Heating, Ventilation and Controls at Montgomery School

Efficient building services are at the heart of a well-implemented Passivhaus strategy and compliment perfectly the airtight, super-insulated construction with high thermal-mass.


VENTILATION


The main benefit of the airtight building is to prevent uncontrolled air leakage – which brings with it associated draughts and heat losses – and alternatively pass on full control of air movement through the building to the ventilation systems.


Winter
In the case of Montgomery School, the ventilation systems operate in two modes. During the winter, where heat losses from the building are a concern, the building is predominantly mechanically ventilated, operating with a variable air volume (VAV) strategy.

Each occupied zone is provided with automatic control dampers in the distribution ductwork which alter the volume of air provided to each space. This system is controlled via the sophisticated Building Energy Management System (BEMS) under the dictates of temperature and pollution (CO2) sensors located in each space.

In conjunction with varying the local dampers, the BEMS also varies the speed of the fans in the main air-handling unit to ensure that the system is in balance and the minimum of energy is expended.

In this manner, it can be ensured that ventilation is only provided to spaces when required and at the duty required. This leads not only to more efficient operation at any given time, but also allows diversity to be built into the ductwork sizing and air handling unit selection for both increased efficiency and reduced costs.


Summer
During the summer, the building operates on a predominantly naturally ventilated strategy. Large roof lights located over the central corridor/atrium space are automatically opened based on the local environmental conditions to create a low-pressure ventilation “stack.” Each classroom adjoining the corridor is provided with an acoustically rated transfer grille at high-level, which allows warm exhaust air to rise into the adjacent corridor when operated in conjunction with the manually opening windows in each classroom.

The combination of both automatic controls to the central “stack” provision and manual local control in each room affords the occupants full control over their local environment.


Equipment
In conjunction with the sophisticated BEMS controls governing the system, the central plant serving the ventilation systems at Montgomery is also of the highest efficiency. Not only are the fans themselves “EFFE 1” rated for reduced electricity consumption, the main air-handling unit also offers ultra-high efficiency heat recovery, operating on the reversing regenerator principles as follows.


In the winter, warm exhaust air is passed through a large aluminium regenerator (heat sink) prior to its discharge to atmosphere. The warm air heats up the regenerator for a period of 60 seconds, at which point the direction of flow through the unit is reversed via a series of control dampers. The cold intake air is therefore warmed by the regenerator, prior to distribution through the building. This system operates with a heat recovery efficiency of around 93% - i.e. 93% of the heat which would otherwise be exhausted to atmosphere is recovered and returned to the building.


This system offers huge energy savings over more conventional systems (or indeed in comparison to a fully naturally ventilated building), since if there were no heat recovery the energy required to heat the incoming air to the required temperatures would dwarf that of the electrical energy required to power the ventilation system.


HEATING


Due to the super-insulated construction of the building fabric and the ultra-high efficiency heat recovery of the mechanical ventilation system, extensive thermal modelling of the building has proven that virtually no heating will be required when the heat gain from the occupants has been taken into account. As such, the only heating provided is by means of local electric heater batteries within the ventilation system, set to operate on restricted manual control or fabric frost protection only. Such a basic heating system has the benefit of zero-loss when not in use and near 100% efficiency when in operation (unlike conventional low temperature hot water based systems).


To offer some degree of user control of the heating, a manual “boost” button is to be provided. To prevent the over-use of this feature however (something that was predicted to be very likely if left entirely uncontrolled), the boost feature is restricted to bringing the building back up to the design set-point temperature should it fall below. In this manner, repeated use will be to no avail (although it may have a certain placebo affect).


It is predicted that the only prolonged use of the heating systems will be to bring the building up to temperature following long periods of absence (i.e. after a holiday period).


COOLING


In a conventional airtight and highly insulated building, issues with summertime overheating can often be encountered unless mitigated by a mechanical cooling system – clearly something entirely inappropriate to a building of this type and nature. To alleviate this, the high thermal mass of the building structure itself can play a part.


Due to the heat absorption and emission properties of the heavyweight concrete structure, the building fabric itself will ‘iron out’ some of the small fluctuations in temperature which would be seen by a lighter weight building. In this manner, ventilation air drawn in via the natural ventilation system will cool down the structure which will then itself absorb heat gains from the occupants.


Although not intended as a main control strategy, in extremes of temperature the mechanical ventilation system could even be brought into effect to pre-cool the building over night. The dense thermal mass would then retain the ‘coolth’ to keep the building at a more even temperature than if just daytime ventilation were utilised.


Further information about Montgomery Zero Carbon School