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Getting Heat Into Your Home


Radiant heat is actually an old technology. It's common knowledge that the ancient Romans used it to heat their public baths. In more modern times, the Europeans have relied upon radiant heat for over 60 years. In fact, it was servicemen returning from World War II who first spread the word about under floor heat to their fellow Americans. Many radiant floors, most using copper tubing buried within concrete slabs, were installed and used successfully in the 1960's and 70's. But they all suffered from one primary problem...longevity. Copper within concrete is highly susceptible to corrosion and a life span of 50 years for a radiant floor was considered exceptional. Today, modern plastics not only share many of the heat emitting properties of copper, but also provide greater flexibility, corrosion resistance, and a life span of over 100 years.

Of these modern plastics, polyethylene is by far the best and most commonly used material. Below is a photo of our most versatile and highest output tubing. With 7/8" Durapoly XL PEX (XL stands for cross-linked) you can expect a heat output of at least 50 BTU's per foot in a slab on grade installation and 40 BTU's per foot in a floor joist application. Our 7/8" PEX is potable (Underwriters Laboratories ANSI/NSF-61) and ultraviolet resistant for protection against sun damage during installation. It is available as either a standard PEX tubing or as a PEX tubing with an oxygen barrier.

polyXL 7/8" Durapoly XL is rated for 60 psi at 180°

The 7/8" PEX is a large diameter tubing, made in America and manufactured by NIBCO, Inc., with the same wall thickness as the commonly used 1/2" PEX. It's main advantage over 1/2" PEX lies in the fact that it holds more fluid, and consequently, more heat. It has a slightly lower temperature and pressure rating than 1/2" PEX, but it can be spaced as far apart as 16" on center and still heat a room insulated to modern standards (R-19 walls, R-27 ceilings). It would take twice as much 1/2" PEX to do the same job.

That makes 7/8" PEX the first choice for any application where it can be feasibly used. It is ideal for joists bays spaced 12", 16", or 24" on center, or virtually any slab on grade installation. It is the only cross-linked 7/8" PEX tubing on the market and its bending diameter is less than 20", making it easy to thread through floor joists. This flexibility factor makes 7/8" PEX much less prone to kinking than other 7/8" Poly tubing. Also, minimum tubing is required to gain maximum heating results. That saves money on materials and time.

tubing layout

7/8" PEX tubing in slab on grade layout, 16" on center

The 1/2" PEX is also a polyethylene tubing with a very high temperature and pressure rating (180 degrees at 100 psi). It emits about half the heat of the 7/8" PEX, but its bending diameter is tighter. Using 1/2" PEX for small zones, tight crawl spaces, or snow melt applications makes sense. It has a bending diameter of 15" and should be spaced 8" to 12" on center.

Various other types of tubing such as rubber, soft copper, polybutylene, or even plain, so-called "High Density Polyethylene" (not cross-linked) are used for radiant heat. But the limited longevity of rubber, the difficulty and expense of installing copper, past problems with polybutylene, and the tendency of plain, "High Density Polyethylene" to shrink and crack in high temperature applications, make PEX the tubing of choice for most applications.

Of course, regardless of which type of tubing is used for your radiant system, consult with your local building department to guarantee conformity with local codes.

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What is Cross-Linking?

According to the Radiant Panel Association, cross-linking is:

"a three dimensional molecular bond created within the structure of the plastic which dramatically improves a large number of properties such as heat deformation, abrasion, chemical and stress crack resistance. Impact and tensile strength are increased, shrinkage decreased and low temperature properties improved. Cross-linked tubes also have a shape memory which only requires the addition of heat to return it to it's original shape when kinked".

There are three types of cross-linking: electron, peroxide, and silane. Radiant Floor Company's Durapoly XL tubing is cross-linked with the electron process. It's the cleanest, most environmentally friendly of the three methods.

If you'd like to see a graphic demonstration of how cross-linked Polyethylene differs from non-cross-linked Poly tubing, see the photos below.

Tubing in Oven

The tubing on the left is black because the Polyethylene contains a 2% carbon element for ultraviolet protection. The milky tubing on the right is a 7/8" ID "natural" Polyethylene. It is not cross-linked, nor does it contain the pigment necessary for ultraviolet resistance. The 7/8" Durapoly XL PEX in the middle is both cross-linked and UV protected.

The Oven Test

tubing in oven

The cross-linking process greatly increases the pressure and temperature characteristics of the Poly tubing. When all three tubes were subjected to 30 minutes of 250 degree temperatures, only the Durapoly XL survived the experience.

Heating Your Water

So what is the easiest, most efficient way to make hot water? The answer depends upon your needs. If the combined requirements of both your domestic hot water and space heating needs are less than 300,000 BTU's then a domestic water heater can do the job. Just remember that not all water heaters are created equal. Some are specifically engineered for domestic and space heating applications. A water heater purchased from your local hardware store may very well do the job...as long as your BTU requirements are low. But the cost of running such a unit may be alarming. Until recently the efficiency of many water heaters has been as low as 60%. That means that a full 40% of your fuel costs are going up the flue stack. The long-term result is enough wasted money to pay for a high-efficiency water heater...but you won't own one! It's always better to search the market for the best water heater you can afford and size it for your heating requirements.

On the other hand, if your home uses a combination of baseboard radiators and radiant heat, a situation common to retrofit projects, a boiler is your best bet. Boilers are designed to deliver lots of heat energy. They heat relatively small amounts of water to very high temperatures and vary in size from about 100,000 BTU's to as high as you want to go.

Other options include indoor and outdoor wood boilers, ground source heat pumps (sometimes called geothermal heating systems), electric boilers (if electricity in your region is inexpensive), and increasingly, on-demand water heaters.

Unlike standard, tank-type water heaters, on-demand units eliminate “standby loss” by heating water only when you need it. This feature can save up to 10% on fuel costs because standard water heaters leak heat to the surrounding air 24 hours a day. Electronic ignition eliminates the wasteful pilot light common to standard water heaters.

In addition, on-demand heaters are lightweight, install in tight spaces, provide limitless hot water, and the most sophisticated brands, like Takagi, monitor inlet water temperature and modulate the heater’s burner up or down for maximum energy efficiency. A digital remote displays incoming and outgoing temperatures, flow rate, flashes error codes for troubleshooting, and allows the user to set the unit for a variety of different temperature settings. Maintenance is simple – basically the fine-mesh inlet screen that should be checked periodically and kept clean.

Note: New installations may require daily cleaning until soldering flux and minor debris has been screened from the system.


On-demand water heaters are efficient (95%) powerful (up to 199,000 BTUs/hr), and affordable. A compact unit like the Takagi T-H3 can provide both radiant floor heating and domestic hot water and the heater vents with PVC pipe.





Polaris Water Heater

The Polaris water heater is somewhat pricey, but it's a time tested heat source with an efficiency rating of 96%. Models range from 130,000 to 199,000 BTU's. A Polaris can provide plenty of hot water for both domestic use and space heating.

Polaris Water Heater


Another excellent option is solar energy. With fuel prices rising, solar water heaters are not only clean, efficient, and environmentally sound, but they deliver years of service long after they have paid for themselves. Radiant Floor Company can design a solar system to meet your needs. See our Heating Water With Solar Energy pages for more information.

solar hot water panel schematic flat panel collector

Standard flat plate solar collectors

Evacauated Tube Collectors Evacuated Tube Collectors

Evacuated tube solar collectors

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Fuel Sources

Radiant floors are heated by every known heat source. The Brietenbush Resort in Oregon pumps water from natural hot springs through their floors. The rest of us probably aren't so lucky and end up using gas, oil, solar, mechanical geothermal (aka, ground source heat pumps), wood, or electricity to heat our water.

As long as the water flowing through the tubing is a steady temperature between 120 and 135 degrees, the method of heating it is up to the homeowner. However, a few guidelines are important. Primarily, recovery rate should be given great consideration. Natural gas, propane, oil, and wood offer the highest recovery rates.

To show the importance of recovery rate, imagine the following scenario. When the heated fluid in a radiant system satisfies the room's requirements, the circulator pump shuts off. After a short time, the fluid cools down to room temperature. Some time later, when the room again calls for heat, many gallons of 70-degree water flood the water heater. This has the effect of lowering the overall temperature of the heating system. Normally, this is not a problem because most water heaters can raise the temperature of the water very quickly.

However, this is not the case with electric water heaters. Electric heaters are very efficient because most of the energy going into the unit becomes hot water. But the electric heating elements don't do their job quickly. The radiant floor system basically limps along with 90 or 100 degree water as the elements struggle to raise the water back to the desired temperature level.

So, if you live in a region of the country where electricity is so cheap that heating water is feasible, then compensate for the slow recovery rate with volume. Minimize the impact of the cooler returning water by storing a large amount of hot water. A 120-gallon water tank in a radiant floor system is not unreasonable.

Unlike standard electric water heaters, electric boilers heat water quickly. But be prepared to install a second service panel to handle the power flow. However, if electricity in your region sells for .06 per kw or less, heating water with an electric boiler may be an option.

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We've seen that virtually any method of heating water will work for radiant floors and that Polyethylene tubing is the best heat transfer medium. So, how do we spread that heated fluid to the living space?

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 be tiny or huge. A zone can consist of many circuits or loops of tubing, or can be a single circuit. Circuit lengths should not exceed 400' of tubing (300' for 1/2" PEX), but a zone may contain any number of circuits.

So, the question becomes: How many zones do I need? The answer depends upon your lifestyle, the size of your heated space, and the unique architectural characteristics of the building. As a rule, keep zoning to a minimum. There's nothing wrong with treating an entire floor as one zone. By one floor, we mean one elevation. The first and second story of one home shouldn't share a single zone. So, if you have a two story house, your system will be a minimum of two zones.

Minimum zoning is important because radiant heating is very even. You are warming not only the floor, but also every object in the room. As a result, the entire space tends to equilibrate. Treating every room in the house as a separate zone is not only a waste of time and money, but it won't give you much control over a space that tends to seek the same even temperature.

Zoning entire sections of a floor makes more sense. A block of rarely used bedrooms could be on their own zone, for example. Also, many people prefer to maintain their master suite at a cooler temperature than the rest of the living space. If there's a lifestyle reason to maintain one section of a given floor at a noticeably warmer or cooler temperature, then zoning is appropriate.

Another example would be architecture features like sunrooms or great rooms with lots of glass. These rooms have a heat signature unlike the rest of the living space. During the day, a sunroom can be 20-degrees warmer than the living room. If the thermostat controlling the zone is located in the living room, the sunroom will receive heat it doesn't need. The reverse is also true. At night, the sunroom will give off much of its heat due to its large amount of glass. Trying to keep the sunroom warm during a cold winter night would overheat the rest of the living area if both spaces shared the same zone. Of course, it goes without saying that window shades greatly reduce nighttime heat loss in high glass areas and should be installed whenever possible.

A garage would always be on its own zone.

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Multiple Circuits

So, if a zone can be any size, and each zone uses only one circulator pump, how far can the hot water travel before it looses all its heat?

The answer depends upon the size of the tubing used. The smaller 1/2" PEX is limited to a 300 ft. run, the 7/8" PEX to about 400 ft. So, if a zone is large enough to require more than, say, 400 ft. of PEX, the heated area must be broken up into multiple circuits, all approximately the same length. Similar lengths are important because you never want to give the water a path of least resistance to follow. If your zone consisted of three circuits, one 200 ft. long, and two 100 ft. long, the two shorter circuits would steal the water from the longer 200 ft. circuit because they would offer the pump less resistance. An inefficient heating system would result.

This is how it should be done. Let's say that your entire first floor is one evenly heated space, one zone. You need 1200 ft. of 7/8" PEX tubing, spaced 16" on center, to cover the whole area. If you tried to run the hot water continuously through 1200 ft. of tubing, you'd end up with ice water by the time you got back to your heat source.

Instead, you can break up the zone into one of these configurations:

(6) 200 ft. circuits
(4) 300 ft. circuits
(3) 400 ft. circuits

You can see that none of the circuit lengths has exceeded 400 ft. In each case, the water is returning to the heat source before or at the 400 ft. point.

Keep in mind also that these circuit lengths are just examples. A circuit length should conform to each individual situation. The installer can be flexible within the above guidelines. If you are installing tubing in your floor joists, and you determine that the ideal circuit length for your situation would work out well as (5) 240 ft circuits... then by all means do it that way. (see The Floor Joist Installation for detailed installation instructions)

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The Zone Manifold

A tank of efficiently heated water won't do much heating unless it can be distributed effectively to the zones. For this we use a Zone Manifold. This is simply a factory built manifold containing all the gauges, valves, pump flanges, etc. necessary to mount multiple circulator pumps in one central location. It's normally installed very near the heat source so that any pump, when activated by a signal from the zone, can draw hot water and send it to the floor.

If for some reason, the Zone Manifold must be located more than six feet (three feet for on-demand units) from the heat source, the pipe size between heat source and manifold should be increased in direct proportion to the distance. Contact one of our technicians for specifics and never use any piping material except copper to connect the heater to the Zone Manifold, i.e. never use PEX, PVC, ABS, black iron, or garden hose for this purpose.

Every zone always has its own circulator pump. That way, the pump can be sized to match the amount of tubing in the zone. The fluid from the pump then enters the supply line to the zone, travels through the floor, then returns to the heat source.

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Supply and Return Lines

The Supply Line

In a floor joist installation, a supply header feeds the circuits within a zone. This header is simply a 3/4" copper supply pipe that comes from the circulator pump. (See The Floor Joist Installation for detailed information on building the supply header.)

In a slab installation, the supply line is run to one side of a slab manifold (see photo below) that has already been installed as part of the slab pour. (See The Slab Installation for detailed information on installing the slab manifold.)

The Return Line

Every circuit of tubing will have a beginning (supply) and an end (return). After traveling the entire length of the circuit, the fluid flows into a return pipe, also 3/4" copper. This return pipe leads back to the heat source where the water is re-heated and sent back to the supply side of the circuit. This cycle repeats until enough heat has entered the space to be warmed. Only when the thermostat in the zone has been satisfied will the zone pump shut off.

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Using Adaptors and Couplings



Normally, all supply and return lines are insulated with foam pipe insulation or fiberglass. This is to prevent heat loss as the water travels to and from the heated floor. Most of the time, supply and return lines are made of 3/4" copper pipe. This is because you are already in copper pipe mode when you leave the circulator pump. Getting from copper pipe mode to plastic tubing mode requires a brass fitting called an adaptor.

Adaptors are very handy fittings. The ability to convert from copper to plastic, and back, at any time, gives the installer a great deal of flexibility when it comes to running supply and return lines. Even while threading the radiant tubing through floor joists, adaptors can be used to overcome obstacles in the joist bays, make super tight bends, etc.



Whenever the installer needs to connect plastic to plastic, use a coupling. Badly kinked, punctured, or crushed tubing can be easily repaired using couplings. Like adaptors, couplings are brass fittings.

See Connecting Adaptors and Couplings in the Installation Details, Details, Details section of this site.

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The Slab Manifold

Slab Manifold

Notice that the supply side of the manifold is joined to the return side by the pressure testing assembly.

Whenever radiant tubing is poured into a concrete slab, a slab manifold should be used. More than merely a method of splitting a supply feed into two or more branch circuits, the slab manifold also doubles as a pressure test kit. Built into every manifold you'll find a pressure gauge and an air stem. Once the tubing has been installed and all the connections have been tightened with a wrench, use the air stem and an air compressor to pressurize the system to 50 psi. If several hours pass without any significant drop in pressure, you can rest assured that your tubing is ready for the pour.

The manifold is also useful as a safety gauge during the pour itself. If at any time you question the integrity of the system, checking the pressure gauge will tell you instantly whether or not the tubing has been damaged.

Slab Manifold on site

The box used for shipping the slab manifold becomes the form to pour your concrete around. This prevents the concrete from coming into direct contact with the manifold and creates a "manifold well" in the slab to protect the tubing and connections from damage during later stages of construction. (note: the above photo shows the manifold box without the front panel installed)

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Mounting the Mechanical Components

Zone Manifold

Zone Manifold

The zone manifold is divided into two sections...supply side and return side. Both sections should be mounted fairly close to the heat source. In multiple zone systems containing many pumps, the supply side of the manifold can be quite heavy so care should be taken to mount it securely. When you receive your manifold, you'll notice that we've included a piece of plywood. The plywood should be removed and mounted on the wall next to your heat source. This will provide a secure foundation for both sides of your manifold and give you a board already precut to the width of your particular manifold.

Also note the "test caps" soldered onto the ends of the zone manifolds & the Expansion and Purge Kit. We install these for pressure testing purposes and to prevent debris from entering the manifold during shipping. These caps should be removed before plumbing the manifold.

In the bag of mounting hardware included with your order you will find two excellent mounting assemblies: 1) 1 1/4" bell connectors, and 2) split ring hangers, complete with precut all-thread rods and the cast iron plate that the all-thread screws into.

The bell connectors are mounted to the plywood and attach to the main body of the manifold.

The split ring hangers are also mounted to the plywood, of course, but they attach to the 3/4" copper pipe just below the circulator pump. The piece of all-thread is used to bridge the gap between the plywood and the split ring hanger and provide a strong support.

You'll also notice identical brass tee fittings on each end of the zone manifold.

The outlet of the tee is threaded. Depending on how you decide to orient the manifold in relation to your heat source, this threaded outlet will contain either a drain valve or an in-line thermometer.

Obviously, you'll want to install one of the in-line thermometers at a point where the hot water first enters the supply side of the manifold. This way you can monitor the fluid temperature on its way to the floor. The second in-line thermometer is installed so that you can monitor the temperature of the water as it leaves the return side of the manifold and travels back to the heat source.

Install the two drain valves in the tees opposite your in-line thermometers. These valves are provided as a means of draining the radiant system should that ever become necessary.

Circulator Pump(s) Circulator Pump

A conventional, high-quality radiant circulator (above) manufactured by Grundfos


The revolutionary ALPHA series circulator. For a slightly higher initial cost, the ALPHA pumps use 50-75% less electricity.


Once the zone manifold is mounted, the circulators are very easy to install. The pump flanges are built into the manifold so attaching the pump is simply a matter of aligning the pump with the flange and bolting it in.

For single zone systems (i.e. one pump) always orient the circulator so that the arrow on the body of the pump is pointing upward. This way any air rising in the system is pushed up and away from the pump. Note: Radiant Floor Company has designed what we call "Radiant Ready" packages. These are pre-plumbed and pre-wired single-zone plumbing assemblies mounted on a plywood board. Depending upon which package is needed, i.e. for an "open" or a "closed" system, only four or five solder connections are needed to complete the mechanical part of the radiant system.

Radiant Ready "J", a single zone package for use with an existing boiler.


For multiple zone systems, you'll want to pre-wire the pumps with either 12 or 14 gauge wire before you install them into the Zone Manifold. Follow your local codes for specific wiring guidance. Some codes require flexible conduit from the relay box to the pumps, others allow a simple Romex connection.

Pump Controller

Pump Controller

The pump controller box is usually mounted fairly close to the rest of the mechanical components. However, some people choose to locate it some distance away. The controller displays a green light to indicate power to the system and red lights to indicate which zone or zones are currently operating. It can be a very handy way of monitoring your system and you may want to install it in some location that you frequent often. Radiant systems are silent. If you're like me and are curious about the rhythms of your system, you'll want your controller somewhere that you can conveniently see it.

Wiring the controller box is generally very simple and schematics are included with every system. But, if you have questions, just contact one of our technicians and they'll happily walk you through the process.

Expansion and Purge Kit

Expansion and Purge Kit

For Closed and Heat Exchanger systems, you'll need the Expansion and Purge Kit (EPK). This consists of an expansion tank, an air eliminator, fill and drain valves, a pressure gauge and a pressure relieve valve. The EPK is pretty much factory assembled. You'll have to screw the expansion tank into the bottom of the air eliminator because the tank is shipped in a separate box, but that is a simple operation.

The EPK makes it very easy to fill and simultaneously purge air from your newly installed tubing, and the expansion tank component acts as a sort of "shock absorber" in a closed system. When water heats up, it expands. The flexible membrane in the tank absorbs that expansion.

In addition, the pressure gauge in the EPK helps you charge the system with the proper 15-20 PSI, and during the life of the system, lets you know when more fluid needs to be added by indicating a pressure drop below 10 PSI.

The pressure relief valve is a safety device very much like the pressure and temperature relief valve on your water heater. It protects the tubing from excessive pressure.

Mounting the EPK is similar to mounting the zone manifold. Use split ring connectors and all-thread to secure the EPK to a wall next to your heat source. In Closed and Heat Exchanger systems the EPK is installed between the heat source and the zone manifold, so that the plumbing itself offers a great deal of support to the assembly. Always install the expansion tank after the EPK has been mounted.

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A world of Possibilities


One of our customers, CAD Master Dan Willis of Grant's Pass, Oregon sent us this schematic of a system we designed for him utilizing solar and wood, with propane for back-up.

It illustrates a basic fact: Whether you're heating a single small zone with a standard, tank-type water heater, or, as in Dan's case, blending multiple heat sources into a harmonious, multi-layered system, Radiant Floor Company can help.