RPA - Radiant Panel Association

What Is Hydronic Heating?

Hydronic heating systems use water to move heat from where it is produced to where it is needed. The water within the system is neither the source of the heat nor its destination; only its “conveyor belt.” Heat is absorbed by the water at a heat source, conveyed by the water through the distribution piping, and finally released into a heated space by a heat emitter.

Modern hydronics technology enables heat to be delivered precisely when and where it is needed.  Hundreds of system configurations are possible, each capable of meeting the exact comfort requirements of its owner. Some may be as simple as a tank-type water heater connected to a loop of flexible plastic tubing for warming a bathroom floor.  Others may use two or more boilers operated in stages, releasing their heat through an assortment of heat emitters.  The same boiler(s) may also provide the building’s domestic hot water.  They might even heat the swimming pool, or melt snow as it falls on the driveway. Well- designed and properly installed hydronic systems provide unsurpassed comfort and fuel efficiency for the life of the building.

THE BENEFITS OF HYDRONIC HEATING

Modern hydronic heating has a lot to offer. The following is a brief description of the key benefits it offers.

Unsurpassed Comfort

Providing comfort should be the primary objective of any heating system designer or installer. Unfortunately, this objective is too often compromised by other factors, the most common of which is cost. Even small residential heating systems effect the health, productivity, and general contentment of several people for many years. It only makes sense to plan and install them accordingly.

The average building owner doesn’t spend much time thinking about the consequences of the heating system they select. Many view such systems as a necessary but uninteresting part of a building. When construction budgets are tightened, it's often the heating system that’s compromised to save money for other, more impressive amenities.

Heating professionals should take the time to discuss comfort as well as price with their clients before decisions on system type are made. Often people who have lived with uncomfortable heating systems simply don't realize what they have been missing. In retrospect, many would welcome the opportunity to have truly comfortable buildings, and would willingly spend more money (if necessary), to achieve it.

Maintaining comfort is not a matter of supplying heat to the body.  Instead, it’s a matter of controlling how the body loses heat.  When interior conditions allows heat to leave a person’s body at the same rate it is generated, that person feels comfortable.  If heat is released faster or slower than the rate it’s produced, some degree of discomfort is felt.

The interior environment significantly effects the processes by which the body loses heat. For example, most people will not be comfortable in a room containing many cool surfaces such as large windows, even if the room's air temperature is 70 ºF.  For optimum comfort, the interior environment must provide the proper balance of air temperature, average surface temperature, and relative humidity to accommodate the various processes through which the body releases heat.

Properly designed hydronic systems control both the air temperature and surface temperatures of rooms to maintain optimal comfort.  Hydronic heat emitters such as radiant floors or ceilings raise the average surface temperature of rooms. Since the human body is especially responsive to radiant heat loss, these heat emitters significantly enhance comfort. Comfortable humidity levels are also easier to maintain in hydronically-heated buildings.

Several factors such as activity level, age, and general health determine what is a comfortable environment for a given individual. When several people are living or working in a common environment, any one of them might feel too hot, too cold, or just right. Heating systems that allow various “zones” of a building to be maintained at different temperatures can adapt to the comfort needs of several individuals. Although both forced-air and hydronic heating systems can be zoned, the latter is usually much simpler and easier to control.

Energy Savings

Ideally, a building's rate of heat loss would not be effected by how that heat is replaced. Experience, however, has shown that otherwise identical buildings can have significantly different rates of heat loss based on the types of heating systems installed. Buildings with hydronic heating systems have consistently shown lower heating energy use than equivalent structures with forced-air heating systems.

A number of factors contribute to this finding. One is that hydronic systems do not effect room air pressure while operating. Small changes in room air pressure occur when the blower of a forced-air heating system is operating. Increased room air pressure often results from the lack of an adequate return air path from the rooms back to the furnace. This condition drives heated air out through every small crack, hole, or other opening in the exterior surfaces of the room. A study that compared several hundred homes, some with central forced-air systems, others with baseboard convectors, found air leakage rates averaged 26% higher and energy usage averaged 40% greater in the homes with forced-air heating.

Another factor affecting building energy use is air temperature stratification; the tendency of warm air to rise toward the ceiling while cool air settles to the floor. In extreme situations the difference in air temperature from floor to ceiling can exceed 20 ºF. Stratification tends to be worsened by high ceilings, poor air circulation, and heating systems that supply air into rooms at high temperatures. Maintaining comfortable air temperatures in the occupied areas of rooms plagued with a high degree of temperature stratification leads to significantly higher air temperatures near the ceiling. The hot air increases heat loss through the ceiling.

Hydronic systems that transfer the majority of their heat by thermal radiation reduce air temperature stratification, and thus reduce heat loss through ceilings. Comfort can often be maintained at lower air temperatures when a space is radiantly heated. This leads to further energy savings. Zoned hydronic systems provide the potential for unoccupied rooms to be kept at lower temperatures, which also lowers heat loss and reduces fuel consumption.

Design Versatility

Hydronic heating offers almost unlimited possibilities to accommodate the comfort needs, usage, aesthetic tastes, and budget constraints of just about any building. A single system can supply space heating, domestic hot water, and specialty loads such as pool heating. Such systems reduce installation costs because redundant components such as multiple heat sources, exhaust systems, electrical hookups, and fuel supply components are eliminated.

The heating loads of some buildings are best served through use of different heat emitters.  For example, it’s common for hydronic radiant heating to be used on the first floor of a house while the second floor rooms are heated using panel radiators or fin-tube baseboard.  Commercial and industrial buildings often require heat emitters that are more resistant to physical damage in comparison to those used in residential buildings.  Modern hydronics technology makes it easy to combine different heat emitters into the same system.

Clean Operation

A very common complaint about forced-air heating is its ability to move dust and other airborne pollutants such as pollen and smoke throughout a building. In buildings where air-filtering equipment is either low quality or poorly maintained, dust streaks around ceiling and wall diffusers are often evident. Eventually duct systems require internal cleaning to remove dust and mold that has accumulated over several years of operation.

In contrast, few hydronic systems involve forced-air circulation. Those that do create room air circulation rather than building air circulation. This reduces the dispersal of airborne particles, which is a major benefit in situations where air cleanliness is imperative, such as for people with allergies, or in health care facilities.

Quiet Operation

A properly designed and installed hydronic system can operate with virtually undetectable sound levels in the occupied areas of a home. Modern systems that use constant circulation with variable water temperature minimize expansion noises that can occur when high temperature water is injected directly into a room temperature heat emitter.

Noninvasive Installation

It is often very difficult to conceal ducting out of sight within a typical house. The best that can be done in many cases is to encase the ducting in exposed soffits. Such situations often lead to compromises in duct sizing and / or placement.

By comparison, hydronic heating systems are easily integrated into the structure of most small buildings without compromising their structure or the aesthetic character of the space.

The underlying reason for this is the high heat capacity of water. A given volume of water can absorb almost 3500 times more heat as the same volume of air for the same temperature change. The volume of water that must be moved through a building to deliver a certain amount of heat is only about 0.03 percent that of air! This greatly reduces the size of the distribution “conduit”.

Here’s an example of what that means: A 3/4-inch diameter tube can carry the same amount of heat as a 14 inch by 8 inch duct.  The following figure depicts both options to scale.

Sawing into the floor joists to accommodate the 14 inch by 8 inch duct would destroy their structural ability.  The small tubing on the other hand is easily routed through the framing, especially if it happens to be one of several flexible tube products now available.

If these distribution system will be insulated, which is now a code requirement in many areas, considerably less material is required to insulate the tubing compared to the ducting. When insulated with the same material, the heat loss of the 14 inch by 8 inch duct is almost ten times greater than that of the 3/4 inch tube.

Hydronic systems using small flexible tubing are much easier to retrofit into existing buildings than is ducting. The tubing can be routed through closed framing spaces much like electrical cable.

For buildings where utility space is minimal, small wall-hung boilers can often be mounted in a closet. In many cases, these compact boilers supply the building’s domestic hot water as well as its heat. The entire system might occupy less than ten square feet of floor area.

As you can see, modern hydronic heating has many benefits to offer. Appropriate Designs can help you maximize these benefits by designing a system exactly right for the needs of your building.