Figure 1. Image courtesy of Myson Inc.


About 20 years ago I helped remove an old cast iron radiator from our church. It was about 30 inches tall, 12 inches deep and at least 6 feet long. When I got there the unit was lying on the floor and simply needed to be “thrown” onto a waiting truck. 

Never having had a close encounter with a similar unit, I squatted down, grabbed hold of one leg, straightened my back, bent my knees and pushed hard with both legs. The radiator didn’t move one millimeter. 

Now I’m not exactly an Olympic lifter, but up until then, most things had moved at least a little when given similar physical effort. In the end, it took four men, several planks and some movers’ dollies to coax that monster onto the truck. 

I came away from that experience thinking that such radiators were doomed to extinction. How could something so big, unattractive and HEAVY ever survive in the modern hydronics heating market? 

I’m not the only one to share this opinion. With all due respect for those who install and service these hydronic heavyweights, most people think of a radiator as a clunky, visually unappealing “thing” that must be tolerated if one chooses not to be cold during winter.

Figure 2.

A New Era

Since that time, the concept of what constitutes a radiator in a modern hydronic system has changed considerably, largely through efforts by European manufacturers. Modern panel radiators are now turning the heads of discriminating owners as well as architects and interior designers. 

For many North Americans the first encounter with one of these modern panel radiators is one of disbelief. …That’s a radiator? 

This question is followed by curious amazement, especially if warm water is flowing through these modern meldings of technology and art. It doesn’t take long to discover that these are not your father’s radiators.

The market for European-style panel radiators continues to grow in North America.  There are now several companies offering a wide range of products from a “utilitarian” fluted panel to exquisite panels that are far more likely to be viewed as sculpture rather than heat emitters.

Panels are available in dozens of sizes, shapes and colors. Some project less than two inches from the wall. Others squeeze into the narrow wall spaces often present in kitchens and bathrooms - places where conventional fin-tube baseboard is out of the question. Many panel radiators can also be “accessorized” with towel bars, mirrors, garment knobs and boot racks.

We’ll start with a look at several common styles of panel radiators, and go on to discuss the benefits they offer. I hope to convince you that panel radiators are currently one of the most underutilized hydronic heat emitters on the North American market and an opportunity at hand for savvy suppliers.

Figure 3. Courtesy of Runtal North America

Fluted Panels

One of the most common types of panel radiators is manufactured from press-formed steel sheet. The forming process produces “flutes” or channels that join to a formed header at the top and bottom. The front and back sheets are welded together to create a sealed unit with lots of surface area yet relatively low water content. This portion of the radiator is called the water plate. Steel fins are attached to the rear side of this plate to enhance convective output. The completed assembly is pressure tested, chemically cleaned, and finished with high quality powder coat or enamel paint.Figure 1shows a typical installation of a fluted panel radiator.

For high heat output requirements, two single-water plate/fin assemblies are connected back to back. Common side panels and a top grill make for a clean, simple appearance. Many radiators of this type also have an integral flow-regulating valve in the upper right corner. This valve can be fitted with a non-electric thermostatic actuator to allow flow modulation through the radiator in response to room temperature. More on this later.

Figure 4.

Flat Pipes

Another common type of panel radiator is built using flat steel tubes arranged in parallel formation, and connected to headers at both ends. The tubes can be oriented horizontally or vertically. Units from 3 to 70 inches wide and up to 29.5 feet long can be manufactured, allowing for hundreds of possible sizes. Most panels of this type also have several optional locations for piping connections. 

Horizontally oriented panels are commonly installed under windows or low on exterior walls as shown inFigure 2. Panels with heights of 6 to 12 inches are installed as an alternative to fin-tube baseboard. They are equipped with steel fins on their rear side to enhance convection and are well-suited to counteracting drafts under large windows. Taller but narrower horizontal panels pack higher heat output into less horizontal wall space.

Vertical panels are great problem-solvers in tight confines such as bathrooms and kitchens where empty wall space is hard to come by (seeFigure 3). For example, a 12-inch wide by 7-foot tall panel has a heat output equivalent to about 7.5 feet of fin-tube baseboard, yet could easily fit on a narrow wall that’s too narrow for other uses.

Figure 5. Courtesy of Vasco

Round Tubes

Still another variety of panel radiator is the tubular towel warmer (seeFigure 4).  Again the concept is simple: A rack of horizontal steel tubes connected to vertical headers at each side.  By leaving out a few tubes here and there, a space is created for a towel to be draped over the panels. Special standoff mounts hold the panel about 3 inches away from the wall so the towel has space to drop down behind it.  Besides their obvious use in bathrooms, this type of radiator is great in entry foyers or mudrooms, where it quickly dries and warms coats, umbrellas and scarves.

One manufacturer offers tubular towel warmers in sizes from 20 inches wide by 30 inches tall, to 30 inches wide by 69 inches tall. Their heat output ranges from about 2000 to 7000 Btu/hr at an average water temperature of 170 ºF. The bottom of each header has 1/2 inch FPT connections. Most panels are mounted with a set of angle valves at the bottom of the headers. These nickel-plated brass valves allow flow through the panel to be regulated.  Because they are equipped with unions, they also allow the panel to be isolated and removed from the wall if necessary for painting. Simple, durable, and cozy - take all three.

Figure 6. Courtesy of Heat Line

I Can't Believe It's A Radiator

Beyond the basic pressed steel and tubular assemblies lies the realm of “designer radiators” - a seemingly endless variety of shapes, colors and accessories guaranteed to surprise and amaze those who think of radiators as ugly 600-pound chunks of cast iron. Perhaps even more amazing is the silent comfort these works of art deliver as warm water circulates through them. A few examples are shown inFigures 5-8.

Beyond their basic function as heat emitters, panel radiators have been creatively integrated as architectural elements. They have been used as room dividers, coat racks, benches and even stair railings.

Some panel radiators can be ordered with inconspicuous electric heating elements. They can operate as stand-alone electric towel warmers or as combination hydronic/electric units. This option allows for warm towels even if the boiler system or central heating plant is off during the summer.

Figure 7. Courtesy of Cordivari

Benefits Galore

One preeminent feature of panel radiators is their fast thermal response. Because they contain far less metal than their cast iron predecessors, and relatively little water, panel radiators can warm up quickly. Equally important, they can cool off quickly as internal heat gains develop in a room. 

This fast response allows panel radiators to “track” the required heat input requirements of a room far better than higher mass radiant floor panels. This concept is shown inFigure 9.

This characteristic makes panel radiators ideal for passive solar buildings where solar heat gains can change rapidly, and over a wide range. It also makes them perfect for room-by-room zoning. 

Another benefit of panel radiators is the ability to operate them with a large difference between supply and return water temperature. While radiant floor circuits serving rooms expected to have barefoot-friendly floors can operate with perhaps a 15 ºF temperature drop under design load conditions, panel radiators can operate with twice this temperature drop. This allows half the flow rate to carry the same amount of heat. Lower flow rates mean smaller piping and lower power circulators, both of which reduce first cost and life cycle operating cost.

Figure 8. Courtesy of Jaga

Piping Preferences

Even the most stylish radiator can’t deliver its anticipated thermal delights without proper piping. Although there are several ways a group of panel radiators can be piped into a system, I strongly advocate use of homerun distribution systems. In this approach, each radiator has its own supply and return tube that originates from a manifold station as shown inFigure 10.

This piping method offers several benefits. First, the size of PEX or PEX-AL-PEX tubing runs to each panel radiator seldom needs to be larger than 1/2-inch I.D.  In many situations, 3/8-inch tubing can be used. The underlying reason is the higher temperature drop that the panel can operate with.

Small diameter PEX or PEX-AL-PEX tubing is easily routed through framing cavities. If you can pull an electrical cable from point A to point B chances are you can also pull through a length of 3/8-inch PEX or PEX-AL-PEX tubing. This makes the homerun approach ideal for retrofit jobs where framing cavities usually have limited access. This is also a great place to use up shorter segments of tubing left over from a radiant floor installation.

Figure 9.

Homerun distribution systems also deliver thesame water temperatureto each panel. The size of the panels doesn’t have to be adjusted to compensate for supply temperature drops as it does with series or diverter type systems. If the supply water temperature is adjusted by outdoor reset, all panels benefit equally.

Finally, homerun systems used in combination with thermostatic valves at each panel radiator are ideally suited for the next generation of pressure-regulated circulators now gaining a foothold in the North American market. Each time a thermostatic valve adjusts flow in a panel the circulator senses the attempted change in system pressure drop, and quickly reacts by changing its speed to cancel out the change. With this approach there is no need of a differential pressure bypass valve in the system. The savings associated with eliminating this valve goes a long way toward covering the higher cost of the pressure-regulated circulator. In short, it’s a marriage made in hydronics heaven.

The market for panel radiators in all their various proportions will continue to grow in North America.  Wholesalers are in an ideal position to communicate the benefits of these products, and allow them to “sell themselves” through operating display panels in showrooms.

Towel warmer panel radiators are the perfect complement to higher end bathroom suites.They are rugged enough for commercial and rental properties, yet elegant enough to turn heads in high-end custom homes. Their compact form, quality and luxurious warmth convey the essence of modern hydronics.

Figure 10.

Installation Pointers

Here are a few other things to keep in mind when working with panel radiators:

  • When installing long horizontal panels allow for thermal expansion. A rule of thumb is to allow approximately 1/8 inch of expansion per foot of length for steel panel operating with water temperatures up to 180 ºF.  Be sure the attached piping can move without scrubbing against adjacent materials. Use escutcheon plates to cover oversized piping holes.

  • Use piping details that allow the panel to be temporarily removed when walls need painting. Most panel radiator suppliers offer isolation valves with integral unions that make this a simple task. Another possibility is to allow some “slack” in the PEX or PEX-AL-PEX tubing supplying the panel that allows it to be lifted an inch or two to release it from its wall brackets.

  • Most panel radiators are hung from steel wall brackets. These clips should be attached to either solid framing or masonry. Do not support the panels on drywall. In many situations this requires blocking to be installed before the walls are closed in.

  • Panel radiators release heat through convection and radiation. Panels with rear side fins will favor convection. Panels with large surface areas favor radiant output. Operating the panel at lower water temperatures increases the percentage of heat output by thermal radiation. It also increases the size of the panel needed for a given heat output.