxide content. The carbon dioxide content is also measured centrally on a continuous basis from the 5th floor.

Experiences

The property´s tenants are satisfied with the new ventilation system. The air is perceived as pleasant. The sunlight-and weather-controlled awnings and the extra night cooling, when needed, give a good indoor climate even in the summer. The tenants have no problem with adapting to the building´s conditions.

It is important that no more workplaces are set up than the room´s ventilation is dimensioned for. Those who work here must be aware that if the door to a room is shut, the ventilation decreases. When using the assembly hall and meeting rooms, meetings should be held with intermittent breaks for opening doors and windows.

When stairways and corridors are used for supply air, as they are in this building, it is very important to have efficient and thorough cleaning so dust and dirt is not spread around. After the building´s new ventilation system had been in use for a year, it was discovered that decayed mortar had sometimes loosened and fallen on the axial fans and caused interruption of service. Experience has shown that the flues above the fans must also be sealed.