During the early 1980s, America was enjoying video-cassette recorders and cable television and marveling at the invention of cellular telephones and personal computers. Meanwhile, the average hydronic heating-boiler was only slightly more sophisticated than a bucket of water and campfire.

At the time, the controls on many mid-range (400,000- to 2.5 million-Btuh input) boilers consisted of a basic single-pole/single-throw thermostat that turned gas valves on and off. Overheat protection consisted of manual reset high limits, and pilots were started with a match.

Evolution of Boiler Technology

During the mid-1980s, fan-assisted combustion became the first significant new boiler technology introduced since the invention of natural gas. Sealing a boiler's combustion chamber and blowing in only the air needed for combustion eliminated efficiency-robbing excess air. Fan-assisted combustion was the breakthrough needed to increase thermal efficiency from the high-70-percent to the high-80-percent range.

A new combustion process meant new controls. Over-the-counter ignition controls managed the ignition process, but did not have blower connections, so a special ignition module had to be commissioned to control the ignition process and power blowers. Previously, most boilers had been operated with over-the-counter controls that were mechanically operated and basically designed. Special electronic boards were needed to control increasingly more complex boiler designs.

As the industry was toying with fan-assisted combustion, it simultaneously was developing stage control. Instead of firing an entire burner tray fully on and off, the boiler was divided into smaller sections, each of which was fired separately. Over-the-counter stage controllers originally were established to stage the firing of boilers separately. Multiboiler step controllers with a minor modification or two now could be applied directly to boilers. Banks of burners could be fed by smaller, more cost-efficient redundant gas valves and controlled with a built-in stage control.

Still major players in the product offerings of every boiler manufacturer, fan-assisted combustion and stage firing were historical changes that laid the groundwork for dramatic new products that now are starting to take over the mid-range commercial-boiler market. These new products have two design features that could not have been achieved without newer and smarter controls. The first design feature is a new heat-transfer process: Boilers have moved from non-condensing to condensing. The European market led the development of stainless-steel and cast-aluminum heat exchangers that could extract heat from not only a flame, but the warm, moist flue products produced by combustion. This technology was the push the industry needed to propel thermal efficiency from the high-80-percent range to 98 to 99 percent, which is possible when boilers are fired in optimal condensing conditions.

There also have been developments in burner modulation. Modulation has been around since atmospheric boilers were invented, but its latest advancements have required greater component improvements. On an atmospheric rigid-tube burner, modulation could reduce only so much before the flame touched the burner's metal. This inadequacy spurred the development of ceramic and micro-metal-fiber burners, which can burn infrared cleanly across a greater modulation range.

Modulation also required changes to the gas train. Today, a new and more precise method — negative regulation (neg-reg) — is used. Neg-reg is a design that combines blower and gas-valve operations. Negative pressure created at the blower's intake pulls gas through the gas valve. As the blower speeds and slows, intake pressure rises and falls, pulling more or less gas from the valve. With this configuration, the air-to-gas ratio remains constant across a wider band of input.

Boilers received new electronics for neg-reg combustion, as well as smarter blowers and gas valves. Now, blowers and gas valves have their own circuitry to perform their balancing act. For example, blowers have electronics that employ power inverters and pulse-width modulation to regulate revolutions per minute without damaging or overheating a motor.

Advances in combustion and heat-transfer technology would not have been possible without corresponding control changes.