Architects, designers and engineers are striving to meet heating, ventilation and air-conditioning (HVAC) loads with optimum comfort and minimal energy. As we have previously seen on this blog, radiant systems constitute a strong choice to heat and cool buildings in an economical and efficient fashion. While radiant systems provide heating and cooling through pipes, ventilation and any humidity control requirements may be met by a Dedicated Outdoor Air System (DOAS). Let us explore this combination in more detail.
Radiant Technology Overview
Radiant distribution systems utilise piping, distribution pumps and control valves to deliver heated or chilled water to the space; typically via two methods which may be singular or combined in a building application. These methods are:
1. Radiant Slabs
Radiant slabs have embedded tubing (typically flex) in large mass areas of the building; most commonly concrete floors but also via ceiling slabs. Valves regulate the amount of heated or chilled water delivered to the zone. Radiant slabs are usually configured as two-pipe systems – attention should be paid to the changeover method between the two modes of operation.
2. Radiant Panels
Radiant panels are ceiling or wall-mounted panels that affix water tubing to an insulated tile. Water is supplied to this tubing by piping located above the ceiling. Valves regulate the flow of water to the panels based on the temperature needs of the space. An advantage is that radiant panels can be used to supply both heating and cooling through the use of a four-pipe system.
The Dedicated Outdoor Air System
The DOAS gets its name from being ‘dedicated’ to bringing in outside air for ventilation and can be designed to meet 100% of the ventilation requirements for all spaces. There are various DOAS configurations and features available – a design choice which will largely depend on the latent and sensible loads of the application. Dehumidification, the use of pre-cooling (which may allow some downsizing of the radiant system), and heat recovery are all system options to be considered based on the climate and building characteristics.
Design and System Considerations
To correctly size the Radiant + DOAS system to meet thermal loads, the analysis must consider the interaction of the two systems to ensure opportunities for pre-cooling and energy recovery are seized and that control strategies will work together. The following lists enumerate the usual criteria taken into account for the optimisation of the radiant system and DOAS.
Radiant system optimisation
- Rules of thumb for heat exchange rate from radiant slab to space:
- 47-50 W/m2 in best conditions
- 25-38 W/m2 is typical
- 13-16 W/m2 worst-case scenario
- Factor in the expanded range of temperature set points to the space compared to a standard forced-air (VAV) system.
- All pumps included in the distribution system should be selected to include either variable-speed drives or electronically commutated motors (ECMs) and be controlled to remove the need for continuous pumping.
- Minimise pumping energy by reducing pressure drops across the distribution system. It is typically recommended to choose for larger piping, shorter distances and smooth transitions.
- When using both radiant heating and cooling in a slab, include a significant deadband (neutral zone) in the control sequence to avoid rapid changeover.
- Radiant systems are best for spaces with predictable schedules and load characteristics as radiant systems typically have a longer warm-up and cool-down period.
- Scheduling can use less energy than running the slab at lower loads in high cooling climates.
- Ensure the system is equipped with a highly rated filtration system.
- Utilise passive methods (natural ventilation) and indirect evaporative pre-cooling first.
- Identify candidate areas (e.g. conference rooms) for localised demand control ventilation to reduce ventilation air when not occupied.
- If energy recovery strategies are included, ensure there is no significant pressure drop from the intake to the exhaust side as this can result in cross-contamination of the air streams.
- In colder climates, the design of energy recovery strategies need to account for frost protection.
All in all, radiant heating and cooling systems provide gains in three main areas:
- By combining radiant space conditioning with a DOAS, we significantly reduce the amount of HVAC energy use, often by 50% when compared to a similar building using a VAV system.
- Thermal comfort is provided in a more even and steady manner by heating or cooling surfaces (slabs or panels). This combination controls two parameters: air and mean radiant temperatures.
- Decoupling space conditioning from space ventilation effectively eliminates a common occurrence in buildings with VAV systems – simultaneous heating and cooling.
By following the above recommendations, we hope your future radiant cooling system will work at an optimal level.