Hydronic Heating

A Brief History of Hydronic Heating

Underfloor heating has a long history that dates back to the Neoglacial and Neolithic periods. Archeological digs in Asia and the Aleutian islands of Alaska reveal how inhabitants drafted smoke from fires through stone covered trenches that were excavated in the floors of their subterranean dwellings. The hot smoke heated the floor stones, which then radiated into the living spaces.

The ancient Romans used an underfloor heating system called hypocausts that heated houses with hot air. Hypocausts were used for heating hot baths, houses and other buildings, whether public or private. The floor was elevated above the ground by pillars with a layer of tiles and concrete on top. Spaces were left inside the walls so that hot air and gases from the furnace would pass through the lower floor and other enclosed areas and out of flues in the roof, thereby heating the interior of the room.

Dating back to 1000 BC, Korean houses traditionally used ondol to provide floor heating on principles similar to the hypocaust, drawing smoke from a wood fire typically used for cooking. Ondol heating was common in Korean homes until the 1960s.

In the 12th Century, engineers in Syria developed an improved central heating system that included underfloor pipes, which were connected to the furnace room. This was very popular in bath houses throughout the medieval Islamic regions.

The early 19th Century saw the start of the European evolution of the modern water heater/boiler and water based piping systems that include studies in thermal conductivities and specific heat of materials.

In 1905, Frank Lloyd Wright made his first trip to Japan where he was introduced to heated floors. He later incorporated various early forms of radiant heating in his projects.

Today, millions of American homes and buildings are equipped with baseboard units and/or radiators for space heating.


    Whether buying a new home or remodeling an existing home, utilizing hydronic heating provides the ultimate in heating comfort, efficiency, and cleanliness.

    Hydronic heating has several benefits:

    • Energy Efficient Savings. Because hydronic systems are sealed, there is virtually no loss of heat as it is distributed throughout the home.
    • Design Versatility. There are dozens of hydronic heating methods using floors, walls and ceilings as the radiating surface. Slim baseboard heaters and sleek wall-mount panels come in a variety of colors and styles. Or, one can choose underfloor heating with no visible pipes, heaters or ducts.
    • Healthy, Clean Heating. Hydronic heat is gentle and draft-free, so it does not circulate dust, odors, spores, or germs around the house.
    • Quiet Operation. Hydronic heating is quiet because there is no noise from air movement.
    • Easy Zone Control. Another advantage is the option of a thermostat in every room. Individual zone controls or circulators and thermostats automatically warm up the daytime living areas or let them cool at night, maximizing comfort and providing energy savings.
    • Low Maintenance. A hydronic system has few moving parts so maintenance is minimized.
    • “Expandable” Heating. A hydronic system can be expanded to include reliable, efficient space heating and domestic hot water, heating for swimming pools and spas, ice and snow melting for driveways and sidewalks and even towel rack warming. Air conditioning can be added in a separate system for year-round comfort. A hydronic system can be the heat source for a forced air system.
    • Easy Integration. Homeowners can convert or supplement their existing warm air or heat pump systems with hydronic systems.
    • Operates with All Heating Fuels. No matter which fuel used to heat your home — natural gas, propane fuel oil, electricity, or even solar energy — hydronic heating offers efficient and convenient comfort.

    Hydronic Heating Applications

    A hydronic heating system offers numerous applications for whole-house heating and can satisfy many heating needs. Hydronic heating applications both inside and outside the home include:

    • Radiant Heating — In this fast-growing use of hydronic heating, hot water circulates through extremely durable tubing installed in or under floors, walls or ceilings. This heating method warms people and furniture, not the air space in the room. Plus, radiant panels can be used in combination with other forms of heat distribution, including baseboard convectors and forced air.
    • Baseboard and Radiator Heating — Millions of American homes use baseboard units or radiators to provide heat. A central boiler circulates hot water to baseboard heating units throughout the house, delivering gentle, comforting heat.
    • Instant and Plentiful Hot Water — Utilizing an indirect fired water heater in conjunction with an existing boiler provides hot water for bathing, cooking, laundry, and dishwashing.
    • Snow and Ice Melting — Durable tube embedded in sidewalks and driveways distributes warm water that melts ice and snow. These snow melt systems eliminate plowing, backbreaking shoveling and icy spills, and prevent potential damage to the concrete caused by snow-removal equipment and corrosive de-icing methods.
    • Spa and Swimming Pool Heating — Indoor or outdoor swimming pools and hot tub spas can be heated with the same boiler by using a heat exchanger. Adding this feature allows for exact temperature control and can extend the swimming and whirlpool season by months.
    • Towel Warmer & Wall Panels — In this application, decorative and efficient panel radiators are mounted flush to the wall and connected to the boiler. This allows the warm boiler water to circulate quietly through the panels, heating the surrounding area, as well as providing warmth to towels, mittens, and wet outerwear for quick drying.
    • Multiple Room Thermostats — Numerous heating zones put the right amount of heat where it is needed to maximize comfort and energy use.
    • Hydronics and Air Conditioning — It is quick and easy to install a separate air conditioning system in a hydronically heated home to assure comfortable heating and cooling. Options include a complete whole-house cooling solution through mini-ducts or a ductless system for localized spot cooling.

    For consumers looking to install a hydronic heating system, to-do’s include selecting the proper boiler, considering costs, choosing the heating method, and selecting the installer.

    Boiler Selection — Efficiency

    The first step is to review available boilers. Gas-fired boilers are rated according to annual fuel utilization efficiency (AFUE) percentage, which is a measure of how efficient the boiler is in converting the energy in its fuel to heat over the course of a typical year. These minimum rating requirements vary based on the type of fuel used and the heating medium. The minimum AFUE rating for a gas-fired hot water boiler is 82%; the minimum AFUE rating for a gas-fired steam boiler is 80%; the minimum AFUE rating for an oil-fired hot water boiler is 84%; and the minimum AFUE rating for an oil-fired steam boiler is 82%.

    High efficiency condensing boilers, which sometimes feature a secondary heat exchanger to extract more heat from the condensation process, can offer AFUE ratings over 90%. Energy savings from a high efficiency boiler will vary based on other factors such as the overall efficiency of the home, the cost of natural gas, and local weather conditions.

    For those residing in a cooler climate, it usually makes sense to invest in a high-efficiency system. Look for the ENERGY STAR label when shopping for high-efficiency boilers.

    Boiler Selection — Sizing

    A properly sized boiler will operate more efficiently. Select a boiler that heats enough water to supply the system with consistent heat, and does so with as little heat and energy loss as possible. A highly trained HVAC contractor will determine sizing to match the output of the boiler’s BTU rating to the total heat loss of the home.

    A boiler that’s sized too small will not provide adequate heat for a home. It will also run longer and waste energy. A system that’s too large will cost more and have a shorter lifetime due to excessive cycling.

    Sizing specifications are typically available on hydronic equipment manufacturers’ websites.

    Boiler Selection — Review Warranties

    Be sure to choose a dependable boiler and compare the warranties of each unit and system you’re considering.

    Cost Considerations

    The initial cost of hydronic heating is typically more expensive than a traditional heating system. However, the savings from reduced energy consumption will make radiant heat less expensive in the long run.

    With an array of boilers, hydronic heat emitters, heat sources, and control options available, the cost of an installation will vary. Because every home and building is different, one size does not fit all when it comes to the installation of a hydronic heating system.

    Costs will differ for new construction installations versus additions or an extensive remodel. The most economical time to include radiant heating is when the house is being built.

    After a review of building blueprints and discussions, the contractor will develop a scope of work, time frame, and cost estimate for the installation.

    Choosing a Heating Method

    Underfloor heating is typically installed by either a dry or wet installation method. During wet installations radiant tubing is placed into a concrete. This method is effective as the concrete protects the tubing while acting as a radiator to warm a room evenly. The dry method consists of prebuilt panels with tracks for the plastic tubing that are then covered by the flooring material.

    Baseboards and radiators are easier to install because less tubing and piping are necessary. These heat emitters are placed within the walls or through floor joists to connect with their units. Be sure to avoid furniture placement locations when installing these units.

    Selecting An Installer

    One of the more difficult decisions is finding and choosing a qualified hydronic heating installer. Choose an experienced, well-trained installer. AHRI recommends using a NATE-certified technician.

    After selecting the boiler and heating method, an installer will help with the following elements:

    • Conduct home energy audit to determine heat loss
    • Choose control system hardware
    • Provide tubing layout drawings for radiant panel heating systems
    • Design piping routing, schematics, and control systems
    • Conduct on-site inspections to confirm design
    • Install system with support from the manufacturer

    As outlined above, the installer should factor in a room-by-room heat loss analysis of the home or building as well as a step-by-step sizing process into any design. Most companies will conduct this work with computer programs and then share the results with the homeowner.

    By conducting some research, asking questions, and selecting a reputable installer, you will end up with a level of heating comfort you may not have thought possible.

    Consumers should visit the Hydronics Institute resources section of the AHRI website for more information on hydronic heating suppliers and manufacturers.

    Q. What is hydronic heating?
    A. Hydronic heating systems use a boiler to heat water that is used as the heat transfer medium. The heated water is then circulated to heat distributors located throughout the home. These heat distributors can be radiators, finned tube baseboard, radiant tubing, towel warmers, and kick space heaters. The boiler also can be used to indirectly heat domestic water, pools, and whirlpool baths, and can be used for ice and snow melting on sidewalks and driveways.

    Q. Is hydronic heating appropriate for any residential home?
    A. Yes, hydronic heating is appropriate for any residential home or building type. For larger homes, two smaller boilers may be used in sequence, with the second boiler only operating when it’s needed.

    Q. How does radiant heating work?
    A. Radiant heating circulates warm water through tubing that is installed in or underneath floors, as well as in walls and ceilings. This form of hydronic heating warms people and objects and not air. Heat flows through the room gently and completely without breezes, hot spots, or cold spots.

    Q. Can I add hydronic heating during a remodel or only during new construction?
    A. The most economical time to install hydronic heating, especially radiant heating, is when the house is being built. However, hydronic heating also can be installed during a remodeling or retrofit project. Many builders today install radiant tubing in the floors, sidewalks, and driveways of homes they are building, even if the home buyer does not ask that the radiant heating be hooked up. This makes it a simple and cost-effective matter to switch to the radiant heating system whenever the homeowner wishes. For home remodels, hydronic baseboard units and radiators are easier to install because they require less tubing below the floor. They are often placed along the walls of a room in areas that make them less conspicuous.

    Q. How can I use hydronic heating to warm tile floors in the bathroom and kitchen during winter?
    A. Radiant heating warms floors in the bathroom, kitchen and throughout a home. This form of hydronic heating sends warm water through extremely durable tubing that is installed in or underneath floors, as well as in walls and ceilings.

    Q. Can hydronic heating be used outside the house?
    A. Yes. Hydronic heating can be used to heat swimming pools and whirlpool spas by use of a boiler or pool heater and a heat exchanger. In addition, durable tubing also can be installed in sidewalks and driveways to melt snow and ice.

    Q. Can I have air conditioning (A/C) in a hydronically-heated home?
    A. Air conditioning can easily be added in a separate system for year-round comfort. There are several AC systems that work well with hydronic heating:

    High Velocity Mini Duct System: This system transfers air to small (2" inner diameter), round outlets in each room of the home through flexible tubing that weaves in between spaces in the walls, ceilings or floors. Installation is easy, economical and nearly unobtrusive.

    Split-System Central Air Conditioner: In this system, a condensing unit is installed outside the home and a matching indoor air handler unit or furnace with coil is inside the home. Ductwork transfers the cooled air throughout the home.

    Separate Standard Air Conditioning System: Many homeowners choose to install a separate A/C system along with their hydronic heating system. In a typical installation, a rooftop A/C unit sends cool air to the home through small flexible tubing (similar to a High-Velocity A/C System), and is distributed by small vents in the ceilings of each room of the home. A mini-split A/C system can also be used.

    Q. How do high efficiency condensing boilers work?
    A. Condensing boilers achieve high efficiency by capturing and using heat from the combustion process that would otherwise be wasted. The boiler’s heat exchanger uses the exhaust gases from the combustion process to preheat water as it enters the boiler. In addition, the water vapor produced in the combustion process condenses back into water and also releases heat that is then re-used. Together, these two processes create the high efficiencies associated with condensing boilers. Just as important as the efficiency of the condensing boiler is the efficiency and proper installation of the overall heating system to which the boiler is a component. To achieve maximum efficiency, a total heating system approach is required.

    Q. Can hydronic heating be used if my home has another type of heating system?
    A. Yes. There are hydronic heating solutions for converting or supplementing existing warm air or heat pump systems.



    Hydronic heating systems, also referred to as radiant heating, circulate heated water throughout a space to provide comfort heating to an entire home. In this type of system, a boiler heats water and circulates it through pipes or tubing to a heat distributor such as radiators, finned-tube baseboard, radiant tubing, towel warmers, and kick space heaters. In addition, the boiler can be used to indirectly heat domestic water and whirlpool baths, or to melt ice and snow on sidewalks and driveways.

    Recent advances in cooling technology also allow for the successful operation of air conditioning with a hydronic heating installation. Hydronic heating is highly energy efficient because it is delivered through completely sealed systems with a minimal loss of heat.

    Hot water from a boiler or water heater is the medium for heat transfer in a hydronic system. Radiators and finned tube baseboard are the most common source of heat transfer, but radiant loops are gaining popularity.

    Hydronic Heating — How It Works

    Modern hydronic heating is an energy efficient home heating system that enables heat to be delivered precisely when and where it is needed.

    Hydronic heating is ideal for any residential home or building. The system has become popular with homeowners who seek a quiet, clean, efficient, and comfortable space.

    To understand hydronic heating, it is important to first review how radiant heating works using these heat transfer processes:

    1. Conduction is the transmission of heat through a medium without perceptible motion of the medium itself. Put your hand on the hood of your car after it has been out in the sun for a while. Your hand gets hot from direct contact with the hot metal. Density plays a major role in an object’s conductivity.
    2. Radiation is the emission or propagation of energy in the form of invisible rays, waves, or particles that are absorbed by a person or object. On a clear day in mid-winter, step out from the shadow of a building into the bright sunlight. Feels good, doesn’t it? You are being warmed by solar radiation. Radiation is a very effective means of transferring heat as it can’t be disturbed by wind or moved.

    Hydronic heating systems are effective because the boiler uses conduction to efficiently and quickly heat the water. Hot liquid is circulated throughout piping in the home via conduction and heats concrete, tile, and other types of flooring. The heat from the liquid radiates warmth into the room at a comfortable and consistent temperature. Thus, a radiator is a good example of conduction.

    Elements of Hydronic Heating Systems

    It’s important to review the various types of hydronic systems to determine the ideal unit for an application. Hydronic designs can be customized to meet most residential needs, whether it is a new home or a remodel. The flexibility of this type of advanced home heating system, as well as the high levels of energy efficiency, have made hydronic heat an increasingly popular choice for both contractors and homeowners.

    The following basic elements are required for hydronic heating in a home:

    • Boiler — The device used to heat the liquid.
    • Liquid — The common form used is water although some also utilize a mixture of water and antifreeze.
    • Tubing — PEX or another type of small, flexible plastic tubing is commonly used to circulate water.
    • Manifold/Thermostat — The manifold directs water flow and room temperatures and is connected to the thermostat.
    • Heat Emitter — The radiator, finned-tube baseboard, radiant tubing, towel warmer, kick-space heater or other heat distribution element.
    • Pump — Used to keep the heated liquid moving throughout the tubing so that the warmth is consistent.

    Hydronic Heating Options

    A variety of hydronic heating options exist to heat water. These include a natural gas or propane water heater or boiler, electric boiler, wood boiler, heat pump, solar water heater, or a geothermal heat pump.

    Condensing boilers achieve very high efficiencies by capturing and using heat from the combustion process that would otherwise be wasted. The boiler’s heat exchanger uses the exhaust gases from the combustion process to preheat water as it enters the boiler. In addition, the water vapor produced in the combustion process condenses back into the water and releases heat that is then re-used. Together, these two processes create the high efficiencies associated with condensing boilers.

    Several options can be used in conjunction with a boiler to improve output and efficiency. These include a tankless hydronic water heater, combined hot water and heating systems boiler, solar water heater, and a geothermal heat pump.

    • Tankless water heaters are designed to provide on demand, energy efficient hot water for domestic uses, such as showers, faucets, and dishwashing.
    • The combined hot water and heating systems boiler — often called a combi-boiler — is growing in popularity for installations in homes and commercial buildings. These units are relatively new to the industry and can be designed for use with water tanks and tankless water heating. The unit is a high-efficiency water heater and a central heating boiler, combined within one compact unit. Therefore, no separate hot water tank is required, offering space savings within the property.
    • A solar water heater will not provide enough heated water by itself, but it can increase the energy efficiency of the system and work in tandem with a boiler.
    • Geothermal heat pumps use the natural warmth of the ground several feet below the soil to preheat water. This method can reduce the amount of natural gas, electricity, or oil that a boiler uses to save energy.

    Hydronic Heating Emitters/Applications

    Heat emitters include underfloor radiant loops (sometimes also in walls and ceilings), baseboard heaters, and radiators.

    The most common system is often called "hot water base board heat" because most of the hydronic heating systems installed in the United States use hot water running through base board heating units to heat the living spaces of the home.

    Other hydronic systems use cast iron radiators or tubing in the floors, ceilings, or walls to circulate heated water and radiate the heat to the living space.

    Radiant flooring is a popular option for new home construction as it can be easily added into the concrete floor or joist system during construction. Tubing for a hydronic system may be installed in a conventional concrete slab or in a lightweight, gypsum-cement slab. Or it can be stapled to the undersides of subflooring.

    For home remodels, hydronic baseboard units and radiators are easier to install because they require less tubing below the floor. They are often placed along the walls of a room in areas that make them less conspicuous.

    Radiant panels for walls and ceilings can heat a broad area. These panels are most often used with electric radiant heat versus hydronic due to the possibility of water damage.

    There also are several outdoor radiant heating systems that can be a helpful addition to one’s home. Options include the ability to melt snow on sidewalks and driveways in areas that receive significant snow fall or ice. Radiant roof heating can help protect a home’s roof from future leaks due to snow and ice buildup.

    Radiant heating loops can be placed beneath pool walls to allow for an energy efficient way to keep the pool warm and lengthen the swimming season.

    A variety of applications and system configurations are capable of meeting the exact heating requirements of the owner. A simple design might be a tank-type water heater connected to a loop of flexible plastic tubing for warming a bathroom floor. A more complex application would include the use of two or more boilers operated in stages that distribute heat through an assortment of heat emitters.

    A well designed and properly installed hydronic system will provide comfort and fuel efficiency for the life of the building.

    hydronic heating


    One of the most significant decisions for consumers regarding a choice of heating system is its efficiency. The efficiency is measured by an AFUE (annual fuel utilization efficiency) rating, which describes the percentage of heat generated that ultimately goes to heating the house. For example, an AFUE of 85% means that 85% of the energy in the fuel becomes heat for the home while the other 15% is wasted.

    Hydronic heating is an energy efficient method that is eco-friendly for the planet and provides cost savings for the homeowner. It provides large amounts of heat transfer with low costs for transporting energy. High-efficiency boilers, for example, can have AFUE ratings greater than 90%. Installing such a system will reduce energy usage and may result in lower energy bills. Homeowners with hydronic systems often report energy cost savings of 30% and more on their heating bills.

    There may be tax incentives or local utility rebates that homeowners can receive by upgrading to a more energy efficient heating design. Consumers should research government programs that can provide savings to reduce the initial installation cost of a radiant heat system.

    Hydronic heating is an energy efficient method that is eco-friendly for the planet and provides cost savings for the homeowner.