Hot Deck/Cold Deck Systems

What are they?

Hot deck/cold deck systems are an air handler based solution where the flow for the building is split into two, with one part being heated and one part being cooled.  These two airflows are then mixed together to create the right amount of heating and cooling for each space.  The main types of hot deck/cold deck system are:

Multizone systems:  For these systems, the hot air and cold air are mixed together in the plant room and the mixed air is ducted to each zone using dampers known as “face/bypass” dampers.

Dual duct Systems:  For these systems, the hot air and cold air are ducted to the zone and mixed together at the zone in a mixing box that is controlled from the zone temperature.

Dual duct systems are most often configured to serve many floors from a single unit, while multizone systems are more commonly configured to serve a single floor.

Advantages / Disadvantages

  • Highly flexible, and designed to service diverse loads from a single AHU
  • Industry standard technology with plenty of industry experience in operation and maintenance
  • Generally poorer efficiency
  • Face bypass dampers and mixing boxes prone to leakage, causing undesirable mixing of hot and cold airstreams
  • Large multi-story air-handling systems often less efficient at servicing after-hours loads.

Energy efficiency

These systems are generally poorer performers in terms of energy efficiency due to simultaneous heating and cooling, poorer fan control and reduced effectiveness of free cooling.  It is possible to reconfigure a dual duct system to be as efficient as a VAV system but this is relatively uncommon.  Multi-zone systems are difficult to operate efficiently especially when they service multiple diverse zones.

Running costs

Running costs for these systems are high due to poor energy efficiency.  Dual duct systems also have a high component count due to the mixing boxes.

A building management system (BMS) is essential to achieve efficient operation with either system type.

Retrofit / improvement opportunities

For hot deck/cold deck systems, the key opportunities are:

  • Reconfiguration of controls to minimise simultaneous heating and cooling
  • Improved fan control
  • Replacement/repair of face/bypass dampers to reduce leakage and possibly to provide independent hot/cold damper control (multizone systems).  This can be achieved withut disturbance to tenants.
  • Replacement/repair of mixing boxes to reduce leakage and possible to provide independent hot/cold damper control (dual duct systems).  This may need to be undertake after hours or in vacant spaces
  • Dual fan operation – one for the hot deck and one for the cold deck
  • Ducting of free cooling to the cold duct only to improve free cooling operation.

Control improvements can be implemented with the tenants in-situ.

Applicable buildings

Dual duct systems are often found in older large office buildings.  Mutlizone systems are found in a wider variaety of building types and re typically on a smaller scale.

Floor plate implications

The size of riser ducts can be significant for multi-storey buildings served from a central AHU (located, for example, on the roof or in the basement).  The risers may restrict flexibility on floor plates. In buildings with AHUs on each floor, the AHU plant room will take up a significant amount of space.

Temperature control / Occupant comfort

Hot deck/cold deck systems can offer a high degree of temperature control when well controlled

Maintenance implications

The mixing boxes and face/bypass dampers s require routine maintenance.   Central plant requires standard levels of maintenance.  Controls require careful configuration and management; arbitrary adjustment of temperature set-points within these system types is a leading cause of energy waste.

How to identify them

Hot/cold deck systems are found in older office buildings and in institutional buildings, but are largely indistinguishable from other AHU based systems to the non-technical observer.  Maintenance or operational staff will be able to advise whether a system is a hot deck/cold deck type.

Questions to ask

  • Does the system have independent control of hot air and cold air for each zone?
  • Is there a building management system (BMS)?
  • Is the system regularly maintained (at least annually) and seasonally commissioned?

More information

AIRAH Guide DA08 HVAC&R An Introduction

CIBSE Ventilation / Air conditioning / Technical Glossary