Indoor Air Quality This paper describes an in-situ comparison study of four different HVAC systems. The main tasks were to study the capital costs, the energy consumption, indoor environment and operation and maintenance of the mechanical systems.

The three functions of heating, ventilating, and air-conditioning are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality and reasonable installation, operation, and maintenance costs. Indoor air quality has a major influence on the health, comfort and well being of building occupants.

Poor air quality has been linked to problems of Sick Building Syndrome and shown to reduce productivity in offices and learning in schools. In Europe people spend about 90% of their time in indoor environments and their exposure to many air pollutants is largely dependent upon the exposure that occurs indoors.

Good air quality depends upon a wide range of parameters including the presence in air of volatile organic chemicals, gases such as nitrogen dioxide, ozone and carbon monoxide, biological particles including bacteria, fungi and pollen. The perception of these components can be influenced by the temperature and humidity. Effects on health depend upon the toxicity of the pollutant, its concentration and the exposure period. The effects may therefore range from odour, to irratation as well as carcinogenic and non-carcinogenic toxic effects.

Importance of ventilation

It’s important to realise that ventilation is one of the most crucial processes in determining the indoor air quality. While many of us tend to perceive ventilation as either routine air movement within a closed room or the intermittent introduction of outdoor air, it is actually a complex process.

Ventilation needs immense planning, and hence demands expert inputs. Let’s get more insight into ventilation. It’s a combination of processes that results in the supply / removal of air from inside a closed structure.

The ventilation processes typically comprise:

• Bringing in outdoor air

• Conditioning, mixing the outdoor air with some amount of indoor air

• Distributing this mixed air throughout the area

• Exhausting some amount of the indoor air outside

The quality of indoor air may fall when one or more of these processes are faulty or inadequate. For e.g., carbon dioxide (a gas produced during breathing), may accumulate in a room if sufficient portion of outdoor air is not brought into and distributed throughout the closed space. Carbon dioxide acts as a surrogate for indoor pollutants, which may cause the room occupants to feel drowsy.

Comparable ventilation systems

Air conditioning is often confused with comfort cooling. Air conditioning heats or cools fresh air as it comes into a building, the air is filtered to remove dirt and can also have the humidity adjusted the treated air is distributed around the building and finally adjusted to local temperature requirements in fan coil unit. Comfort cooling generally is cheaper to install, systems typically consists of an indoor unit and external unit connected by two or three pipes, with piping to a fan coil unit either in the ceiling or mounted on a wall. These simply heat or chill the air although modern VAV systems can heat one area and cool another at the same time. Generally the systems circulate the air already contained within the building and do not bring in fresh air, although just to confuse, some units will. As we have already mentioned, temperature control and ventilation (as well as lighting) have a dramatic effect on the well being (and productivity) of the occupiers of a workspace. Very often when a building undergoes modification, or use is intensified after the design of the system, the original system will need checking to ensure that it is still fit for purpose. Moreover, new techniques such as mixed mode systems are becoming popular, where natural ventilation is reinforced by a cooling system for hot days, and also where the mass of the building, is used to cool the air down during the day.

This study looks at the four different systems, taking information and research from a variety of source materials, and independently tries to determine the benefits and drawbacks of using each system.

We have only compared the properties of an active chilled beam system. The active chilled beam can be described as a fan coil unit with no fan or filter, working as a ceiling mounted induction unit, it provides ventilation cooling and sometimes heating into a space. It is totally reliant upon other factors such as chillers, ground source water, heat sinks etc. It is in these areas that the free cooling and energy efficiency of the systems is realised.

Active chilled beams are more complex than passive chilled beams. In addition to a finned cooling coil, they have an integral air supply designed to meet minimum outdoor air requirements. In this way they differ from fan-coil units, which blow indoor air over cooling coils located in the conditioned space and rely upon a separate system to meet OA requirements. Due to forced convection, active chilled beams achieve cooling densities about twice those of passive chilled beams.

Fan coil systems are either 2-pipe, cooling only, or 4-pipe, heating and cooling. For this report we have selected as preference the 4 pipe system, with hot water heating coil, chilled water cooling coil, and an outside air mixer, as it offers good levels of comfort air conditioning at a competitive installed price. It is also simple to maintain and allows a good level of flexibility in relation to any reconfiguration of the occupied space.

VAV systems are primarily cooling systems. However, VAV boxes can be equipped with reheat coils, especially if the required VAV box minimums are substantial. Fan powered boxes incorporate the use of a fan as part of their operation. The fan operates to pull air from the plenum space. In a typical VAV system there is no return air duct, per se. Return air grilles are mounted in the ceiling grid, yet are not connected to anything.

Variable refrigerant flow (VRF) systems, were introduced more than 20 years ago and consist of a number of floor, wall or ceiling or mounted indoor units connected to a common outdoor unit by refrigerant pipe work. Some areas may be served by packaged, split and multi-split packaged comfort cooling equipment, in addition to a centralized system.

Conventional systems transfer heat from the space to the refrigerant by circulating air (in ducted systems) or water (in chillers) throughout the building. VRF systems are larger in capacity, more complex versions of the ductless multi-split systems, with the additional capacity of connecting ducted style fan coil units. They require many evaporators and complex oil and refrigerant management and control systems.

Also, they need a separate ventilation system. The term variable refrigerant flow refers to the ability of the system to control the amount of refrigerant flowing to each of the evaporators. This enables the use of many evaporators of different capacities and configurations, individualised comfort control, simultaneous heating and cooling in different zones, and heat recovery from one zone to another.

Additional costs will be incurred for both the VRF and Fan Coils, as prior to installation a seperate ventilation system will be required.

Whole life costs over 25 years for a 6000m2 new office

Chilled beams

Fan coil units

VRF

VAV

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 Net Present Value (£000s) Key

Capital cost Major asset replacement

Sub asset replacement

Maintenance Energy costs

Comparison

VAV Systems Chilled Beams Variable

Refrigerant Flow

Fan Coils

Use of refrigerant

No refrigerant No pipe inspections No danger of leaks

No refrigerant No pipe inspections

X Refrigerant used X Possible leaks X Pipe inspections Refrigerant is more efficient than water

No refrigerant No pipe inspections

Installation

X Ductwork needed X Ductwork and pipe work needed

X A qualified and registered installer is needed – very costly X Because of this there is less risk of leaks

X Ductwork and pipe work needed

X Diffusers and grilles needed

Does not need diffusers

X Diffusers and grilles needed

X Diffusers and grilles needed

Free cooling

Can provide some cooling load in more moderate climates.

Significant free cooling from sources such as outdoor air or ground water heat sinks.

X No "free" cooling. X Adaptation required for some free cooling systems and ground-source heat pumps.

Cooling

Building load diversity allows cooling system to be downsized as much as 30 percent depending on building.

Cooling via pumped chilled water means a reduction in fan energy by a factor of seven.

Air systems can be dramatically reduced if chilled beams are used to handle the cooling.

X The cooling capacity available to an indoor section is reduced at lower temperatures.

X Must choose unit with capacity meeting or exceeding each zone's maximum.

X The cooling capacity available to an indoor section is reduced at lower temperatures.

Good for applications with intermittent medium to high sensible cooling loads and where close humidity control is not required.

Commissioning

They can be set in the factory.

Comfort control significantly reduces time and cost Controls are fairly simple.

Relatively easy to commission due to pre set control and packaged units.

X Commissioning on-site include control of the fan coil unit cooling connections and flush, vent and balance.

Flexibility

X Larger ductwork required.

Smaller ducts, saving space in false ceilings.

Easy retrofit to existing buildings.

Small ventilation plant and system.

X However, no fresh air.

Can break system into smaller sub-systems for phasing

Zoned control can easily be provided on the supply air to every level.

Can phase installation to serve portions of the building as funding or program requirements allow. Can move diffusers to adapt to room requirement

Individual zone control of space temperature, if suitable controls fitted. Combination of comfort control and FPT allows full flexibility

Maintenance and inspection

Units require little maintenance as few moving part inspections to prolong their life.

Chilled beams contain no moving parts, filters or fans. X May require in-room cleaning as visible to the eye.

X FGAS regulation annual inspection of system above 3kg X Above 30kg - 6 monthly X Above 300kg - 3 monthly

X They are difficult to maintain, requiring regular engineer visits.

No filter changes needed

No filter changes needed

X Requires filter changes often

X Requires filter changes often

Life expectancy

20-30 years 20-30 years X 10-15 years X 10-15 years

Efficiency

Variable volume means demand control ventilation

Efficient water distribution means lower energy consumption and operating costs.

X Fans/motors efficiency about 30-40 per cent. At least one per room rather than one per zone.

The use of EC fan motors can reduce electrical input by about 55% (against older systems).

X But still uses a lot of energy

Using pressure sensor, essential plant can be turned down to reduce energy

Self regulating output due to room occupancy loads

Variable speed compressors enhance part-load efficiency using inverter control

Modern systems feature multiple scroll compressors for highly efficient operation, in relation to the chiller.

Heat Recovery

X No heat recovery X No heat recovery Can recover heat without going back to condenser

X No heat recovery

Heating

X No inherent heating ability, water and electric reheat

Heating can be supplied through 4 pipe unit

Three-pipe heats and cools during the same period.

Heating can be supplied through 4 pipe unit

available.

Ventilation

System has demand control ventilation

Tempered ventilation through ducting in the beam itself

X Will require separate ventilation system

X Limited fresh air can also be introduced via the fan-coil unit.

Greater ability to condition only occupied spaces.

X May require air flow greater than comfort requirement Can respond to

staged occupancies, only conditioning rooms as they become occupied.

Air re-conditioned and fresh air introduced from a central air-handling unit.

Indoor Air Quality

Can provide various levels of temperature and humidity control.

Produces extremely stable environment

X A separate ventilation system is normally included. Provides only the required minimum quantity of outside air.

Provides good air distribution throughout space

Delivers enough air to meet respiratory requirements, with a consequent reduction in capital cost, electricity consumption and maintenance cost.

X A separate outside air fan and control system is generally required for larger buildings.

X Can create drafts.

Flexibility

Can mix and match various manufacturers on any given project.

Must use the same products throughout the project.

Good level of flexibility in relation to any reconfiguration of the occupied space.

© Fläkt Woods Group