Never having had a close encounter with a similar unit, I squatted down, grabbed hold of one leg, straightened my back, and pushed hard with both legs. That 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 mover's dollies to slowly coax that beast onto the truck. By now it must be the anchor for an aircraft carrier.

I came away thinking these behemoths were doomed to extinction. How could something so big, so unattractive and so heavy ever survive in the modern comfort heating market?

I'm not the only one to share this view. 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 aesthetically tolerated if one chooses not to freeze during winter.

Fortunately, this image is being rapidly transformed. Rather than being seen as an unavoidable necessity, modern panel radiators are turning the heads of discriminating owners as well as architects and interior designers.

For many North Americans the first encounter is one of disbelief.

"?hat's a radiator?"

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

From Europe With Warmth

Most manufacturers offer dozens of sizes, shapes and colors. Some models project less than 2 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 also can be "accessorized" with towel bars, mirrors, garment knobs and boot racks.

Figure 1

Fluted Panels

For high output requirements, two such panels are often 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 nonelectric thermostatic actuator to allow flow modulation through the radiator in response to room temperature.

Figure 2

Flat Pipes

As with fluted panels, these panels have lots of surface area and low water content. A typical 18-inch by 6-foot. panel holds less than 2 gallons of water. With the proper controls, these low thermal mass panels can respond quickly to internal heat gains or desired changes in room temperature.

Horizontally oriented panels are commonly installed under windows or low on exterior walls. Panels with heights of 6 inches 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. For example, a 12-inch wide, 7-foot tall panel has a heat output equivalent to about 7 1/2 feet of fin-tube baseboard, yet could easily fit on a narrow wall that's too small for other uses. (See Figure 2.)

Figure 3

Round Tubes

One manufacturer offers tubular towel warmers in sizes from 20 inches by 30 inches to 30 inches by 6 inches. Their heat output ranges from about 2,000 to 7,000 Btu/hr. at an average water temperature of 170 degrees F. 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 4

I Can't Believe It's A Radiator

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.

For those who don't start every shopping jaunt examining price tags, towel warmers are even available with 24-karat gold plating. You can even get gold-plated valves to match. Just be careful with those pipe wrenches!

Figure 5

Piping Matters

One approach is a simple series circuit. Although arguably the simplest to install, series circuits have significant limitations. Chief among them is the lack of individual heat output control from each radiator. Series circuits should only be considered for areas that can be controlled as a single zone. Avoid series circuits if one or more of the radiators is located in a room with high internal heat gains relative to the others. You can change the water temperature and you can change the flow rate, but the change always effects the entire series circuit rather than a single panel.

Another potential limitation of series circuits is high head loss. The individual head losses of all radiators, valves and interconnecting piping add together in a series circuit. Too many panels in series will lead to high head loss and low flow rates. The result will be cool panels near the end of the circuit.

Series circuits containing several radiators must also be designed to accommodate the drop in water temperature from one heat emitter to the next. In most cases, the size of downstream radiators must be increased to compensate for the lower water temperatures.

All things considered, the limitations imposed by series circuits typically outweigh their ease of installation.

Figure 6

Diverter Tees

A schematic of diverter tee piping is shown in Figure 7. The diverter tee is typically mounted as the return tee reconnecting the branch riser to the main. The tapered venturi in the tee creates a pressure differential that pulls water through the panel radiator as the TRV opens. As the room approaches the set comfort level, the TRV slowly closes off flow through the panel. It's a simple, silent and fully modulating approach to comfort control.

If TRVs are used on each panel, the distribution circulator must operate continuously during the heating season. This can be done several ways. If the system has a boiler reset control (which I highly recommend), the distribution circulator can usually be turned on by that control whenever the outdoor air drops below a preset warm weather shut-down temperature. A similar option is to use a separate setpoint control to turn on the circulator and allow the heat source to operate below a given outdoor temperature. A manual switch is also a possibility, although it is not automatic.

With nearly continuous circulation during the heating season, it's important to insulate the distribution piping with this type of system.

Figure 7

Bypass Valves

Bypass valves make it possible to string several panel radiators together in what appears to be a simple series circuit, yet retain individual heat output control.

Some panels have an integral bypass chamber built into the lower portion of the panel that allows similar piping and control using TRVs.

Figure 8

To Each Its Own

This approach offers some nice benefits. First, the size of the tubing runs to each panel seldom needs to be larger than 1/2-inch I.D. In many situations, 3/8-inch or even 1/4-inch nominal I.D. tubing can be used. The reason is that panel radiators can operate with a temperature drop of 30 to 40 degrees F. High temperature drops mean low flow rates. For example, a panel delivering 5,000 Btu/hr. with 170 degree F inlet and 130 degree F outlet water temperature only needs about 0.25 gpm of flow. This could easily be handled by a 1/4-inch I.D. (10 mm O.D.) tube.

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 those shorter segments of tubing left over from a radiant floor installation.

Homerun systems also deliver the same water temperature to 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 reset, all panels benefit equally.

Homerun systems also allow accurate heat output adjustment of each panel using the balancing valves on the manifold station. When individual room temperature control is desired, each radiator can be fitted with a TRV. This approach is ideal for situations where some rooms might experience high internal heat gains at various times of the day. Another alternative is to install low voltage valve actuators on the manifold and wire them to individual room thermostats. The end switches of the actuators are wired to operate the circulator and heat source when any one or more of the thermostats call for heat.

Figure 9

Some Other Pointers

• 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 panels operating with water temperatures up to 180 degrees F. 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. A suggested detail is shown in Figure 9.

• Most panel radiators are hung from steel wall clips. 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.