Boilers come in all shapes and sizes—from mini units designed to heat small spaces to monsters that warm up vast areas, like the nine 9.6 million BTU boilers installed in the Anchorage International Airport’s Concourse C, a massive structure with acres of glass. 
   “There’s been a lot of technology change in the last few years,” said Ed Nordstrom, Western regional sales manager for Heat Transfer Products Inc., in East Freetown, Mass. “The advent of condensing boilers, boilers that have modulating burners, boiler controls that are weather-responsive, different pumping and circulation techniques like variable speed pumps—all of these have come into play, options that didn’t exist in the U.S. market 10 or 15 years ago. Knowing how to use these products correctly is important.”

Burnham Hydronics’ Alpine mod/con boilers are available in seven sizes that are stackable for modular applications. Photo courtesy of Burnham Hydronics.

On the residential side, in the West and Northwest, “what we’re seeing a lot of is more of a whole-house approach,” said John Sparks, the Western region technical and training manager for Michigan City, Ind.-based Weil McLain, the company responsible for producing those massive Anchorage airport boilers. 
   “Our boiler becomes part of the energy savings in the home,” Sparks said. “They’ll take our boiler and an indirect water heater and use the modulating/condensing unit year-round rather than just in the heating season.”
   There’s often a blurring of the lines between what boilers are installed in residential and commercial radiant products, said Mike Juhnke, director of product management for Lochinvar, the Lebanon, Tenn. manufacturer of Knight boilers. 
   “What’s been considered a commercial boiler is being installed in residential projects and vice versa,” Juhnke said. “We have large homes in Colorado, for example, where there are commercial boilers installed. A large commercial boiler might be installed in a whole-house scheme rather than linked, stacked or teamed residential boilers,” he said. 
   By industry definition anything less than 300,000 BTU is deemed a residential boiler, but that doesn’t mean someone’s large home doesn’t have a 400,000 BTU boiler, said Ken Niemi, marketing manager, Burnham Hydronics, Lancaster, Pa. “You can have a smaller BTU boiler in a doctor’s office, for example; so, it becomes a commercial application for a residential boiler.”

Heat Transfer Mod/Con boilers can be stacked to double capacity while maintaining a small footprint. Photo Courtesy of Heat Transfer Products.

Even the fuel-oil heavy Northeast may be changing its ways when it comes to space heating. “We’ve seen a large shift due to the price difference between fuel oil and natural gas,” said Mark Avron, Buderus’ regional sales manager for New England based in Londonderry, N.H. “Last heating season you were talking twice the cost per BTU for oil versus natural gas, so a lot of conversions happened. I think all manufacturers have seen a large demand for the mod/con boilers to the extent that manufacturers are having trouble keeping up with the demand.
   The surge in residential radiant heating has come full circle for Los Angeles-based Raypak Inc., which started putting together radiant packages, hence the company name, in 1948. 
   “The company was formed to better address a then-new and fast-growing market for high end residences in Southern California,” namely radiant heating for the large homes of the Hollywood crowd—according to Raypak marketing consultant Stan Young and technical consultant Larry Ashton. Film stars are among the customers for today’s radiant systems, but now they service second and third homes in mountain areas where they do everything from heat the lodge to melt snow.
   “Raypak’s innovation for hydronic systems was the adoption of copper fin tube waterways, which afforded nine times the heat transfer coefficient of the smooth cast iron or steel in use until then,” Young said. The capability of copper fin to operate with return water temperatures as low as 105 degrees Fahrenheit without condensing, to modulate down to that level, and then be shut off and restarted instantly without thermal shock, were ideal characteristics to further enhance the comfort and energy savings of radiant heating.
   “Later, these same copper fin tubes were used to advantage for pool heating and snow melting,” Young said. “Today copper fin waterways are still particularly adaptable to low system temperature radiant systems, both commercial and residential. With the introduction of Hi Efficiency boilers, they allow true 90 percent or more condensing efficiencies under normal system operations. This results in radiant installations being able to offer their acknowledged high comfort levels with economics more comparable to condensing forced air systems. This benefits the owner, the installer, and the ability of the radiant heating industry to grow in both the commercial and residential marketplace.”

The wall-mounted Knight mod/con boiler from Lochinvar features Smart System operating controls, night setback, and password security. Photo courtesy of Lochinvar.

Matching Boiler to the Job
Efficiency and energy savings in radiant installations can be enhanced by how boilers are sized and linked. Typically, when a boiler is designed, it’s designed for the worst-case condition—the single day it’s the coldest ever, Niemi said. Ninety-nine chances out of 100, it’s never that cold day. So, consequently, in the fall and spring you don’t need all those BTUs of heat. 
   “If you have one big boiler and you have to fire it, that’s like taking a car and say I’m going to give you an option, you can go 150 or you can sit still,” Niemi said. “What if I had five cars and each one could do 30 miles per hour. I can still get to my 150 mph multiplying those.” 
   Most of the time you only need a small part of that boiler, Niemi said, so you can run smaller boilers, stage them, exercise each one—run one, then the next one, then the next one—and get better utilization and longer life out of them and far more efficient fuel usage. “Multiple boilers are very big in any commercial application; it creates redundancy and it also creates efficiency.”
   A commercial project like a hotel might cascade a series of six larger residential boilers, which come on in response to demand, Juhnke said. The energy savings there comes because, rather than having a large unit running all the time, the linked equipment comes on to meet the immediate demand, from one boiler to every link in the chain.
   “As an engineer you have to design for the coldest days. If you choose one big boiler, you’re always going to have too much boiler for the other days of the year. Instead, let’s say I put in four 500,000 BTU mod/con boilers,” Juhnke said. “Because I cascade, the single boiler can go down to 100,000 BTUs, so you have a range from 100,000 to 2 million BTUS available to run that building. By cascading a series of boilers, you’re not wasting energy and it extends the life of the equipment.”
   In residential applications, Sparks says probably 80 percent of radiant systems are going into new construction. “I think technicians have grasped the concept of low-temperature heating and they’re running with it. I’ve seen some very creative applications out there. With that said, the two main applications are space heating and snow melting. For the typical installation, as it relates to the boiler or power plant, I see the trend going to higher-efficiency units—whether it’s a mid-efficiency or a mod/con in multiple boiler systems even in the residential applications.”
   There seems to be a point where it does make sense to do a multiple-boiler installation in a residential structure, Sparks said, around 200,000 BTUs in output. Determining factors include the size and location of the structure. “Obviously, the farther north we go, the larger the heating load hours, so there’ll be a higher demand. It’s all load dependent, but in a residential structure, typically the heating load is not that large; in fact we’re sizing boilers to pick up the domestic hot water load which far exceeds the heating load of the structure. 
   “A typical installation might have a heating load of 80,000 to 100,000 BTUs, but a domestic hot water requirement of anywhere from 150,000 to 300,000, depending upon the type of application.” One powerplant can be utilized to heat the space, heat water for both and kitchen use, heat the pool or hot tub and melt snow. And that is where the modulating/condensing boiler really comes into play. 
   Sparks cites a condominium project in the Aspen area where price tags on a 1,400-square-foot condo are in the $2.5 million neighborhood—“very high-end, very exclusive, real attention to detail on the structures. They could have a soaking whirlpool bath tub that might have a flow rate on the fixture of between 10 and 20 GPM and showers in the master bathroom suite with multiple shower heads and a 20 GPM flow rate.”
   This is a perfect example of why you would want to use multiple boiler systems, Sparks said: “If we are radiant heating the structure, I might have four small zones, the low as low as 20,000 BTUs, yet my domestic load could be somewhere around 400,000 BTUs” he said. “There would we turn on two large boilers—in the neighborhood of 250,000 BTU gross input. When they’re filling that tub, the worst phone call in the world is ‘I don’t have enough hot water’.” 
   

The Ultra Series 2 gas boilers from Weil-McLain are designed to operate in low temperature condensing applications. Photo courtesy of Weil-McLain.

The Shift to Mod/Con
Modulating/condensing boilers and radiant heating seem an ideal match. Let’s say your house is 40 F, and you need a certain amount of heat. Your furnace is going to come on, heat, shut off, come on heat, shut off, because probably it’s capable of providing much more heat than what you need, Juhnke said. If it drops down to zero or 10 F, the furnace is going to come on for quite a long period of time, keeping your house steadily heated. 
   “That’s what we’re essentially doing with our modulating/condensing products. But we’re not providing a set amount of heat; what we’re doing is measuring the amount of heat that’s needed and adjusting our piece of equipment to put out that amount of heat,” Juhnke said. “We’re not turning our machinery on and off. As you know with any mechanical device, turning it on and off is what puts the wear and tear on the product, heating it up, cooling it off expansion and contraction.”
   The condensing part of it—from an energy efficiency standpoint—“we’re just sucking every possible bit of energy. Anytime you burn something there is latent heat in that exhaust. In a mod/con boiler, we convert the steam to water and extract that extra 10 percent of energy.”
   A big reason why mod/cons are well suited to radiant is “that, in a lot of cases, if you are just running a single water temperature with radiant you can modulate the water temperature in the boiler,” Avron said. “Most of these boilers use outdoor reset controls as standard and you can actually set a heating curve on the boiler control itself and eliminate external mixing valves, external controls, so you simplify the installation.” 
   When you’re talking about radiant heating systems, you’re talking about return water temperatures which can be anywhere on the low side between 80- to 130 F, Avron explained. Cast iron boilers face a couple of problems with water temperature that low. One issue is the risk of condensation of flue gases and the other is thermal shock or a fracturing of the iron.
   “With conventional cast iron boilers, you really are required to add additional piping and controlling to prevent that from happening,” Avron said. “When you look at it, if you were to take a conventional cast iron boiler and add the piping and controlling required to safely handle a radiant system, it’s actually cheaper to install a modulating/condensing boiler, and you end up with a more efficient system,” he said.
   If you want super-high efficiency, absolutely you’re talking modulating/condensing boilers, Niemi agreed. “A lot of cast iron boilers are EnergyStar-rated; that means they are 85 percent efficient or greater. If you’ve got a 40-year-old boiler in your house, you’d be lucky if it was 60 percent efficient because that was the design of those boilers back then. A typical good cast iron boiler today will be between 85 and 87 percent. If you want to get into the 95 percent range, you have to get into a modulating/condensing boiler. And that’s simply because of pure physics. The way that you get efficiency is to keep the heat in the house, not up the chimney.”
   Sparks makes a comparison to cast iron boilers—“which still work very well, by the way”—that have a single fixed-rate gas control. “The typical load on an average structure is about 100,000 BTU output per hour net load,” he said. “When the boiler fires up, it’s firing 100,000 BTUs all the time, every minute that it’s firing. With the modern condensing boiler that has a modulating combustion system, the turndown range is typically 4 or 5 to 1 (the difference between high fire and low fire). If my high-fire output is 100,000 BTUs, if I have a 5 to 1 turndown, the minimum I could respond to is a 20,000 BTU load. That greatly reduced energy consumption and extends the life of the equipment.”

Out West in general, with the cost of construction escalating on an annual basis, there’s certainly justification for hydronic systems versus forced-air systems based on their inherent efficiency, Sparks added. “The cooler we run the water temperatures from the power plant (the boiler) the more efficient the system becomes,” he said. “If we look at the modulating boiler that can pick up load-specific demands, various firing rates depending upon the heat demand, and also vary the water temperature, we have a really efficient system. Outside of the efficiency—which is a tremendous advantage, I can’t underscore that enough—the biggest advantage I think a radiant system offers is comfort. The next big advantage of hydronics is the ability to target individual zones.”
   Boilers play a role even if an alternative energy source—like solar or geothermal—is the primary heat source for a radiant system, Ashton noted. “They generally never cover 100 percent of the load on peak days. Because of the upfront costs, systems may be sized to carry a majority of the load and then back it up. That could be electric or a radiant heating boiler. The energy sources thought of as ‘non-deplete-able’ almost always have backup.”
   Nordstrom agreed. “One trend we’ve seen is the interest of tying solar into the heating system. You can accumulate the heat in a storage tank. There are controls that allow priorities for what is going to heat what. If there is a need for heat in the accumulation tank you can either heat it first with the solar, and if that is not available you heat it with a boiler. You always need a backup because, obviously, you get very little solar radiation at night and you get less solar radiation on a cold winter day because of shorter daylight time periods.” When solar isn’t enough, a boiler takes up the slack.
   A lot of contractors deal with the same products all the time because they understand what they’ve experienced in terms of reliability and ease of installation, Nordstrom added. 
   “I think that the thing most contractors should look at more is efficiency and comfort of the ultimate user or homeowner. People are buying radiant floor systems because they want comfort, and comfort isn’t something that’s quantifiable. But that’s where manufacturers can come into play, helping the contractor to design good systems.”

The Xtherm commercial boilers, available in three sizes from Raypak, are the predecessors of a pair of high-efficiency residential boilers coming to market from the company during the first quarter of 2009. Photo courtesy of Raypak.