Walking on [Hot] Water With Radiant Hydronic Heating

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by Planning Technician Carol Borck

What is it and how does it work?

The cold winter months are upon us, and my old, gas-fired, forced-air furnace is again chugging away, heating my house as inefficiently as ever while spewing out ucky blue dust into every room. I cringe with each arriving utility bill, and I sneeze just about the first thing each morning. Conventional forced-air furnaces are not very efficient systems. Heat is lost as the air travels through pipes and ducts, into unoccupied rooms, and as it floats up to the ceiling above you. However, if you are remodeling or building new, there is an alternative that can provide you with comfortable, efficient heating and quiet, clean operation, while also allowing for programmable heating zones.

Radiant Hydronic Heating (RHH) systems heat your house by circulating hot water through piping embedded in the floor. Typically, a boiler or hot water heater is used to heat the water, and pumps transport it through plastic tubing that is installed in a concrete slab floor, attached under the subfloor, or on pre-fabricated boards attached to the subfloor. The system is closed-loop, i.e., the same water is heated and re-heated as it circulates. Warmth from the hot water is conducted upwards to the floor surface where it heats objects in a room (radiant heating - very efficient) rather than the air itself (convective heating - not as efficient). To better visualize how effective radiant heating is, imagine standing outside on a sunny, but chilly morning. Stepping out onto your shaded patio, you feel cold, but when you walk out into the sunlight and soak up some rays, you warm up even though the air around you is still as cold as it was when under your patio. This is radiant heating.

RHH, more popular in Europe, was introduced to the U.S. in the 1940's although it has been used for centuries (Roman bathhouses were heated in this manner). These systems are becoming more common as the desire for healthier homes and energy efficiency continues to increase.

Why are RHH systems better?

Efficiency: RHH systems are more efficient in heating your home in several ways. First, the small size of the hydronic tubing, as compared to traditional forced-air ductwork, greatly reduces the quantity of heat lost as it is sent to various parts of your home. The amount of electrical energy used to circulate the heated water is also less than that required to move air through ducts.

Additionally, with forced-air systems, because air is heated directly to high temperatures, it rises upward to the ceiling creating temperature stratification. So, while you sit on your couch trying to thaw out, your ceiling is warm, cozy, and losing heat to the attic. RHH systems heat your room directly and evenly from the floor up. Because it warms your body and not the air, you feel comfortable at a lower air temperature and can keep the thermostat lower.

Zoned hydronic systems (using separate temperature controls for chosen areas) allows you to direct heat where you need it and keep unoccupied rooms at lower temperatures. This reduces both heat loss and energy consumption.

Clean & Quiet Operation: Without the need for ducts and fans used in forced-air heating, hydronic heating systems eliminate dust and other air-borne pollutants from being distributed throughout your house, providing you with healthier indoor air quality.

Boilers and circulation pumps are usually placed in the basement or separate mechanical room allowing for virtually undetectable operation in the occupied areas of your home.

Superior Comfort: Through heating objects in a room, radiant floor systems offer comfort that cannot be matched by a forced-air system. Heat is concentrated near the floor, furniture, and you. Even when a door is opened and you feel a draft, radiant heat recovers quickly; you don't have to worry about hot air escaping.

Installation & Costs: Radiant heating is easily installed during construction of a new house or addition. Existing floors can be retrofitted if the subfloor is exposed (and if an existing basement is unfinished). RHH can be added to as many rooms as you'd like, whether you choose to install it in a bathroom, bedrooms, or the entire house.

  Wet installation
  Dry installation
There are two types of RHH installations, called "wet installations" or "dry installations." Wet installations involve embedding the tubing within the concrete foundation slab or in a lightweight layer of concrete or gypsum that is installed on top of the subfloor.

In dry installations, the tubing is attached under the subfloor or fit between the subfloor and the floor covering. Additionally, new products have recently been developed that consist of manufactured, non-structural boards covered with aluminum that incorporate grooves for the hydronic tubing into the individual pieces. The boards are applied to the subfloor and may spread heat more evenly and quickly.

The most common and effective floor covering used with RHH systems is ceramic tile because it is a good heat conductor. Other floor coverings such as linoleum, wood, or carpet can be used; however, these materials act as insulators between the floor and the room and will therefore decrease the efficiency of the heating system.

An RHH system is more expensive to install, and depending on your specific needs, can be about double the price of conventional forced-air. However, when comparing the two options, it is important to distinguish between the initial price of installation and the overall lifecycle costs (installation price combined with the operational costs over the product's lifetime). While you will be paying more upfront for the hydronic system, the long-term operational costs will likely be considerably less when you factor in the greater energy efficiency and durability. In general, the price to install RHH by a licensed contractor can range from about $4 to $9 per square foot. However, RHH can reduce the cost of heating 20% to 40% over using forced-air.

Maintenance & Lifespan: Hydronic systems have advanced significantly since the middle of the last century when metal pipes were used and controlling temperatures was iffy. Today's systems are of superior design and extremely durable.

PEX tubingOrdinarily, maintenance will focus on the boiler and pump. Pumps are practically maintenance free and should last 10 years or more. Hot water boilers outlive their hot air counterparts. The boiler will come with a warranty and likely a maintenance package for cleaning and "tune-ups." Most installers will recommend annual maintenance to keep your system working properly with maximum efficiency.

The tubing frequently used to carry the hot water through the floor is cross-linked polyethylene (PEX). This material is flexible, non-toxic, and leak-resistant. PEX is highly durable; it doesn't become brittle and can withstand concrete encasement and variable water conditions over time. PEX tubing set in concrete can be expected to last at least 50 years and up to 100 years.


  • With RHH, it is just heating, not air conditioning; it does not cool or clean the air
  • Response time is slower - it takes longer to build up enough heat to increase room temperature
  • Not ideal for carpeted floors as the carpet acts as an insulator
  • Care must be taken when installing under hardwood floors to avoid warping and lifting

Disclaimer: The products pictured in this article do not constitute an endorsement, approval, or recommendation of the product by the Town of Portola Valley. These product images are provided only for informational purposes and as visual examples.

If you have a green building topic you would like explored further in an article on this web page, please feel free to provide me with your suggestions at cborck@portolavalley.net or via telephone.

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