Installing a Hydronic Radiant Floor Heating System (PEX Tubing)
These installation recommendations are provided for general information only. The architect or HVAC contractor is responsible for all design details and installation procedures for the specific radiant floor heating system. The architect or contractor is also responsible for maintaining the work in compliance with all applicable building codes, local and national.
Note
Install all the components of a hydronic radiant floor heating system in accordance with the equipment manufacturer’s instructions and all applicable codes. Failure to do so could result in severe personal injury, death, or substantial property damage.
Installation Recommendations
The following installation recommendations are provided as a general reference. Each manufacturer will provide instructions specific to its product.
System Inspection
After the PEX tubing has been embedded or concealed, it becomes a relatively permanent part of the structure. Because of the difficulty of servicing embedded or concealed loops, it is essential that a final inspection be performed to make sure the tubing or piping has not been damaged during construction and that all tubing or piping loops have been installed in compliance with local codes and ordinances. Check the following:
• Check to make sure the tubing or piping loops have been installed according to the layout (coil patterns) in the building plan.
• Inspect the tubing or piping for kinks, scrapes, slits, or crush damage.
• Inspect the tubing or piping for correct spacing.
• Make sure all manifolds are correctly located and provide easy access.
• Check to make sure the tubing or piping connections to the manifold are tight.
• Make sure the tubing or piping is properly fastened and there is a correct spacing maintained between the fasteners.
Tubing Length and Diameter
It is important to know the length and inside diameter (ID) of the tubing when creating a circuit (loop). Excessive circuit lengths will result in a significant temperature drop in the circuit. The temperature drop is the difference between the supply (hotter) water entering the circuit and the return (cooler) water leaving the circuit. In residential heating systems, the temperature drop is normally 15–20°F. If the temperature drop is greater than 15–20°F, it will result in insufficient heat and/or uneven heat being delivered to the room or space.
Long loops also result in increased friction in the tubing, which slows the flow rate of the water. This pressure drop must be over- come by the circulator (pump) in order to maintain a uniform flow rate for the water in the tubing.
A typical residential hydronic radiant heating system uses 1⁄2- inch-ID tubing. The maximum recommended length for this diameter is 300 feet. Most circuits (loops) in residential heating systems are shorter (about 100 to 250 feet long). Tubing with an ID of 5⁄8–inch or 3⁄4–inch, on the other hand, can be used in circuits up to a maximum of 450 feet in length.
In addition to the tubing ID, the length of the tubing required per square foot of floor will also be affected by such variables as the type of slab used, the heat load for the structure, the type of appliance (boiler, water heater, or heat pump), the type of controls used, and even the climate.
Tubing Spacing
Another important factor to consider when designing and installing a hydronic radiant floor heating system is the spacing of the tubing in the loops. Most residential heating systems are based on the use of 1 to 11⁄2 linear feet of 1⁄2-inch-ID tubing per square foot of floor area with the tubing spaced 9 to 12 inches apart. That is only a general rule, however, because there are situations where the tubing must be spaced closer to increase the heat output (for example, under windows, along cold exterior walls, and so on). A 3-inch to 6-inch spacing of the tubing will require 2 to 4 linear feet of tubing per square foot of heated floor area.
Loop Continuity
The tubing loop extending from the manifold supply port to the manifold return port must be one continuous length. Never splice together two lengths of tubing to form a loop. Doing so will weaken the loop.
Insulation
Install insulation beneath the tubing to prevent the downward loss of a portion of the heat. In uninsulated slab-on-grade construction, for example, a portion of the heat will be lost to the ground. The ground becomes a heat sink if there is no insulation installed. Use 1- to 2-inch- thick rigid polystyrene to insulate a slab-on-grade radiant heating sys- tem. Batt and blanket insulation are also in other types of radiant heating systems. See “Radiant System Construction Details” for examples of the use of the different types of insulation.
Vapor Barrier
A vapor barrier of 6-mil polyethylene sheeting should be installed between a thin slab and the wood sheathing to limit the transfer of moisture from the slab to the wood. Check the local building code for the use of a vapor barrier. Not all codes require it.
Panel Testing Procedures
Radiant heating coils should be tested for leaks after they have been secured in position but before they are covered with concrete or some other covering material. Both a compressed-air test and a hydraulic pressure test are used for this purpose.
The compressed-air test requires a compressor, a pressure gauge, and a shutoff valve. The idea is to inject air under pressure into the radiant heating system and watch for a pressure drop on the gauge. A continually dropping pressure is an indication of a leak some- where in the system.
The pressure gauge is attached to one of the radiant heating coils, and the shutoff valve is placed on the inlet side of the gauge in a valve-open position. The air compressor is then connected, and compressed air is introduced into the system under approximately 100 psi. After the introduction of the air, the shutoff valve is closed and the compressor is disconnected. The system is now a closed one. If there are no leaks, the air pressure reading on the gauge will remain at approximately 100 psi. A steady drop in the air pressure reading means a leak exists somewhere in the system. A leak can be located by listening for the sound of escaping air. Another method is to use a solution of soap and water and watch for air bubbles.
The hydraulic pressure test requires that the coils be filled with water and the pressure in the coils be increased to approximately 275 to 300 psi. Care must be taken that all air is removed from the coils before the system is closed. The system is then closed, and the gauge is watched for any change in pressure. A leak in the system will be indicated by a steady drop in pressure on the gauge. The source of the leak can be located by watching for the escaping water. If a leak is discovered, the coil should be repaired or replaced and a new test run on the system.
Installation Guidelines
Guidelines
• Run the tubing parallel to the wall or walls with the greatest heat loss. (continues)
Guidelines (continued)
• Maintain a 12-inch gap between the outermost tubing and an exterior wall.
• Space tubing 6 inches o.c. between the first two loops along the wall or walls with the greatest heat loss.
• Tie tubing every 3 feet or less with plastic tie wraps. Note: Never tie tubing anywhere within the end of a loop.
• Always use a vapor barrier under the slab. Note: Place the vapor barrier between the ground and insulation, if the latter is used under the slab.
• Place a vapor barrier between the soil and any insulation installed under the slab.
• Insulate under the slab if groundwater comes within 3 feet.
• Always install edge insulation along the foundation walls to prevent edgewise (horizontal) heat loss.
Whenever possible, follow the radiant heating system manufacturer’s installation guidelines. The procedure described here for installing a hydronic radiant floor heating system (using PEX tub- ing) is offered as a general guideline. It may be outlined as follows:
1. Attach the manifold wall brackets to the wall.
2. Assemble the manifold (if it is not a factory-assembled unit) and clamp it into position on the wall brackets.
3. Mount a pipe bend support directly below the manifold to hold the supply pipe.
4. Connect the supply pipe to the manifold and lay out the pipe loop by following the layout plan.
5. Mount a pipe bend support below the manifold to hold the return pipe.
6. Create coil pattern.
7. Cut the return pipe and connect it to the manifold.
8. Mark or number the first loop for identification.
9. Check the length of the first loop against the layout plan by using the length markings on the outside of the pipe. A significant deviation in overall length between the layout plan and the installed pipe loop will require an adjustment of the loop balance settings.
10. Repeat steps 1 through 8 for the remaining loops in the system.
11. Close the supply, return, and shutoff valves on the first manifold.
12. Connect hoses to the end caps on the manifold.
13. Connect the end of one of the hoses to the main and the end of the other hose to a drain.
14. Open the end cap valves for filling and draining the system.
15. Open the supply and return valves on the manifold for the first loop.
16. Turn on the water and allow it to flow through the loop until all the air has been expelled. Purging the air from the system is a very important step. Air trapped in the loops will cause the system to operate inefficiently.
Note
If the water will not flow through the loop, the pipe may be buckled or crimped or there may be a blockage at the manifold connection. Check and repair before proceeding to the next step.
17. Repeat steps 10 through 15 until each loop in the heating system has been filled with water and any air trapped in the piping has been removed.
18. Open all the system valves and perform a pressure test (at 3 to 4 bar pressure). The pressure will drop during the first few hours and then remain stable if there are no leaks and the ambient temperature remains constant.
19. Install the floor covering (cement, carpet, tiles, and so on).
20. Close all the loop valves and open the shutoff valves.
21. Fill the boiler and the supply pipes with water, and purge the air.
Open every valve and fixture (faucets and so on) in the system and continue purging until all the air trapped in the pipes has been pushed out of the lines and the water flows freely from the fixtures. Purge the air from the end caps at each end of the manifolds. In a structure with several floors, purge the air from the manifold located at the lowest level first.
Note
There must be shutoff valves on the manifolds to properly purge air from the loops.
22. Open all the loops in the heating system and check to make sure the air has been removed. If there is still air in the tubing, repeat steps 20 and 21 until all air has been removed.
23. Place the system under pressure by starting the boiler and circulator.