Gas modulation systems have evolved in the last 20 years, spreading in use from the large-scale boilers of a couple decades ago to applications today such as residential furnaces, commercial food service equipment, water heaters and other gas appliances.

Some modulating systems modulate gas only, but more efficient systems modulate both air and gas that is inputted to a gas-fired appliance. Variable control of gas flow can be accomplished by means of a multi-position valve or pressure regulator to control the flow of gas into the combustion chamber, or by using a variable-speed premix blower to regulate the air/gas mixture headed to the burner.

By using modulation, the unit stays on for a greater length of time, but at a lower firing rate, reducing the number of on/off cycles and reducing the amount of time the burner spends firing at full rate. This is unlike traditional gas-fired systems that are either 100 percent on or 100 percent off.

The core concept is already well established in large-scale commercial heating applications. Now, the roster of modulating appliances includes commercial foodservice appliances and residential appliances such as gas furnaces, water heaters, cooking equipment, and even fireplaces. This short, and certainly incomplete list is most likely to grow as the benefits of modulation such as improved performance, precise control, better energy efficiency, lower emissions and consumer’s personal comfort in heating applications, all begin to outweigh the drawbacks of higher cost and complexity.

Burner for steam generator. Source: GTI

The technology migration may be sped along as the experience from the  earlier applications provides a knowledge base from which to evolve. “It isn’t a great leap like it was for other technologies such as going from direct-spark ignition to hot-surface ignition systems,” says Denny Gambiana, vice president at Varidigm Corp., Plymouth, Minn., a manufacturer of burner controls. “Those were different technologies that had to be proven throughout the industry. Modulation had already existed on boilers and big systems for decades, so the change should be more intuitive.”

There are two approaches to modulation — multi-stage and variable modulation. For multi-stage products, which some experts say is not “true” modulation, the unit may have one or more fixed levels of output that are less than full. For example, an appliance that can operate at either 70 percent or 100 percent, can often spend most of its time at the 70 percent stage, then kick up to 100 percent when more heat output is needed.

Even a simple two-stage design can provide significant energy savings. In one example, a single-stage furnace of 100,000 BTUs that operates at 92 percent efficiency wastes 8 percent of its fuel, or approximately 8,000 BTUs. A two-stage unit will operate the majority of time at 70 percent capacity or 60,000 BTUs and will lose only 5,600 BTUs. Plus, it has the remaining capacity for those few times when the temperature drops below normal and additional heat is needed.

Now, imagine if those stages were not just two, but many. That is variable modulation — being able to more precisely regulate the amount of fuel supplied to closely meet the energy needs of the appliance.

Exploded view of the ebm-papst premix gas blower and venturi.

In addition to energy efficiency, the modulating systems allow for precise control of temperatures. This control offers myriad benefits such as improving food quality in cooking applications and enhancing personal comfort in heating applications. When modulation is used in an oven, bakers can make a confection at a certain temperature knowing that the temperature is within a degree or two of the temperature for which it was set. When used in water heaters or furnaces, the same is also true — the system hits a specific temperature target and holds it consistently.

In research by Frank Johnson of the Gas Technology Institute (GTI), a Des Plaines, Ill.-based technology center, the temperature of a modulated oven was kept to within 5 DegF of the set point, as opposed to a traditional system that had swings of as high as 50 DegF.

“A modulating system gives a nice even heat so that if a baker wants to bake loaf of bread at 450 degrees in a commercial oven, he is actually baking it at 450 degrees,” says Johnson. “Modulation takes the variability of the temperature out of the equation.”

By reducing on/off cycles, the units can also help reduce emissions of CO. “Every time a burner ignites, there are puffs of unburned, natural gas and CO released,” says Caroline Duphily, technical manager in the Research & Development office at The Natural Gas Technologies Center (NGTC) in Quebec, Canada, which researches gas technologies. “There is a puff when you ignite the burner and another when it is extinguished. The more cycles that occurs during an operation, the more of these puffs will emit the gases into the atmosphere.”

Enlarge this picture Fig. 1. Comparative drying curves show how drying times are reduced by using a gas modulated clothes dryer. The technology was developed by the Natural Gas Technologies Centre, Quebec.

Modulating systems are not without drawbacks, however, and Duphily says that modulation systems should only be considered when an appliance requires varying loads. The disadvantages of modulating systems can be summed up by two words — complexity and cost. Both stem from the need to use specialized equipment such as modulating valves and modulating burners and can often also require more sophisticated controls and sensors, not only to control the combustion process, but also to provide the necessary feedback to determine the required fuel load.

This is opposed to the tried-and-true traditional open/closed gas valve that is an “inexpensive, very good piece of equipment,” says Johnson. “Millions are sold and they work reliably for a long time. When you get to modulating, you are adding a complexity and cost to the product.”

Despite the increased cost and complexity, Johnson believes in modulation, and every gas-fired product that comes through his center is looked at with an eye toward integrating this technology. One of his most successful projects is a pizza oven, which is being actively marketed, that makes a pizza in six or seven minutes. It features an airflow and combustion system that increases pizza production rates while maintaining cooking quality. “A lot of the big chains want consistency, they want the same pizza whether you get it in Wichita or Moscow,” says Johnson. “That is where that precision really comes in handy.”

Commercial cooking equipment in general is a growing area for modulation, ovens in particular. But the concept has also shown up in an upscale residential gas cooktop.  Caldera Corp., Stowe, Vt., uses a modulation system to control its Arrow Gastop, which features five burners, with each providing 21 different settings. The lowest setting delivers 185 BTUs per hour and the highest cranks out 18,000 BTUs/hr. At the simmer settings, the unit does cycle on and cycle off to keep the BTUs at a consistent point.

The DDM3 from Maxitrol controls valve pressure in minute increments.

Company founder Al Shute says the Arrow Gastop uses a touch-control interface to control the flame. A variable-capacity gas valve that could be electronically controlled was needed to make the product a success, but Shute could not find one on the market and so began a multi-year research project to develop a valve that would work with his system. The modulating valve worked so well that Shute created a spin-off company called CompuValve to license the technology.

While the modulating systems can make cooking better, it can also make homeowners more comfortable in heating applications. York and Rheem, for example, are among those gas furnace companies offering two-stage and variable modulating units. York’s 9.M and 9.C Series Affinity Modulating Furnace continually adjusts the firing rate in 1 percent increments between 35 percent capacity and 100 percent. Rheem offers two-stage furnaces that can run between 70 and 100 percent capacity and a variable-modulation furnace that operates between 40-percent and 100-percent capacities.

While modulating furnaces do offer the users more precise temperature control enhancing personal comfort, they also illustrate the issue of  added complexity, as they often employ variable-speed conditioned air blowers and variable-speed exhaust blowers, both of which require more complicated control schemes than constant-speed blowers. So adding variable firing capacity to a gas appliance requires more than just substituting a modulating burner in the design. The whole system needs to be redesigned.

Bosch and Rheem offer tankless, gas water heaters that use modulating systems. Bosch features it on about 60 percent of its product line. Rheem offers the technology in all of its tankless water heaters, says Peter Blaha, product manager for specialty products, Rheem Water Heaters.

The modulating valve allows the tankless heater to adjust for the amount of water flowing through it. Rheem’s RTG-74 model, which has a maximum flow of 7.4 gallons per minute, would not require the heater’s maximum output if only one faucet was in use. But, if additional faucets, showers, or clothes washers were running, and more hot water was needed, the system would detect the increase in water flow and send a signal for more heat. The modulating valve would adjust to provide more fuel and a hotter flame to deliver the increased demand for heat. With this type of unit all of the hot water needs can be met, says Blaha.

“If we utilized an ‘on/off type device’ for our tankless heaters, it would be extremely inefficient,” says Blaha. “With the modulating gas valve, Rheem's tankless heater range for the 74 model is from 19,000 BTUs to 199,000 BTUs.”

Bosch tankless water heaters have many of the same capabilities. On its modulating units, the company uses ceramic fiber mat burners. Because of the porous structure of the burner, the mixture of gas and airflows through the burner mat and the flame stabilizes on the burner’s surface.

The Bosch 2400E tankless water heater uses a gas modulating system.

When the system activates, the gas valve opens, a fan turns on and draws in a volume of gas and a volume of air and mixes it. “As the flow of water increases or decreases, or the temperature selected is changed, the fan changes its speed and will induce either more or less of the air and gas mixture to the burner,” says Steve Barnard, field engineer for Bosch.

Modulation also has the potential to be used in clothes drying appliances where the technology has been proven to reduce drying times in a research project. The NGTC worked with Camco, a Canada-based appliance manufacturer, on a modulating combustion system clothes dryer concept that dries clothes about 30 percent faster. (See Fig. 1.)

Modulation was achieved using a two-step gas valve from Italian valve manufacturer SIT. The valve combined with a boosted-capacity burner achieved the reduced drying time. “At the beginning of the drying cycle, when there is a lot of water present, the dryer uses a lot more heat without damaging the clothes,” says Duphily. “As the clothes dry, the heat is reduced so you don’t over heat the clothes. Globally, we use the same amount of energy, more at the beginning and less at the end, but the system can dramatically reduce drying times.”

Although the modulating gas dryer never made it into production for marketing reasons, the technical feasibility of the concept was nonetheless established during the research.

Residential gas fireplaces represent another application area for modulating gas valves. For example, Sherwood Industries’ Sonnet Fireplace features a 75 percent turn down with automatic modulating gas valve and a programmable remote-control thermostat. The remote control allows the user to remotely adjust the flame height, which requires a modulating system.

Maxitrol's GV60 e-flame remote ignition and control system fully modulates between low fire and high fire in a residential fireplace.

One of the suppliers that offer technologies to the fireplace industry is Maxitrol of Southfield, Mich. The company is one of the main players in gas modulation systems and their modulating valves can be found in commercial, industrial and residential products around the world. The valves already offer precision control, says Mark Masen, senior engineer, research and development for Maxitrol, but as demands increase, the company felt it needed to increase precision throughout its entire range of modulation.

It is currently in lead user testing of a system called Direct-Digital Modulation, or DDM. The unit can be installed on any of the company’s valve products. It is a microprocessor-controlled system that helps regulate valve pressure and make minute percentage changes to the pressure level to help meet the turndown ratios, says Masen.

This is important because as turndown ratios reach as high as 20:1, the gas flow rate must be even more precise. Turn down is the ratio of the maximum, or 100 percent, firing rate as compared to the minimum firing rate. If the turn down ratio is 10:1, the minimum is 1/10th of the maximum, explains Masen. To reach these the flow rates, the pressure required to move the fuel becomes more exacting.

The pressure amount is a square root of the flow rate, says Masen. “If we say we are taking down the turn down to 2:1, which is not a big deal, we are taking the flow from 100 percent down to 50 percent, and we have to take the pressure from 100 percent to 25 percent. Imagine where this goes when people start getting into 10:1 or 20:1. We could be down to the 1/100ths of the pressure that it was at a high fire rate.”

The ebm-papst NRG118/0080-3612 premix combustion gas blower.

CompuValve’s modulating valve system is also microprocessor controlled. The valve, which can control gas flow from down to 1 millionth of a liter with a potentially infinite variability for points between lowest and highest position. In the Caldera Arrow gas cooktop application, using the variable capacity allows the appliance to hold a pot of water to within +/- 0.5 DegF, according to the company.

The Arrow’s control system relies on a number of different monitoring approaches including a thermocouple positioned in the flame of each burner to detect flame presence or absence and a thermistor implanted inside each burner body to measure burner temperature. This last sensing approach permits the Arrow to compensate for expansion of the gas, which occurs when the burner heats up. This alone can cause the loss of as much as 20 percent of energy output unless the flame is readjusted.

An algorithm within the controller looks at the burner temperature, predicts gas expansion, and, as needed, opens the gas valve a little more to prevent the effect from occurring.

Another variable-capacity burner control system that is beginning to make a name for itself is Varidigm. The company’s Variable Burner Control (VBC) concept began at the Gas Research Institute where a modulating furnace was under development in the 1990s. That technology was spun off into Varidigm, says Gambiana.

Varidigm’s patented VBC is an integrated burner control system that modulates the firing rate from 10 to 100 percent in either furnaces or boilers. Varidigm’s modulating furnace technology includes premix burners, variable-speed motors, and combustion controllers. The combustion controller sequences, times and monitors the combustion process using a sensor located in the flame.

Enlarge this picture Results for modulating vs. non-modulating burners. Source: GTI

“It controls the entire combustion process from proving air levels in the combustion chamber, mixing the air and gas, operating a feedback system and using recirculating blower motors,” says Gambiana.

In addition to the typical advantages of energy efficiency and home comfort, Gambiana points to low emissions of harmful gases as well as low noise. Gambiana says that the units typically have a 25 to 30db reduction below standard furnaces.

Varidigm’s technology is just one of many innovations that came out of GTI. While Johnson did not work on this process, he has worked on a wide variety of units including the pizza oven discussed earlier, a steam generator, and building humidifier.

GTI’s system features three types of parameters, a High Firing Rate (HFR), a Low Firing Rate and a Modulation Bandwidth (MB). The high and low firing rates are based on the maximum output of the burner.

“The way the system works is it looks at how far away from set point the temperature is at and automatically adjust the firing rate at that point,” says Johnson. For example, if the temperature in the oven is set at 350 DegF, if the oven is less than 300 degrees the system will run high fire full out, then once it gets to that defined range (MB), it will slowly start cutting back the gas flow in correspondence to how fast it is approaching that final temperature. “So, if I’m within 50 Deg, I’m full out, and within 40 degrees I might be 75 percent of full, so just as you get close to the temperature, you start dropping the burner firing rate,” says Johnson.

Another way to achieve a variable-capacity gas fired system is to utilize a variable-speed premix combustion blower combined with a specialty gas valve and venturi to regulate the flow of the air-fuel mixture to the burner. In this scenario, burner output is regulated by decreasing or increasing the speed of the blower while the air-fuel ratio remains constant.

One such product is a premix-ready combustion blower from ebm-papst of Farmington, Conn. The ebm-papst model NRG118/0800-3612 is the next generation of the company’s premix-ready combustion blowers that have been used in the European hydronics market since the 1980s and the North American hydronics market since the late 1990s.

The new blower was sized to deliver the air performance currently supplied by larger blowers installed on boilers rated up to 102,000 Btu/h (30kW) without sacrificing efficiency. Though the blower was initially designed for the European hydronics heating market, but Tom Costello, product manager for ebm-papst, says that the technology can be applied to a variety of gas-fired applications.

One advantage of this approach in a high efficiency gas-fired boiler is that, combined with an advanced control system, it can be designed to automatically adjust its firing rate or energy input (modulate) to compensate for a change in the heat demand due to a change in the outside air temperature, says Costello. “If the outside air temperature increases, the boiler will continue to run, but at a reduced firing rate,” he says. “The result is an increase in the thermal efficiency due to an increase in the ratio of the heat exchanger surface area to the heat input as the firing rate is reduced. Therefore, efficiencies will range between 87 percent to more than 95 percent depending on the load and boiler design.”

Since first developing the product, ebm-papst has made a number of improvements to it, including an upgraded microprocessor, a reduced size of the scroll housing, new molding techniques, the use of new materials, and a revamped impeller system. “The system went from a 130 mm diameter impeller down to 118 mm diameter, and achieved the same performance as the larger product without increasing the blower speed, without increasing the power consumption and without increasing the noise level,” says Costello.

He adds that it was important to achieve the same results as the larger unit, but make it in a smaller package. The impeller redesign was key to this process. They first developed a design for a one-shot injection molded part, whereas the previous design was a 2-shot injection molded part that was ultrasonically welded together. Costello says that not having to mate the two parts together improved the balance, reduced manufacturing time and simplified manufacturing operations of the component. The impeller blades were also redesigned. They now feature a hybrid vane design that provides equivalent air, speed, power, and noise performance in a smaller diameter.

All of these changes made for a more efficient blower, which in turn helped improve the condensing boilers to be more efficient and quieter. This is important because of condensing boilers used in Europe are generally installed near living quarters. “This of course demands a product with a high level of balance to insure quiet operation,” he says.

One of the products’ benefits, Costello says is that it can be made on an automated assembly line and customized at the final stage to the OEMs requirements. “The design of the housing allows us to build up the motor on an assembly line and at then at the end of the line install the rest of the housing, which can be OEM specific in its design. This allows us to automate the manufacturing process, further reducing cost.”

This type of cost reduction and automation is key to reducing the cost of the product and making them easier for the OEM to utilize. This in turn may help pave the way for more migration of modulation units from high-end systems down to everyday, household products.

For more information, email:
CompuValve: info@compuvalve.com
Ebm-papst Inc: Jamie.Perkins@us.ebmpapst.com
Gas Technology Institute (GTI): businessdevelopmentinfo@gastechnology.org
Maxitrol Co: mmasen@maxitrol.com
The National Gas Technologies Centre: Caroline.Duphily@ctgn.qc.ca
Varidigm Corp: sales@Varidigm.com