Compliance with building codes is an important phase of any major construction project (“major” being defined as virtually anything beyond building a backyard sand box). As a result, the radiant installer, whether a do it yourself homeowner or a professional contractor, must understand applicable building codes and work with local building inspectors to install the system accordingly.
After many years in the heating and plumbing industry, we have concluded that most building inspectors are well-informed and reasonable people. Many have installed radiant heat in their own homes and understand its operation and many benefits.
However, the industry constantly changes. New heat sources are introduced. New controls, valves, and ways of configuring systems enter the market. And, of course, radiant tubing comes in a wide variety of pipe diameters, wall thickness, and material types. During an average year, a building inspector is likely to see cross-linked polyethylene (PEX), non-cross-linked, so-called “natural” polyethylene, rubber tubing, copper tubing, or polybutylene used in radiant heating systems.
The best, and most commonly used tubing is, of course, PEX. But even within the PEX category the varieties are daunting. There’s PEX processed with the electron beam method, the silane method, or the peroxide method. There’s PEX lined with aluminum, PEX with a plastic coating for protection against oxygen diffusion, PEX colored orange, blue, silver, or red to protect it against damage by UV rays. And in case you’re not confused enough, you can buy PEX in it’s natural, uncolored, milky white state.
Have sympathy for the building inspector who sees different tubing on every job.
Fortunately, MOST of the tubing an inspector encounters is high quality, durable, and designed specifically for radiant heating. Occasionally a poorly informed homeowner tries using black poly water line or garden hose in a radiant system and that’s why the inspector uses code guidelines to certify that the tubing is engineered to meet the design specifications of a specific radiant system.
However, sometimes problems arise when the inspector, overwhelmed by the shear variety of possibilities, mandates a code specification that applies to MOST of the PEX on the market, but excludes other very worthwhile, cutting edge, and well-engineered variations of PEX not yet part of the code.
We make this distinction because one of the products sold by Radiant Floor Company, as well as other reputable radiant heating companies, is a superb variety of PEX that does indeed fall outside of the “standard” code specifications for PEX (180 degrees @ 100 psi).
This design variation was done deliberately, and in association with one of the largest and most reputable PEX manufacturers in the country. But, as mentioned above, it’s design parameters differ from those of “traditional” PEX because it is representative of the ongoing evolution in the radiant heating industry.
Why was this evolution desirable? In one word: flexibility.
We designed the 7/8″ Durapoly XL PEX with a thinner wall to make it flexible enough to thread through floor joists. The equivalent “standard” PEX (that is, 1″ PEX) is way too stiff to bend in such a tight space. As a result, before the dawn of 7/8″ PEX, only “standard” ½” PEX could bend within a joist cavity. But because the small volume of fluid in the ½” PEX results in a rather low BTU output, TWO runs of ½” tubing per joist bay are necessary to heat most zones. This means lots of tubing, lots of heat diffusion plates to hold it up, and lots of connections for many circuits…in short, a lot more expense and labor than installing the same job using a tubing like 7/8″ Durapoly XL PEX. And that’s why we designed it.
So, you may ask, does thinner walled tubing like 7/8″ PEX perform differently than standard PEX?
The short answer is yes. It has a higher heat output because a thinner wall means less resistance to heat flow. Is it a significant enough difference to matter in the real world? Well, more heat is more heat. But the main advantage of the 7/8″ PEX is in its flexibility. And of course, that IS a significant advantage. As mentioned above, the same heating job can be done with half the tubing.
Of course, the thinner walled 7/8″ PEX has a lower pressure and temperature rating than, say, a comparably sized, thicker walled, impossible to bend 1″ “standard” PEX. But fortunately, that doesn’t matter.
Why? Because the 180-degree, 100 psi rating of the “standard” PEX is absurdly over engineered for a radiant heating system. It’s ridiculous to use 180-degree water in a radiant system that functions most efficiently at 120 degrees. Remember, 180-degree boiler water is used in baseboard radiators because fin tube baseboards heat AIR. Radiant systems heat the MASS of the floor. Instead of strips of super hot baseboard units mounted along the wall, the entire floor becomes the heat source. As a result, significantly lower temperatures are used in the radiant tubing.
And the lower pressure rating of the 7/8″ PEX?
The best answer would be another question: Why would anybody pressurize ANY heating system to 100 psi? Baseboard radiators normally run about 15 psi. The same is true with “closed” radiant systems. An “open” radiant system runs at domestic water pressure, i.e. about 50 psi. Imagine taking a shower with TWICE that pressure. The sensation would be similar to standing under a sand blaster.
Besides, all ratings for PEX tubing are nothing more than “benchmarks”. A benchmark is a reference point that changes relative to the pressure and temperature. In other words, the pressure and temperature rating on any tubing slides up or down depending upon what temperature and pressure is being used. Tubing like 7/8″ Durapoly XL PEX is “benchmarked” at 120 degrees at 100 psi. If you raise the temperature to 180 degrees, the pressure rating drops down to 60 psi.
So, even though the benchmark rating is relative, it’s important because the manufacturer of the tubing certifies that the tubing can be subjected to the benchmark pressure and temperature (in this case, 120 degrees @ 100 psi) for FIFTY YEARS CONTINUOUSLY without any deterioration in its performance.
Needless to say, a NORMAL radiant system operates with 120 degrees @ around 50 psi. Tubing operated at this lower, (i.e. normal), pressure and temperature will last considerably longer than 50 years.
It should be obvious from the above explanation that a flexible, user friendly, thinner walled PEX like 7/8″ Durapoly XL is not only suitable for most radiant applications, but also preferable. An exception may be a zone with very short, tight joists, or an odd shaped building with a joist layout that constantly changes direction. In these instances, the smaller diameter ½” PEX would offer even greater flexibility than the 7/8″ PEX and should be used. Of course, you’ll have to use twice the amount of ½” tubing. But that can’t be helped.
So, what’s the best way to work with a building inspector who insists on applying “standard” PEX code guidelines to a “non-standard” PEX like 7/8″ Durapoly XL?
The following excerpt from the International Mechanical Code may be a helpful first step:
105.1 Alternative materials, methods, equipment and appliances.
The provisions of this code are not intended to prevent the installation of any material or to prohibit any method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material or method of construction shall be approved where the code official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety (emphasis ours).
Often, a building inspector has great latitude regarding his decision whether to approve or deny a permit. This feature is built into the Building Codes to allow inspectors to exercise their intelligence and good judgment and prevent them from becoming mere “rubber stamps” subservient to a blanket set of rules. Codes are well thought out guidelines. The thoughtful inspector visits the jobsite, assesses the situation, learns the facts, evaluates the materials in question, and rules according to his informed opinion.
Nobody in the construction industry expects every building inspector to be intimately familiar with every up to the minute development in the trades. If an inspector has never seen or evaluated 7/8″ PEX, for instance, it is the homeowner’s job to provide him with documentation from the tubing manufacturer outlining the pressure and temperature characteristics of the PEX (see downloadable PDF file below).
The 7/8″ PEX from Radiant Floor Company is designed to be part of an “engineered” radiant system. In other words, our systems run within the temperature and pressure guidelines of the 7/8″ PEX tubing and are not intended to perform under the extreme upper limit parameters of “standard” PEX. For that matter, even standard PEX should not operate at the upper limit parameters, because, as stated above, those standards are absurd and inefficient.
In addition, the homeowner should encourage the inspector to perform a “site test” on the 7/8″ PEX. Normally, this involves pressurizing the tubing to 100 psi or higher for a set period of time (usually 30 minutes to 3 hours). This should convince any reasonable inspector that the 7/8″ PEX is in absolutely no danger of exploding, or blowing up like a nickel balloon, or otherwise deforming in any way. If your inspector requires a pressure test for a floor joist installation, a simple to install, inexpensive, pressure test kit is available from Radiant Floor Company. For slab installations, the test kit is already built into the slab manifold.
As a side note, representatives from Radiant Floor Company attended a demonstration conducted in a laboratory testing facility. For this test, a 2 ft length of our 7/8″ PEX was submerged in a bath of 198-degree water, and then pressurized in ever-rising increments until it exploded at 215 psi! Every batch of tubing is subjected to this and other rigorous evaluations.
The building departments of most states recognize the difference between PEX tubing specifically engineered for radiant heating (7/8″ PEX) and tubing bought at the local hardware store designed for cold water delivery only (black Poly, garden hose). In nearly every case, an inspector will recognize the value, efficiency, and high quality of 7/8″ PEX and, as a result, will grant permit approval.