Under Floor Heating

It’s a simple fact—heat goes up. With that in mind, it seems the most effective heating system would be the floor. New products available today allow the floor to provide very efficient heat. And, don’t think radiant floor heating is new. It is probably one of the oldest known methods of central heating. The Romans built a fire below the living space and the heat from the fire traveled through passages or channels under the floors. European kings and queens used a similar system during the “Dark Ages” to heat their castles. Hot water was one of the first “modern” radiant systems. As early as 1942 an American company started testing and experimenting with radiant heat for residential use. After WWII a number of huge housing developments used the technique. The metal pipes of the first units installed in hastily built concrete slabs of the time quickly degraded. And the copper, steel and wrought iron piping deteriorated over the years when placed in a concrete slab.  

Today’s plastic technology has produced products that alleviate the problems of earlier radiant heating systems. The plastic cross-linking process produces tubing that is very strong at high temperatures and high pressures, and the flexibility of the plastic allows it to move with the natural movement and settling of a house without leaking or otherwise being compromised.


Unlike hot water baseboard or forced air, a radiant floor heating system heats objects instead of just the air in the room. Because every building, no matter how well insulated, constantly looses heat to the outside, conventional heating systems work to replace this loss. Our bodies lose heat to the colder objects around us. We feel cold because of this heat loss. Because heat always flows toward cold, if you are standing next to an object that is colder than your body, that object will steal body heat.


A radiant floor heating system does not heat the air directly like a baseboard or forced air system. Rather, a radiant system warms the floor, the chair, the sofa, the tables, and so forth, and this slows the rate at which your body looses heat to these objects. An overall even feeling of warmth and comfort is the result.


Interestingly, the air temperature in the room can be considerably lower if your body is in a room full of warm objects. In fact, many people with radiant heat lower their thermostats to 65 degrees and still feel more comfortable than they did with their baseboard or forced air system set at 70 to 72 degrees. It’s important to note that in a baseboard or forced air system, the warmest air is at the ceiling and the coolest air is at the floor. This, of course, is not efficient. A radiant system that produces warm feet and a cooler head is healthier and feels more comfortable.


Radiant floor heating systems may be hydronic, circulating water through tubes embedded in the floor, or electric, utilizing electric heat cables beneath the floor covering. The latter is available only to specific floor coverings, while the hydronic is not.

Most hydronic floor systems are divided into separate heating zones. (Image courtesy of Uponor Wirsbo) 

Hydronic Systems

Hydronic systems, such as those from the Radiant Floor Company, use warm water to turn your floor into a large radiator that sends waves of radiant energy in all directions, warming everything in the room.


The methods of heating the water are as varied as your imagination. Solar panels, oil and gas boilers, water heaters, wood boilers, geothermal and electric are all viable methods of heating water for a hydronic radiant floor system. The water is then sent through the tubing via a circulator pump. Additional materials such as manifolds, mixing valves, expansion tanks and thermostatic controls are designed into the system to fine tune the heat for optimum comfort.


Before any radiant system is installed in your house, the contractor or the system supplier must perform a heat loss calculation. This is done by determining the amount of heat that your house will lose on the coldest day of the year in your location. This heat loss is expressed in terms of BTUs or British Thermal Units. The supplier or contractor then designs the system so that the heat output from the radiant floor exceeds heat loss from the house. This is done through a combination of tubing spacing and water temperature.


High-tech, cross-linked polyethylene is used these days as the tubing for the hot water. In addition, manifolds and circulators are used to fine-tune the system and direct the water to the various zones.

According to the folks at Radiant Floor Company, “Radiant floor heating is one of the fastest growing segments of the housing market, growing at a rate of 25 to 30 percent a year. In custom designed new homes it is by far the most utilized heat system. Even homeowners doing renovations are using radiant whenever possible. Of course, the one thing radiant can not do (at least with the current technology) is provide air conditioning. If you live in an area where air conditioning is required, then an air conditioning system, minus the furnace component, is installed along with the radiant system.”


Three Hydronic Methods

Three main hydronic methods are used in radiant floor heating. In an open system, one heat source is used, your domestic water heater, to provide both floor heating and domestic hot water. The two systems are basically tied together. The same water that ends up in your hot shower or dishwasher, for example, has passed through the floor first. This is a very efficient system because one heat source is doing all the work. As long as the water heater is sized appropriately and matches your heating and domestic requirements, the need for a “separate” heating system is eliminated.


Two different types of in-floor radiant heating exist. The first is hydronic using hot water pumped through pipes in or under the floor. (Image courtesy of Uponor Wirsbo)

A closed system uses a dedicated heat source for the radiant floor. The fluid in a closed system is re-circulated around and around in a completely closed loop. There is no connection whatsoever to the domestic water supply. The main advantage to this system is that, being closed, anti-freeze instead of water can be used as the heat-transfer medium. Closed systems are often used in second homes or primary residences in areas prone to long power outages. If freeze protection is an issue, then a closed system with anti-freeze is a good idea. The down side to this type of system is the need for two heat sources.


Three types of hydronic systems are available. The first is the open system that utilizes the water heater used for domestic hot water.

All water heaters waste heat energy, even when the burner is off and the unit is sitting idle between heating cycles. The unit dedicated to heating the floor only wastes heat through the winter months, however, standby losses for six months out of the year can add up. In considering these systems, a hot water heater is the primary heat source although solar may be an option in some cases. Regardless, the water flowing through the tubing should be between 120 and 135 degrees F. It’s important to size the hot water heater to the job. As long as both your domestic hot water and space heating needs are less than 300,000 BTU’s, a domestic hot water heater can do the job. Some are specifically engineered for domestic and space-heating applications. Until recently, many water heaters had an efficiency rating as low as 60 percent. That means 40 percent of your fuel is going up the vent flue. It’s best to purchase the best, highest efficiency water heater you can afford and size it to your heating requirements.  


Another  system is radiant zone heating with an existing boiler using baseboard or cast iron radiators. (Diagrams courtesy of Radiant Floor Company)

The third type of system involves connecting radiant floors or “zones” to an existing hot water baseboard or cast iron radiator system. In many instances of this type of installation, a boiler is the water heat source rather than a hot water heater. Boilers heat more efficiently because they tend to heat small amounts of water to very high temperatures and heat fairly quickly.


A hydronic system installed in a concrete slab is probably the most effective method to heat a floor. (Image courtesy of Uponor Wirsbo)

Unless a heated area is very small, it will likely be broken up into several “zones.” A zone is any area controlled by a single thermostat and supplied by a single circulator pump. A zone can consist of many “circuits” or loops of tubing, or can be a single circuit. Circuit lengths should not exceed 400 feet of tubing, but a zone may contain any number of circuits. As a rule, it’s important to keep zoning to a minimum, and there’s nothing wrong with treating an entire floor, or elevation, as one zone. If you have a two story house, your minimum would be two zones.


Minimum zoning, but zoning entire sections of a floor is the best choice, because radiant heating is very even. For instance if you have a block of rarely used bedrooms they should have their own zone. Also many people like to keep their master bedroom at a cooler temperature than the rest of the house. This is easy with radiant floor zone heating. On the other side, if you have a sun room or great room with lots of glass and it is zoned with other rooms whether the thermostat is in that room or in another room, it will not provide comfortable heating for the various rooms.


Hydronic Installation

Hydronic radiant heating can be used in several different construction applications. Installing radiant tubing within a concrete slab, either “at grade” or poured several feet below grade as part of a full foundation is probably the easiest, most effective and highest performing application of the science. The thermal benefits are unsurpassed. Actually, any concrete building pour should contain radiant tubing, even if you have no immediate plans to heat the space. The tubing and manifold are relatively inexpensive and mechanical components can be added even years later.


An alternative to the slab-on-grade installation is the “suspended slab,” and the thermal performance can rival the slab-on-grade. The suspended slab incorporates sand, cement or Gyp-crete to store and diffuse thermal energy. The downside is the added weight to the floor, possible loss of headroom, and (especially in retrofit situations) difficulty making transitions into other rooms and adjusting door thresholds.


The third method is ledger board installed radiant tubing. This works in two primary situations. The first is an existing slab upon which you plan to add a floor joist, for example, converting a garage that is 8-inches or so below the level of the rest of the house into an office or room. You will want to raise the floor to match the rest of the house. The second situation would be a re-modeling project that required the removal of the existing sub-floor, or a room where headroom is at a premium and raising the floor isn’t a problem. In both cases, the joists are exposed and the tubing installed from above. 

Hydronic systems can also be installed beneath floor joists.

The last system is the floor joist installation. This is primarily used when the floor joists are exposed, say in rooms over an unfinished basement or crawlspace. In this case the tubing is run between and through the joists and anchored to the subfloor. This typically presents more challenges, but most are easily overcome.


Watts Radiant Onix hydronic tubing is stapled to the underside of the subfloor on multistory projects. Onix is flexible, cross-linked EPDM tubing with Aramid reinforcing and an aluminum oxygen barrier. Onix conducts heat through the flooring to warm your room without changing your finished floor height.

Electric Cable Systems

Electric heating cables embedded in the floor beneath tile, marble, slate or in some cases laminate floors, is another approach to in-floor heating. An example is the Warm Tiles Products. Warm Tile kits pioneered easy-to-buy solutions to radiant floor warming. The Warm Tiles radiant heat is suited to virtually any room: bathroom, kitchen, nursery, or family room—wherever you desire comfortably warm floors. Operating on ordinary current, Warm Tiles costs less than a penny per square foot per day, when the system includes the specially designed Warm Tiles thermostat. If you are able to install the flooring yourself, installing a Warm Tiles system is simple. For full radiant floor coverage of many shapes, Warm Tiles off-the-shelf warming cable kits meet many needs. Simply match your walkable floor area with the Warm Tiles Selection Charts to calculate which cable system to buy. Or, the system can also be purchased in pre-fabricated, labor-saving mats. Then choose a controller according to your system size and requirements.

Each component of the system includes detailed instructions for a proper and safe installation. Generally, an electric system installation can be completed in five phases. In the first phase the system is designed, calculating the heated area and determining the amount of cables and/or kits required. In the second phase the electrical power is brought to an electrical control box in the room or rooms to be served. In phase three the cable and thermostat sensor are installed and the included accessories as per instructions. In phase four the floor installation is completed in the usual manner, installing ceramic, marble or slate tiles or laminate flooring. In phase five the controller installation is completed using its included accessories and instructions.


Only a couple of companies recommend under-carpet installation. One is the Environ II system sold by Warmly Yours along with some of the Flextherm cables. Although any hydronic or electric cable system installed within a concrete slab could be used under carpet or any other flooring.


Regardless of the system used, it’s important to check with local and state zoning regulations on any installations.


Maybe the Romans had it right from the beginning. Warm floors can translate into warm bodies. With today’s high energy costs, it pays to use the most efficient heating system you can afford. These days, you might want to look into an in-floor heating system for your garage, shop, bathroom or even entire house. And you can do-it-yourself, whether new house or retrofitting an older home, garage or shop.



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