Do we need hot boxes in smoke control?

Posted by Paul Compton on 09/08/16 12:00

Hot box for smoke controlBS 8519 introduced the concept of “hot boxes” to protect damper motors to a wider audience. What are these and what do they offer?

What is a hot box?

A “hot box” is simply an insulated covering for an actuator, intended to keep it cool in case of fire, as most actuators in common use are not themselves rated for high temperature use.

Is it always needed? It depends.

It is of course easy to simply specify hot boxes and cover yourself. After all, if the actuator is protected from fire it must be better, mustn’t it? Well, yes, but it adds cost, bulk and complexity and makes maintenance more time consuming, so is it worth it?

To a large extent this depends upon where the actuator is, what it controls and when it is needed to operate.

Not as clear cut as it might seem

A typical example of when hot boxes might be considered is a smoke control shaft ventilating a series of lobbies. Normally the actuator is within the shaft, initially protected behind the fire rated vent it operates. The vent opens into a lobby, which is again protected from the potential fire room. The vent is opened upon detection of smoke in the lobby and it can be considered as fully protected by virtue of its position until it has operated and the lobby vent is opened. Specifying a hot box to protect this operation is pointless and a complete waste of money.

If however we expect fire fighters to open and close vents later in the progress of the fire and we expect the temperatures in the shaft to exceed the operating temperature of the actuator a hot box makes more sense (and don’t forget to also protect the actuator’s flying lead as well). However, will the fire fighters really be familiar enough with the system to be confident that they are not making matters worse by taking manual control?

Conclusion: we need a debate

In my opinion, hot boxes should definitely not be the default choice, but should only be specified after full consideration of the project requirements. In the absence of much guidance, debate would be useful. What do you think?

Paul Compton Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Smoke Control, Regulations