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Radiant In-Floor Heating Tour

INTRODUCTION
The comfort advantages of radiant floor heating have been known for centuries. In recent years, installation of Hydronic Radiant Heating (HRH) ­ systems has expanded with the development of improved system components and installation techniques. Modern HRH systems circulate heated water through flexible plastic tubing. This plastic tubing is cast directly into a concrete slab floor, attached to the underside of wood flooring, or embedded in a thin layer of lightweight concrete poured on top of the sub-floor. Vanguard's Vanex® PEX (cross-linked polyethylene) tubing, is the material choice for the in-floor heat exchanger. Vanex's light weight and flexibility make it the prime choice for HRH installations. The information presented in this tour will familiarize the reader with HRH systems and the application of Vanguard Vanex PEX as the heat exchanger tubing. Although recommendations are based on recognized design criteria, this is not a design manual. For proper operation and required heat output, HRH systems must be designed by an experienced hydronic engineer or with appropriate design software.

Material

Vanguard Vanex PEX tubing is ideally suited for HRH applications. Its light weight and flexibility make installation of hydronic heat exchanger loops fast and easy. For in-slab systems, the Vanex tubing is simply attached directly to the reinforcement mesh. For under-floor (staple-up) and above-floor (suspended floor with or without poured topping) the tubing is attached to the flooring with tubing clamps or heat transfer plates.

Vanex PEX tubing is manufactured to nationally recognized industry standards (ASTM F876 and F877).

Vanex PEX tubing is available in both standard, non-barrier and premium oxygen barrier* versions. Both tubing types are approved for conveyance of potable water in the event that the hydronic heating system will be connected to the drinking water system.

Vanex PEX is available in 3/8" through 1" sizes. Common tubing sizes are 1/2" for residential applications and 3/4" for commercial and snow-melt systems. Supply/return transfer lines use the 3/4" and 1" sizes.

* Oxygen barrier tubing is used for those hydronic applications where the total amount of oxygen ingress into the system must be limited. Barrier tubing may be required by water heating unit manufacturer. For a more detailed discussion of oxygen barrier tubing see the Vanguard Hydronic Radiant Heating System Installation Instructions.

Choosing HRH as a Heat Source

HRH systems generally offer superior comfort and efficiency when compared to hot-air (forced-air) heating systems. However, as with any heating system, buildings with little or no insulation, or drafty buildings may not be served well by an HRH system. HRH systems will not cure heating problems in these types of structures unless the HRH system is being used as supplemental heat. HRH systems will, however, offer occupants the comfort of a warm floor without the inefficiencies of heating air to a higher temperature to achieve a similar but lower level of comfort.

Since a properly designed and installed HRH system achieves a nearly ideal heating zone for occupant comfort, it relies less on the ambient air temperature of the heated space to provide comfort. On the other hand, forced air systems, since hot air rises, require higher air temperatures for occupant comfort. HRH systems typically have ambient air temperatures that are 5 to 10 degrees cooler than for forced air systems while achieving a higher level of comfort. Also the warmest air in a hydronic radiant heated structure is near the floor and not at the ceiling.

There are, however, a couple of limits to heating with HRH:

1. Generally speaking, HRH systems cannot be used for cooling.
2. There is a lag time from the time a HRH system is first started to the time that it is fully up to heat.

HRH systems may take from 1 to several days to bring a concrete slab up to temperature. Staple-up and suspended systems have faster response times, but may not be suitable if a structure or space is occupied intermittently during the heating season. HRH is best applied when the structure can be maintained at a nearly constant temperature during the entire heating season.

HRH systems don't work well for cooling because when the air close to the floor cools to the dew point, the moisture in the air begins to drop out. The floor may constantly be damp or wet. Some success with HRH cooling has been achieved, but only with proper temperature control and moisture monitoring. For additional information concerning cooling with a HRH system, contact the Radiant Panel Association.

Additional information on choosing radiant floor heating is included in the next four pages of this tour. In each case, however, it is important to review existing or planned design conditions before installing an HRH system.

A total, effective Hydronic Radiant Heating system is designed on a room by room basis with careful attention given to floor coverings and placement of Vanex PEX tubing circuits. The following section will outline suggested general pre-installation conditions for selecting HRH. 

Residential

Residential applications for HRH include new construction, add-ons and retrofit of existing structures. HRH provides even, comfortable heat without the noise and air movement associated with forced air systems. HRH systems are essentially noise free.

To ensure proper heat output, systems are designed on a room-by-room basis with consideration given to numerous factors about each room. Number and size of outside walls, wall and ceiling insulation, window area, floor covering, etc. are assessed for each room when designing a radiant floor heating system.

HRH is compatible with most floor coverings including carpet and heavy linoleum. The type and thickness of pad and carpet, or linoleum should be carefully considered to complement floor heat. Marble, stone and wood offer the best heat transfer requiring the lowest water temperatures.

Typically, floor temperatures for residential heating are in the 80° F to 90° F range. This allows an HRH system to provide up to 30 BTU's per square foot of floor area which is more than sufficient for the vast majority of applications. However, as previously stated, existing heating load conditions such as high levels of air infiltration can overwhelm the system and possibly reduce the expected comfort level. HRH is most favorable in homes that maintain relatively constant interior temperatures throughout the heating season. Since there is a lag from the time the system is initially started until it is up to heat, radiant floor systems are not particularly suited to homes where the interior temperature must vary from day-to-day or week-to-week.

The hot water source must be sufficiently sized to provide the necessary BTU input to the floor. Generally, a standard residential water heater is not capable of generating enough hot water to operate an HRH system except possibly for a small home or addition. There are a number of high output water heating devices that are used specifically for radiant systems. Many of these heaters fall into the classification of boilers and may also be capable of heating or pre-heating potable hot water as well.

In many cases, HRH systems can provide favorable design solutions to troublesome issues with traditional HVAC systems such as placement of bulky ductwork, floor registers and return-air plenums.

A properly designed HRH system takes into account the variables noted above, and others, to provide a system which performs satisfactorily even at the lowest expected outside temperatures.

Commercial

In spaces with large, undivided floor areas, HRH can provide even, comfortable heat to persons working on or near the floor. For example, garage mechanics, students in classrooms, and workers in assembly plants all benefit from the lower heat stratification inherent in a HRH system.

As with residential HRH systems, commercial systems operate more effectively and efficiently with good structural insulation. Although some buildings such as vehicle maintenance or service garages suffer from high infiltration due to opening large overhead doors, HRH can still provide a higher degree of employee comfort by keeping the floors warm. Some supplemental heating may be necessary if overhead doors must remain open for lengthy periods.

Many commercial occupied areas, unlike those in residential, may experience rapid temperature changes due to heat loss or heat gain. While concerns with heat loss are generally obvious, there should also be an awareness of heat gain. Temperature increase from internal gains (heat generated by machinery, lighting fixtures, or even many human bodies) may occur too quickly for an HRH system to respond. Spaces that experience wide swings in occupancy level (conference rooms, ballrooms, churches) can pose problems for an HRH system if the warmed thermal mass continues to give off heat when it is not needed. Special system controls that anticipate significant changes due to internal gain may be required.

Commercial BTU/square foot requirements are generally higher than for residential. Regular opening of outside doors (walk-through or overhead) may require more BTU's per square foot than a radiant floor system can provide without the danger of heating the floor to an uncomfortable temperature to stand or lie on. Localized supplemental heat may be required near doors to offset the heat loss.

A properly designed commercial system takes into account these factors and others to insure that the floor heating system provides the desired level of comfort and efficiency. 

Agriculture

Agricultural applications for HRH systems include livestock or poultry breeding enclosures as well as greenhouse and seed bed applications.

Since many agricultural enclosures are constructed over earthen floors, tubing installation is difficult. Also, poor thermal mass could discourage use of HRH as the sole heat source. Slab-on-grade provides the most effective and efficient heat conductivity, however, an awareness of "floor covering" is still important. Layers of soil, straw, saw dust, etc., will impede good heat transfer. HRH applied in livestock scrub-down or bathing areas may not deliver adequate warmth for heating purposes, but can prevent ice or glaze from forming in freezing weather.

Farm buildings that have little or no insulation, or suffer from high air infiltration can overwhelm a radiant floor system. It may be necessary to provide supplemental heat to compliment the HRH system when these conditions are present.

HRH installed in plant seed beds has been found to augment growth by warming the soil year round even during harsh winter environments. Such systems may be expanded to include greenhouse floors and office spaces thus allowing employees and customers to benefit from the comfortable heat.

It should be noted that BTU/square foot requirements can be high for agricultural applications. The size and output of the hot water source must be sufficiently sized for the area to be heated. Agricultural systems will use either 1/2" or 3/4" Vanex PEX tubing depending on system design and heat output requirements.