The options and limitations of HVAC systems for fire protection

The zoning off of specific areas within a building by means of fire proof or fire resistant walls and ceilings has proved to be an effective, structural means of preventive fire protection. However, the fact that the ducting of ventilation systems has to pass through such structural elements means that their effectiveness is seriously compromised.

Fire dampers have been shown to be an ideal means of preventing the spread of fire and smoke and are a safe means of stopping fire breaking out elsewhere. However, the true danger of smoke spread has only been recognised properly over the last few years. In drawing up suitable concepts for fire protection, the question has constantly arisen as to what the functions of ventilation systems should be in the event of fire.

The zoning principle

The absolute minimum requirement is that fire dampers should be closed in good time to prevent a fire spreading. Automatic thermal trips, an essential item with any fire damper, take care of this aspect.

However, many Regulating Bodies already include a requirement for the prevention of smoke spread. A thermal trip fire damper will actually close sufficiently tightly to prevent hot combustion gases from a fire spreading. However, additional measures will usually be required in order to prevent the spread of smoke when the temperature is less than 72°C.

Fig. 1: Zoning by means of fire dampers with smoke trips

External air intakes and recirculating air systems

In the case of supply air ventilation systems the official guidelines by many Regulating Authorities is for ventilation systems to specify that the external air intakes should be placed in a position where it is impossible for smoke to be drawn in. If this is not possible the spread of smoke mixed with external air must be prevented by means of shut off devices incorporating smoke trips (fire or smoke dampers). This means that there have to be smoke trip devices in the flow of the supply air. The danger of smoke spread is especially great in the case of the recirculating type of ventilation systems. Consequently, here too the flow of supply air must be protected with shut-off devices with smoke trips. In this case the smoke trips can be fitted in the exhaust air, re-circulating air or supply air ducting. Guidelines also require that the supply air fans be switched off when the smoke trips operate.

Fig. 2: Smoke spread with recirculating-air ventilation systems

However, the switching off of the supply air fans can actually be counter productive if, for example, some areas of the building receive their supply of air solely from the ventilation system or the supply air fans are needed to maintain specific pressure conditions (e.g. a positive air pressure for escape routes, clean rooms, etc.). In the event of smoke being drawn in through the external air intakes the ventilation system could continue running in recirculation air mode. In this case, however, the smoke trip would have to be fitted in the flow of external air.

In the event of a fire in the building the ventilation system can continue operating provided there are fire dampers with smoke trips in the zone of the building affected by the fire.
Guidelines by Regulating Authorities do not always specify whether the exhaust air fans should also be shut down. Should the exhaust air system remain operating after the supply air system has been shut down there are two particular factors to be taken into account:

1. There will be a negative pressure in all areas connected to the system. This means that the spread of smoke can receive considerable assistance especially if, in the zone affected by the fire, the fire dampers have been operated by thermal trips and a positive pressure has built up. Although the smoke no longer spreads through the ventilation ducting it could possibly travel along the escape and rescue routes.
2. Due to the uncontrolled negative pressure that arises with the exhaust air system still running the resulting pressure differentials could cause the maximum permitted door closing force of 100 N to be exceeded and it would be impossible to open doors to escape routes and rescue routes.

Consequently, the normal course of action in the event of a fire is to shut down the ventilation system completely.

Stopping the ventilation system

When smoke is detected in a building (e.g. by smoke trips at the fire alarm control room) the whole ventilation system throughout the building is shut down; because of the ducting of the ventilation system, the fire zones will still be open to each other.
As the intensity of the fire increases so too does the positive pressure in the area of the fire. Since the release of pressure takes place towards the adjacent fire zones the smokes spreads in that direction too. In the process the combustion gases in the ducting cool down so much that the temperature at which the thermal trips operate (normally assumed to be 100°C in practice) is reached either very late, or sometimes not at all. Past fires have provided impressive evidence as to how open ventilation ducting systems can make a major contribution to more serious fire damage.
Therefore, the best advice, when a ventilation system is shut down, is to close all the fire dampers too. This normally assumes, of course, that the fire dampers are motorised.

Fig. 3: Smoke spread with the ventilation system stopped

Special case: Overflow fire dampers

However some Regulating Authorities guidelines do not mention the special danger that arises from overflow fire dampers when there is a fire in a building. Due to the absence of the duct connection, smoke inevitably spreads, e.g. if supply air is introduced into a fire zone while the exhaust air is being removed through an overflow damper with no duct connection at the back, the smoke will be able to flow into the adjacent fire zone.

This type of ventilation is often used in school classrooms with the overflow vents then also being designed to discharge into the corridor (the escape route). Consequently, it is absolutely essential for overflow dampers to be fitted with smoke trips.

Fig. 4: Overflow fire dampers

Continuous operation of ventilation systems during a fire

Some Regulating Authorities make reference to it being useful and not dangerous to leave the ventilation system running in the event of a fire, although naturally, the spread of smoke via the recirculating air must be prevented. The comments also mention that while the system is still running, although smoke can be drawn in through the exhaust air outlets, it cannot then spread into the adjacent zones because there is a negative pressure in the exhaust air ducting. Due to the positive pressure, no smoke can spread into the supply air ducting. If the tripping temperature were to be reached however, the closing of the fire dampers would also eliminate any fear of smoke spread. This assumption, which is a logical approach, however does not take into account that the supply air and exhaust air fire dampers are tripped at different times. Since the fusible link of the supply air fire damper is “cooled” by the fresh air, past experience has shown that the exhaust air fire damper closes long before the supply air fire damper. If now there are no limit switches available to pass on the signal, or the ventilation system does not shut down due to some other reason; in addition to the combustion gases building up a positive pressure, the fire zone will also be “pumped up” by the supply air. The smoke and combustion gases will then be forced through the door apertures or any other unsealed points into the adjacent fire zones, corridors and escape routes, so endangering the rescue of occupants.

Shutting down the ventilation system when the exhaust air fire dampers have been tripped does not always prevent the spread of smoke. The positive pressure in the area of the fire causes smoke spread until the supply air fire damper closes.

The sensible course of action is to for the supply air fire damper to be closed immediately after the exhaust air fire damper, which makes an automatic closing device essential. Therefore, motorised fire dampers are normally essential if the ventilation system is to continue running when there is a fire.

Cold smoke extraction and fire service switch

There is undoubtedly an increasing trend for ventilation systems to assume the task of smoke extraction systems. One of the main reasons for this, of course, is the economic gain to be had. Legitimising this course of action, however, is a controversial matter and can be in conflict with National Regulations, which can call for the following statement:

“In commercial premises with sprinkler systems, in the event of a fire it must be possible to operate ventilation systems in shopping areas and shopping aisles so that they only extract air, provided the rules governing the use of shut off devices for the prevention of fire spread allow.“

In practice this section of the regulations has been interpreted to mean that smoke extraction systems can be dispensed with entirely in commercial premises with sprinkler systems with the ventilation system undertaking extraction of the “cold smoke”. In actual fact “cold smoke” means smoke at a temperature below the operating temperature of thermal trips (i.e. fusible links). The definition cold smoke is combustion gases that are only slightly different in temperature from the ambient air and cannot be removed without an additional flow of air.
Even clearer than the commercial premises regulations are the industrial premises regulations for fire extinguishing systems:

“...Instead of smoke extraction systems it is possible to use ventilation systems with control systems which, when there is a fire, only allow air to be exhausted. The fans of these ventilation systems do not have to be designed for fire and in other respects they must satisfy the requirements of the regulations for fire protection equipment in ventilation systems.“

Although the elimination of a smoke extraction system in industrial buildings might possibly appear to be justified (the rooms, etc. are usually high and the occupants are trained people) for commercial premises with large numbers of untrained people inside, it could be irresponsible to apply this model. In addition, the various floor levels are usually of open design (up to 3000 m2 over three floors is allowed), which also helps to promote the spread of smoke. The principle factors opposing the removal of smoke by means of the ventilation system are as follows:

1. Ventilation systems are designed for comfort and the number of air changes they perform, which is usually neither suitable for slowing down to any great extent the increase in the layer of smoke produced by a fire, nor for achieving any noticeable reduction in room temperature, even when, as required, the supply air system provides additional extraction.

2. The cooling effect expected from the triggering of the sprinkler system, will not be able to safely prevent failure of the ventilation system, due to temperature (fire dampers, fans) as the fire continues. Therefore, there is no reliable foundation from which to plan any suitable evacuation scenarios.
3. Without the proper extraction of smoke, triggering the sprinkler system can make it more difficult to rescue people from close to the affected area, because the volume of combustion gas increases, the smoke cools down and is forced downwards.
4. If, in the event of a fire, the ventilation system were to be switched over completely to air exhaust, suitable apertures would be needed to allow pulses of ‘topping up air’ to be introduced. This is almost never the case in actual practice and so the effectiveness of the cold smoke extraction will be reduced even more.
5. The idea of cold smoke extraction is leading to the formal use of building products, in ways other than those approved by the relevant general building regulations. This is especially true of fire dampers.

The removal of smoke by means of the ventilation system must not be continued after the fire dampers needed for producing the fire zones have been triggered. A sprinkler system too will not be able to safely prevent the triggering temperature being exceeded. In order to allow the ventilation system to continue operating an attempt is often made to re-open motorised fire dampers that have been triggered thermally by short circuiting the thermal contacts. This variant is also called the “fireman’s service switch” but it conceals some major risks: Firstly the thermal triggering of a fire damper must be permanent and irreversible. This means that a fire damper with a “fire service switch” would no longer receive general building regulations approval. The only possible verification for use therefore, would be approval granted by the overriding building authority (not the fire service!) for individual cases. Secondly, the triggering temperature of a fire damper would have to be chosen so that the spread of fire through the ventilation system could be safely prevented. The re-opening of a triggered fire damper represents a safety risk that it is impossible to calculate as far as the spread of fire is concerned. Therefore, the use of the ventilation system for smoke removal is only possible within very close limits.

Smoke removal by scavenging

Whereas the classic mechanical means of smoke extraction by drawing in the combustion gases released by the fire and exhausting them to the outside, produce specific layers of smoke and also help to cool the seat of the fire. Pressurising systems for smoke protection is based on the principle of preventing the penetration of smoke, especially into escape and rescue routes. For this purpose suitable fans are installed in the rescue routes (e.g. staircases, corridors, lift shafts) aimed at producing a positive pressure of between 15 and 50 Pa at the place of use.

In principle the ventilation system is also suitable with the aid of an actively controlled build up of pressure differences, to oppose the spread of smoke and provide support for the action of smoke extraction or pressurised smoke protection systems.

In large buildings of open construction in particular the ventilation systems keep thousands of cubic metres of air flowing continuously in specific directions. In the event of a fire it is frequently necessary for those flow conditions to be completely reversed and it is here that the ventilation system can make a valuable contribution. For example, in the areas not affected by the fire, closing the exhaust air fire dampers, will act against the spread of smoke, producing a positive pressure. In the ideal case supply air fire dampers in the zones affected by the fire would be closed in good time by automatic smoke tripping devices and other apertures would be opened up (e.g. windows, open exhaust air fire dampers) to allow air to flow right through the building in the direction of the seat of the fire. The effectiveness of this measure must be verified by suitable trials whereby it is particularly important to remember not to exceed the maximum permitted door closing forces as a result of the pressure conditions that have been created.

Fig. 5: Ventilation system for preventing smoke spread

Therefore, it will usually only be possible to prevent the spread of smoke if smoke tripping devices can detect the fire quickly, pinpoint it and operate the fire dampers automatically by means of remote control devices.

It has also become clear that by motorising fire dampers, it creates the opportunity to respond flexibly to different situations and hazards.

In order to be able to implement intelligent scenarios in the event of fire, fire and smoke dampers are increasingly being controlled through bus systems by higher level instrumentation and control systems. In future these automation systems will need to be the subject of specific requirements regarding functionality and security of data.

Belimo Automation FZE is located in Dubai Airport Free Zone, which serves the entire Middle East. It holds the full range of Air and Water products in stock. The Swiss based company has been manufacturing electric actuators for air dampers and valve technology in heating, ventilating and air-conditioning (HVAC) systems since 1975. Belimo has become a world leader in this specific market.