Mechanical Ventilation  »  Air water/refrigerant systems

In radiant systems heating or cooling occurs through radiation. Conditioning space through the circulation of chilled/hot water is considered more efficient because water can carry 3,400 times the energy compared to air for the same volume. This property is chiefly exploited in radiant cooling systems.

Radiant cooling/heating is commonly used in commercial buildings in Europe but also in residential buildings (especially for heating). Cooling radiators temperature must stay above the local dew point to avoid condensation, which limits their application to low humid climate zones. Sometimes, radiators are used as supplementary heating or cooling systems (in perimeter of the building) along with a primary all air-cooling or heating system.

Wall mounted radiators
Radiators are room terminal units and are generally used for heating. Heating distribution through radiation is typically done through radiators mounted on the wall. Hot water from a central boiler is circulated trough insulated pipes to radiators. For more information, please refer to the section on heating.

Radiative floor heating/cooling
Under floor heating or cooling works on the principle of radiation. Hot water or chilled water is circulated through sophisticated layout of pipes (typically PEX tubes) buried within the thermal mass of the flooring (typically concrete slab flooring). The warm/cool floor then warms up the space through radiation. Special piping and flooring requirements make under floor heating an expensive option compared to radiators. 

Chilled beam/chilled slab
Chilled water or hot water is circulated through pipes located in a suspended false beam or slab through which space is conditioned using radiative heat transfer.

Energy savings
Energy savings of about 40% are possible by using DOAS+Radiant systems when compared to All-air VAV systems. A case study of a 6-storey office building for Infosys Software Development Block-1 has ben done by Sastry (2012), which has radiant cooling system using chilled slabs. The building is divided symmetrically into two parts. One half is served by a VAV system and the other half is served by a radiant system. The data has been monitored for the year 2011-2012 and shows that radiant cooling system saved about 33% energy compared to conventional VAV system for air conditioning purposes for similar load conditions in each halves. Tian and Love (2009) studied the application of radiant cooling in office buildings in different climates through simulation approach. A comparison has been made between a chilled slab systems compared to a conventional VAV systems (with an air side economizer). The results show that radiant system provides 10-40% better energy performance compared to VAV systems. Similar comparison studies have been done by Jeong, Mumma and Bahnfleth (2003) between VAV system and DOAS system integrated with ceiling radiant cooling panels as a parallel sensible cooling system. They concluded that DOAS/radiant panel cooling system consumes 42% less energy than that of the conventional VAV systems with an airside economizer.

Radiant systems are much more efficient compared to VAV systems in comparable applications. This is due to the following factors:

• Compared to all air systems, air water systems only need to circulate less quantum of air (only fresh air) due to which the fan size and fan energy use is considerably reduced.
• Since the temperature of water used in air water systems is higher (for cooling) and lower (for heating) compared to all air systems (a difference of about 4 – 8 °C), the size of cooling or heating equipment is decreased and their efficiency is increased.
• Efficient heat recovery system compounds the energy savings through pre-cooling/heating the fresh air.

Limitations and challenges
The key challenges in the design and maintenance of radiant cooling panels using chilled water is to avoid condensation on radiant panels, first (capital investment) costs, and operating conditions (including schedule of operation).

• Condensation could occur on radiant panels when the space temperature falls below its dew point temperature. This limits the usage of this system in highly humid conditions. Moreover, the DOAS system used for fresh air should be able to take care of the latent loads in the space to prevent condensation.

• Radiant panel cooling system is suitable for spaces with fairly constant pattern of loads. Since the cooling or heating is through energy stored in the thermal mass instant (constant) fluctuations in the load cannot be solely met by a radiant system and needs the support or an auxiliary cooling systems such as a VAV or FCU units.

• Radiant systems incur extra costs in the form of hydraulic systems used (PEX piping in the concrete and auxiliary pumping system). However, studies show that the extra capital costs are neutralized or in fact much lesser when compared to the savings incurred through reduction in fan capacity, duct sizing and cooling/heating plant capacity (Mumma, 2002).

Local Outdoor air units are small size ventilation units. Some systems can also double up as air conditioning units and can be connected to VRV outdoor units or chilled water or hot water coils. Unlike central DOAS systems, these local units are through the wall units are located one per room. They draw air directly from outdoors, filter the air and supply the filtered air to indoors.

To improve efficiency of local outdoor units, they should be fitted with heat recovery units and fans equipped with variable speed drives (inverters) to adjust the volume of fresh air intake. Advanced units also come with dedicated humidity control devices using desiccants or double stage cooling.

Local outdoor units use small fans and operate as per the zone’s demand. This potentially reduces the fan energy usage compared to a central DOAS system. These units are particularly suited for residential buildings, small office spaces, and meeting rooms with intermittent cooling and heating demand. In Europe, they are increasingly used for meeting Ultra-Low-Energy Building, nearly-Zero-Energy Building, and Plus-Energy Building standards.