HVAC Systems, Radiant Cooling

A Hybrid Radiant HVAC System Design

This post outlines a design of radiant cooling coupled with a hybrid HVAC system. Such a design is almost always needed because radiant cooling alone cannot provide ventilation and address the latent loads (although in some dry climates, it may be sufficient to only address ventilation loads). Some common design options in parallel with a radiant system are reviewed below with a focus on the systems used for ventilation, humidity control, and supplemental cooling capacity.

Because this post concentrates on radiant cooling, space heating systems are not covered, although they are usually needed in most buildings.

Dedicated outdoor air systems

radiant cooling optimization using desicated outdoor air system

A DOAS is often used with a radiant cooling system to provide filtration, heating or cooling, and dehumidification of outdoor air for ventilation. In comparison with conventional air systems, which condition the mixed outdoor and return air to meet both ventilation and thermal comfort, the DOAS has the advantage of more accurate delivery of ventilation to spaces and improved humidity control. Vapor compression and desiccant cooling are technologies used for the purpose of dehumidification. Except in mild climates, energy recovery is usually used in DOAS to precondition the outdoor air by exchanging heat with the exhaust air.


Displacement ventilation


DV provides a low-velocity air stream directly to the occupied zone via diffusers in the wall at floor level or through a raised floor. As the cooler air moves upward, stale and warm air is exhausted from the space at the ceiling level. Because of comfort constraints, the temperature difference between the DV supply air and room air is relatively small (less than 3°C). This low-temperature difference and the low air flow rate limit DV cooling capacity.


Conventional variable-air-volume (VAV) systems


Depending on the space condition, the VAV system provides ventilation, space heating, and additional cooling to multiple zones. Such design, however, may complicate the system controls to avoid simultaneous cooling by the radiant slab system and heating by the VAV system.


Fan coil units


FCUs provide cooling supplemental to the radiant system. Because fan coil units do not intend to address any latent load, they are called dry FCUs. It is also possible that FCUs are installed to remove humidity of the ventilation air.

Now that we have described our design options, let us turn to the major advantages of radiant cooling systems.


More energy efficient


The radiant cooling system’s energy efficiency comes from the following sources: 1) the radiant system can reduce fan energy significantly because hydronic system can transport a given amount of cooling with less than 5% of the energy required to deliver cool air to fans; 2) the high supply water temperature is beneficial to the operation of vapor compression equipment and expands the potential of using low-grade energy sources for cooling such as evaporative cooling and geothermal energy; 3) the shift of cooling loads to nighttime would further lower the lift of vapor compression equipment and increase the opportunity of free cooling with water-side economizers and night ventilation.


Improved thermal comfort


The radiant cooling system features a uniform surface temperature close to the air temperature in the conditioned space, reducing the draft impact on occupants.


Improved indoor air quality


By using a separate air system for ventilation, the radiant system usually eliminates the use of recirculation air that can potentially cause cross-zone contamination. In addition, a dedicated outdoor air system is more accurate and reliable than a mixed air system in delivering the required ventilation to multiple spaces.


Reduced capital investment


Capital investment of radiant systems may be reduced because:

1. The reduced duct size means less space used for duct shafts and a lower plenum height requirement to accommodate ducts; hence, building material cost is reduced

2. The HVAC equipment sizes and their initial cost may be reduced because of the shift of cooling loads.

Studies were made to quantify the energy and cost savings of radiant systems relative to conventional all-air systems, mostly with simulation programs. Energy savings seem to improve as more systems are installed and the technologies are better understood.



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