It’s sometimes easy to think of a radiant manifold as simply a dumb box. One stream of water or other fluid goes in and gets directed to several other areas. Choosing the right radiant manifold isn’t quite as easy as one who holds the belief that manifolds are just dumb mixing blocks may think.

The first thing a contractor may want to consider when shopping for a manifold is to have a plausible layout for the system already in hand. Gary Runyan, Zurn PEX manager of product development and engineering, says this factor can be critical. Zurn PEX makes three manifolds for radiant heat: a stainless-steel model, the QickZone modular manifold and the AccuFlow preassembled brass manifold.

“I think he first should consider the loop layouts and, therefore, whether it’s better to have one manifold with all of your loops running to a single spot or is it better to have two smaller manifolds,” Runyan says. “All jobs are different, so it’s difficult to give rules-of-thumb. If your manifold is located in the center of your home, trying to get all the pipe to come together there could create some excessive heat in that location.”

Jeff Halter, sales manager at Mr. PEX, maker of stainless-steel and brass manifolds for the radiant market, agreed that an idea of the number of loops, or circuits, required for the project is a good place to start.

“That would be determined by the square footage of the room,” Halter says. “If you have a 1,000-sq.-ft. room, you don’t go over 300 ft. long on a 1/2-in. loop. That’s the industry standard for residential, anyway. If you’re doing 12-in. on center, that’s about 1,000 ft. of pipe, plus you have your leader length where you go from that room to the manifolding.”

He adds that a 1,000-sq.-ft. room might take four 250-ft. loops, so you don’t go over that 300 ft. rule-of-thumb. Sizing manifolds is a fairly simple exercise, however, because there aren’t many appropriate sizes to choose from.

“A manifold is a good thing to always think about when you’re in a multiple zone situation. For sizing, generally one size fits all applications,” says Woody Dickenson, Caleffi product manager. Caleffi offers the TwistFlow manifolds for radiant applications.

 “And what I’m talking about is the entrance from the primary source of heat. In our catalog, that would be either 1 or 1 1/4 in.,” Dickenson says. “The other thing to be leery of is the feet per second of velocity to make sure you don’t erode anything. A guy is going to know — or should know — what his heat load is. From Btu he can back down to flow rate and that is usually somewhere from one to five gal. per min.

Price is an important consideration, as is the type of material you choose, he adds. Also consider the number of circuits you’ll require for your project.

“TwistFlow, for example, can be had in three circuits up to 13 circuits,” he says. “You can buy them in pieces so you have to put them together or, with today’s modern manufacturing, many major manifold manufacturers are offering them with the supply manifold connected with a bracket so they can be easily mounted on walls. You buy it preassembled from the factory.”


Balancing capability — do you need it?

“Yes you do, especially if you have a lot of different sized loops,” Halter says. “In the first example I talked about there were four 250-ft. loops. But if you had a 100-ft. loop, a 250-ft. loop, a 300-ft. loop and a 350-ft. loop, you would need balancing.”

Runyan agreed: “I think it’s best if you have it. Try as you might, you are going to have different tubing lengths in your loops. In most situations, you really want to use a manifold that has balancing valves on it.”

Is the integral balancing valve the newest thing in manifolds on suppliers’ shelves?

 “Pretty much,” Dickenson says. “The science of manifolds has flattened out to where there’s not going to be too much newfangled you can do with it. The first thing I did with Caleffi when I started five years ago was to help introduce the TwistFlow to the marketplace. It has a flow meter/balancing valve incorporated into the high side so you can actually turn it and control your flow rate.”

Halter offered a practical example from the real world where all your loops aren’t going to be the same size.

“For example, in a 1,000-sq.-ft. garage, do you need manifolds with balancing valves on them?” he asks. “I’m a big believer in them because they help purge the air out of the system and you can do it per loop.”

There are a few other details to consider before running to the supply house and plunking down cash for a manifold. As noted earlier, different body diameters come into play, Runyan says.

“Some are 1 in. and some are 1 1/4 in. The larger ones can accommodate a larger number of loops, so you need to make sure that you’re not trying to put too many loops together for a given size,” he explains. “Most manufacturers will sell preassembled manifolds in 2 through 12 loops and a given body diameter. If you want more than 12 loops on a manifold, then you have to go to 1 1/4-in. body diameter. Those go up to 16 loops or something like that. Just make sure you have a big enough manifold.

“You want to consider valves so you can isolate the manifold so you can do any service you might need to do. Also air vents — you have to have a convenient way of bleeding the air out of the system. It’s possible to do it by loosening the piping connection but that’s a messy way of doing it. It’s a good idea to have bleed valves so you can purge the air out of the system.”

Still another factor to think about is the fluid you’re pumping through the circuit, Dickenson notes. A glycol-and-water mixture will have to be controlled, as too much glycol can wreck a manifold.

“If it’s water- or glycol-based coolant, he’s going to need to know what level of glycol he’s going to be using in there because there’s going to be some limit to it,” he says. “A lot of our products have a 50% glycol limit. The other thing is the construction material. For example, radiant applications generally use a lower temperature. In that case, sometimes people can lessen the costs of using manifolds by utilizing a composite or a plastic-bodied manifold.”

Other features of today’s manifolds include integral flow meters so you can adjust the flow per loop. Some contractors may argue against doing heat loss calculations per room, but Halter says it’s important.

“I still think it’s pretty crucial to do a heat loss calculation by room when it comes to radiant,” he notes. “A lot of contractors would argue with that and I’m not saying the way they do it won’t work, I’m saying why not get the system to work better? If I look at all of the jobs I’ve been on because people are just doing it their own way and kind of rule-of-thumbing it, pumps tend to be oversized. Nowadays that’s not as big a deal because of all the smart pumps out there. Manifolds like our stainless steel have isolators so you can turn them on and off. We also have on/off valves on the return side, but that’s just so we can opt to put on actuators.”

All in all, these simple-looking devices carry with them many considerations and features. It’s not just a matter of diddy-bopping down to the supply house and buying a unit. Some thought goes into it.

“Today’s manifolds represent the cumulative experience of radiant heating people working and refining the way they put in systems,” Runyan says, noting that there may still be some room for evolution in manifoldry.

“Most radiant manifolds, at least in the United States, are metal,” he said. “As in all water-handling systems, there seems to be a trend toward plastics and composites. I know there are plastic manifolds in Europe and a few people have them here in the United States. I don’t think they’ve been widely accepted yet, but that doesn’t mean the right design won’t come along and change that. It’ll probably wind up having many of the same features as currently exist on the metal manifolds.”


This originally appeared in the September 2013 issue of sister publication Reeves Journal.