Why Hybrid Systems Make Sense
Designing an hvac system to heat and cool a space is quite different than designing a project for heating and cooling comfort. True human gratification cannot be accomplished without addressing radiant losses or gains, which means an hvac system designed for comfort should incorporate some form of radiant.
Heating and Cooling Sensible Primary LoadsRadiant systems enclosed in walls, floors, and ceilings (or surface-mounted wall or ceiling types, towel warmers, heated partitions, etc.) raise the mean radiant temperature of the room, affecting the operative comfort temperature.
In general terms, radiant floor heating systems can generate a nominal output of 2.0 Btu/sq f t x 1Â°F. A radiant floor cooling system can generate a nominal 1.2 Btu/sq ft x 1Â°F.
To satisfy most occupants, the maximum surface temperature for floor heating should be limited to a range between 80Â° and 85Â°, and for cooling between 66Â° and 70Â°. This places a floor heating output capacity in a nominal range of 30 Btu/sq ft and a sensible cooling capacity of 14 Btu/sq ft. Walls and ceiling have different limitations, which provide higher capacities.
This flexibility facilitates using greater surfaces or higher surface temperatures to increase the output. However, if the calculated loads exceed these surface temperatures (outputs), supplemental secondary heating and/or cooling is required.
Providing Supplemental Secondary LoadsSupplemental systems can include radiant. However, the need for ventilation, latent cooling, and dewpoint humidity control frequently provides an alternative method for dealing with secondary loads. These air-based systems include fancoils, fan convectors, and furnaces, which may incorporate heat recovery ventilators with preheat and reheat coils. In many cases the cfm required for ventilation is sufficient and suitable for supplemental heat.
VentilationIndoor air quality is a vital component of comfort-based systems and addresses filtration of air pollutants, removal of excess humidity, and introduction of fresh air. Ventilation can take the form of an exhaust air system balanced out with a makeup air system, or it may employ a heat recovery ventilator (HRV).
Larger projects usually use a combination of HRVs and fancoils (furnaces). The ventilation load is calculated on the energy to heat or cool incoming air to a temperature, which is similar to the exhausted air.
Climatic conditions must be considered for equipment selection, which will determine the need for preheat and/or reheat coils and materials of construction. Some heatpipe HRVs can be adjusted for winter and summer operation, which is beneficial in reducing cooling loads.
Air ConditioningThe need to control humidity and cooling requires a set of calculations based on the ventilation and latent load plus the supplemental sensible load from a radiant-cooled project. Cooling is best done using first-stage economizers and second-stage water or refrigerant-based condenser coils.
Humidification on air-based systems, whose primary function is makeup air, must employ some type of steam or vapor-induced system, as the air discharge temperatures are insufficient to evaporate water from drip or drum type humidifiers.
Dehumidification is done with cooling coils, desiccant wheels, or a combination of the two. Control-ling dewpoint in hybrid applications requires experience and skill not only in the load calculation, but also in the selection of equipment and control integration.
Air systems are great for ventilation, filtration, humidification and latent cooling.
The Hybrid ModeHybrid systems have sensible loads taken care of by an external source, which means the air strategy is a fraction of a typical air-only system. This translates to quieter, more energy-efficient systems.
Radiant systems are excellent providers of comfort heating and cooling. They reduce radiant losses and gains from the body with an invisible and silent source of energy. Room temperatures are consistent without noticeable stratification, keeping draft evaporation and cooling at a minimum. Zoning is easily accomplished, further allowing customized comfort in different regions of the project.
The hybrid model demonstrates energy efficiency, and creates a healthy, comfortable environment for the occupant.
Bean is an engineering technologist registered in Alberta, Canada. He works with Danfoss, Inc. as North American manager for Engineering Control Panels.
Publication date: 05/07/2001