This column describes some issues related to hydronic heating for which answers remain nonspecific.

This month’s column departs from the usual focus on a specific technical topic. Instead it describes some issues related to hydronic heating for which answers remain nonspecific. The beginning of a New Year seems an appropriate time to “broadcast” these concerns in hopes that 1997 will see some or all of them rectified. The incentive, as always, is to provide the best possible comfort and quality in the heating systems we provide our customers.

1. Use of wood flooring over heated floors. Inquiries on this topic come up almost daily at my office. I respond by describing the different approaches I have either seen, read about or discussed with others:

  • Laminated/prefinished hardwood strip flooring fully adhered to a concrete slab.
  • Solid-sawn flooring nailed down to wood sleepers with tubing and concrete in between.
  • Solid-sawn hardwood nailed down to one or two layers of plywood installed over a heated slab.
  • Floating floors that are not fastened down to the floor.

Here are some of the concerns: Does the floor heating system have to be operated for several days before the wood flooring is installed, especially if the slab has been under cover for several months? Will the moisture driven out of the slab when the heat is turned on affect the wood flooring, its substrate or adhesive?

Let’s be realistic. Many tightly scheduled construction projects won’t necessarily have their floor heating systems operational by the time the finish flooring is scheduled for installation. Many projects don’t have the luxury of time that allows wood flooring to be laid out on the floor to acclimate for three weeks prior to installation, as suggested by one industry source. And how many projects can simply substitute quarter-sawn wood flooring, which can easily cost one and a half to two times that of plane-sawn flooring, because it has a better chance of remaining flat over underfloor heating?

Are you, as a floor heating installer willing to risk there won’t be problems when $15,000 worth of hardwood is installed over your system? Especially if you realize the flooring’s warranty often sets a temperature limitation to 80-85 degrees as several do. And by the way, does the flooring manufacturer intend that temperature to apply at the top surface of the wood or the top of the slab? A temperature difference of 10-15 degrees or more can exist across the thickness of the wood depending on its thermal conductivity, thickness, and the intended upward heat flux from the floor. If the top of the wood is at 80 degrees and the bottom at 95 degrees, is the product still operating within warranty?

Who Knows? Responses to my inquiries at several wood flooring manufacturers and retailers who handle their products have been mixed. I recently had a rep tell me the floor adhesive they normally use will “turn to jelly” above 85 degrees. I had another tell me there is no temperature difference between the top and bottom of wood flooring when installed over floor heating — he’s apparently discovered a way around the laws of physics. Another assured me his product had been successfully used in many floor heating installations, but then clearly stated the warranty would not apply if the wood was heated above 80 degrees (not 85).

Whose responsibility is it to provide the answers? I feel the wood flooring industry should take the lead, with direction and support from the floor heating industry, perhaps coordinated through the Radiant Panel Association. The flooring is, after all, the product that “fails” if not properly selected and installed over a heated floor. It’s also the product that stands to be passed by in favor of more predictable finish flooring if questions about its suitability in heated floor applications remain unresolved.

I’m convinced there are several viable methods for successfully installing wood flooring over radiant floor heating. But we can’t afford to be the guinea pigs while some flooring manufacturers hide behind warranties that are unrealistic, and, not surprisingly, provide a way out if a problem develops. Let’s push for some answers, and the development of a comprehensive design/installation standard for wood flooring over heated floors. Better to find the answers in a laboratory than a court of law.

2. The effectiveness of radiant insulation within floor heating systems. Most of us who work with radiant floor heating know the benefits of radiant heat output. But do we fully understand the radiant barrier insulation products that may be part of such systems? In principle a radiant barrier reflects a significant portion of the infrared heat that impinges on it back toward the surface it faces.

But what effect does accumulated dust have on the product’s reflectivity over time? Have you ever cut into an enclosed floor joist cavity that was not dusty? How much does a typical 5-, 10- or 15-year coating of dust reduce the insulation ability of such a radiant barrier? Should such a radiant barrier only be part of the underside insulation along with another material? What is the optimal air gap between the radiant barrier and the heated surface it faces? What is the cost effectiveness of a radiant barrier insulation product vs. other materials such as fiberglass in underfloor applications?

3. Thermal traps at primary/secondary piping connection. The use of primary/secondary piping is a very effective means of coupling independent hydronic distribution circuits into a common hot water supply system. But when a secondary circuit connects to the top of the primary piping as shown in Figure 1, hot water can migrate upward into the secondary circuit during its off cycle. This can, for example, cause undesirable space heating in a multiload system whenever the primary circuit is operating to service a domestic water heating load. This so-called “ghost flow” in some secondary circuits has also been known to overheat domestic water heating tanks to the point that their relief valves open.

One solution is to install a weighted or spring-loaded check valve in both the supply and return secondary risers when attached to the top of the primary piping as shown in Figure 1. This however, adds to the cost of the system, and can dissipate a significant portion of the secondary circulator’s head when operating.

Another approach that could potentially eliminate the need for check valves, and conserve circulator head is to “undersling” the secondary piping, thus creating a thermal trap (also shown in Figure 1). This concept is based on the fact that hot water in the primary pipe is less dense than the cooler water in the downward-routed secondary risers, and thus doesn’t “want” to flow downward.

Sounds Too Good: But a couple of things can affect how well it works in a specific system. One is the pressure drop between the primary/secondary tees. We’d like to think it’s zero, but in reality, there is some small pressure drop between these tees. The greater it is, the greater the driving force trying to push hot water downward in the upstream riser of the secondary circuit.

What is the minimum vertical drop of such a thermal trap to ensure ghost flow will not develop? I would expect the answer depends on water temperature, tee spacing and the pipe sizes/lengths involved. Some industry references suggest that one foot is adequate, but I have seen ghost flow develop in systems with more than a foot of thermal trap in the secondary piping. Would a new specialty fitting — let’s call it a “primary/secondary tee” — with two closely spaced side ports, and a very smoothly contoured inner surface help prevent this ghost flow? How about some kind of thermal breaks at the connection to the secondary circuit that could reduce the undesirable heat conduction along copper tubing? Anybody out there want to research this, or maybe even create a new product?

4. Water quality in hydronic systems. We routinely test ground water for various impurities before we consider drinking it. But how often do we analyze the same water before we fill our hydronic systems? Are we always confident the water is not carrying with it chemicals, minerals or particulate that might eventually damage circulator bushings, jam mixing valves, build up as scale on boiler sections or cause dezincification of brass valves?

Several means of corrosion control are now routinely applied in modern hydronic systems. They include oxygen diffusion barriers on polymer tubing, microbubble collection and elimination devices, dielectric unions and the use of antifreezes with corrosion inhibitors.

But even with all these measures in place, how does the installer know when the water onsite is suitable for use in the system? Our industry needs to examine a wide variety of component failures caused by various forms of poor water quality, identify the exact cause of failure and then develop a standard for acceptable water quality, as well as a means of checking and even correcting it on– site. I’m sure various manufacturers of hydronic heating components already have some experience and data in these areas. Let’s share it for the common good of the industry.

An Invitation: Since I’m posing all these questions, it’s only fair that I serve as the “bulletin board” for your input, experiences and solutions. Rest assured I’ll share the results with all readers in future columns. Please feel free to fax your comments to me at 315/865-8903 or e-mail me at

In the mean time have a great New Year.