Hydronic heating and cooling systems utilize fluid, usually water and sometimes antifreeze, as a heat-transfer medium which is distributed through plastic piping systems throughout a building.

Hydronic heating and cooling is a 100-year-old technology that is constantly evolving. Although some hydronic systems use fan coils, chilled beams, convectors, or radiators to transfer thermal energy to or from a space, the most efficient and comfortable hydronic systems use radiant surfaces, sometimes called radiant panels to heat and cool spaces.

Introduction

In a radiant heating system, heated fluid is distributed from a warm-water source (e.g. geothermal system, boiler) through plastic tubing that is embedded in the floors, walls, or ceilings of a space. The tubing conducts heat to the exposed surface, which acts like a large, gentle radiator.

When heated surfaces emit more than 50% of their heat energy through infrared emissions, they are known as radiant heating systems. In a heated floor, the warm surface radiates heat to everything in line-of-sight of the floor, warming the objects and people in the room, while also allowing warm air to gently rise from the floor. The resulting comfort is unmatched with other forms of heat delivery.

Typically designed in conjunction with radiant heating, radiant cooling systems can circulate chilled fluid through the same network of embedded plastic tubing. This network of tubes can turn floors, walls, and ceilings into cooled surfaces that evenly absorb sensible* heat energy, including radiant energy from solar gain, people, lights, computers, etc., in addition to some convective heat transfer from the air.

*Heat that causes a change in temperature in an object is called Sensible Heat.

When dehumidification and fresh air are needed, radiant cooling usually handles part of the cooling load while a dedicated outdoor air system (DOAS) meets fresh air and dehumidification requirements, addressing latent* heat loads. Other dehumidification technology, such as desiccant heat wheels, may also be used. Since the radiant cooling system can address a large portion of the cooling load (depending on the climate, etc.), the demand on the air-side system and the overall energy consumption can be significantly reduced. The result is a more comfortable indoor space with an efficiently-sized hybrid radiant/air-based cooling system.

*Heat that causes a change of state with no change in temperature is called Latent Heat.

Delivering Better Thermal Comfort - Radiant Heating

Since the function of indoor space conditioning systems is to control the heat loss of people in the space, the most effective way to do this is to eliminate cold indoor surfaces through hydronic radiant systems. Air-based heating systems surround people with hot, dry air to offset the heat loss from our bodies, but are never truly comfortable.

Radiant systems are safe, gentle, quiet, steady, and invisible, and provide unmatched comfort through the use of warm surfaces.

Delivering Better Thermal Comfort - Radiant Cooling

When the outside air temperature is hot and solar gain heats buildings through windows, the most comfortable method to address comfort is to surround occupants with cooler surfaces. When radiant cooling is used to condition a space, less cold air is required. This reduces the need for high air volumes, and the resulting noise and drafts, and keeps occupants comfortable. A certain volume of cool, dry, fresh is usually supplied in conjunction with radiant cooling.

Control

In a radiant heating or cooling system, each circuit of tubing is usually connected to a distribution manifold. Manifolds often include flow control valves for each circuit, making room-by-room zoning easy to accomplish with no additional piping or valves. Room-by-room zoning is the most comfortable and efficient control strategy.

Delivering Better Efficiency

Hydronic systems are more efficient than air-based systems due to the high specific heat of water as compared to air, and the fact that hydronic circulators (pumps) utilize a fraction of the electrical energy required by fans.

Hydronic systems are more efficient than refrigerant-based systems due to the high specific heat of water and the fact that hydronic circulators utilize a fraction of the electrical energy required by compressors. Water is the natural refrigerant, with no environmental concerns related to leakage, CFCs, global warming potential (GWP), or flammability.

Modern hydronic circulators typically use variable-speed motors (a.k.a. electronically commutating motors [ECM]) to deliver only the amount of heat energy that is needed at any moment in time. This is one of the reasons why hydronic systems are so efficient, since these small circulators often draw 75-90% less energy to transfer heat as compared with fans.

The warm water may be produced by a variety of heat sources such as high-efficiency boilers, ground-source geothermal heat pumps, air-to-water heat pumps, and thermal solar collection systems. The warm fluid may also come from waste heat of other cooling operations and industrial processes. Many radiant heating systems meet design loads with fluid temperatures of 110ºF (43ºC) or less.

Some radiant heating systems also operate as cooling systems, circulating chilled water through floors, walls or ceilings in the cooling season to absorb heat energy from spaces. Radiant cooling systems improve comfort and efficiency, as the cooling load on the traditional air-based system can be significantly reduced, lowering air movement, noise and drafts. The reduction in size of the air handling equipment is often enough to offset the cost of the heating/cooling pipes.

Radiant cooling systems are usually installed in commercial spaces where the humidity can be controlled by computerized control systems to manage air dehumidification and prevent condensation, or in arid climates where the temperature of cooled surfaces is always above the natural dew point of the ambient air.

In addition, other applications for embedded radiant pipes include warming floor surfaces to dry items on the floor, such as at the Big Bear Area Regional Wastewater Agency, and cooling concrete structures to help them cure quicker, such as in the Wilshire Grand Center.

Advantages

Heating and cooling an occupied space with hydronic radiant surfaces is efficient, silent, invisible, clean, controllable, reliable, maintenance-free and extremely comfortable. This means that radiant systems can improve comfort while reducing energy consumption for heating and cooling, while radiant systems also have reduced operational costs and maintenance costs.

Thanks to its high specific heat as compared with air, water is a much better conductor of heat energy.

• A nominal 3/4 water pipe can transfer the same heat energy as a 14 x 8 inch duct
• A hydronic circulator moving warm fluid can typically deliver the same thermal energy as an air fan with 75-90% less electrical consumption

Because radiant heating systems can heat a space with very low water temperature, the efficiency of the heat source is typically improved. This means that systems with fossil-fuel devices deliver better efficiencies by operating in condensing mode, and air-to-water or water-to-water heat pumps deliver higher coefficients of performance (COPs). In many cases, heat pumps can be smaller and draw lower electrical loads than if distributing heat via an air-based system.

Radiant heating allows a thermostat to be set as much as 4ºF (2ºC) lower than it would be with traditional hot air systems without sacrificing comfort. These lower operating temperatures coupled with the superior heat transfer properties of water can reduce fuel consumption, potentially saving up to 30% per year in heating expenses

Since radiant pipes are embedded in floors, walls or ceilings, there is more available space on floors and less space lost to ductwork.

Many designers use radiant heating systems to improve energy efficiency and comfort, and to achieve green and sustainable certifications and recognition, such as LEED®.

Radiant heating and cooling systems are easily integrated into the mechanical environment of a building and can be combined with geothermal heat sources and traditional HVAC distribution methods for high-performance hybrid systems. Combined radiant heating and cooling systems provide uniform and efficient heating and cooling, and are a cost-effective way for a building to achieve a higher level of energy performance.