Modulating boilers - from commercial use to residential applications.

Imagine driving your car in this manner: Stomp on the gas pedal, accelerate for about 10 seconds, then stop, accelerate, stop, accelerate, stop.

Your car would get really bad gas mileage, wouldn't it? (Not to mention the fact that it would take a long time to get anywhere.) But this is a simple illustration of what happens in a conventional boiler, explains Tom Fallon, technical specialist at Peerless Boilers. They turn on and off repeatedly to heat the water and achieve the desired temperature in a home or building, wasting energy in the process.

But a modulating boiler “throttles” the fuel input, much as a car does, adjusting the heat input to the fuel input.

“In an ideal space-heating system, the heat output from the heat source would always equal the rate of heat loss from the building,” says Matt Prouty, project engineer for ECR International (parent company of Dunkirk and Utica Boilers). “Modulating boilers are designed to provide an amount of heat to match the load on the heating system at a particular time and constantly adjust to do so.”

“While traditional boilers and conventional heating systems function in an on/off mode,” adds Ed Nordstrom, vice president of Viessmann Mfg. Co. (U.S.) Inc., “a modulating water supply temperature coupled with a modulating burner provides continuous circulation at the proper water temperature, so there is a minimum of starting and stopping in the distribution system. The amount of heat that's being lost out of a home's walls and windows is continuously being replaced.”

Achieving Energy Savings

But it's not really the boiler that “modulates,” it's the burner, notes Jack McKeegan, Patterson-Kelley's region manager for western United States and Canada. McKeegan is also the principal systems and applications trainer for the company. “The trick is to use a burner that varies both fuel and air, and maintains a constant fuel-air ratio across the entire modulating range.”

A constant fuel-air ratio means there will be more boiler heating surface than is needed at lower inputs, he adds, and the stack temperature will fall. This indicates that more heat is getting into the water. Energy efficiency will rise as fuel input goes down. (Most units have between 92 percent and 98 percent efficiency.)

But if the fuel-air ratio is not constant, the benefits of burner modulation can be lost.

“If excess air rises as fuel input goes down, the low-fire efficiency can be worse than at high fire,” he warns.

Minimizing the off cycles also makes the boiler more energy-efficient. A short-cycling boiler will use nearly 15 percent more fuel than one that does not, McKeegan says. Modulation helps prevent that, as well as the design of the heating system.

“Longer run times means a shorter period of time where the unit sits idle,” Prouty says. “Most of the heat that is generated by the boiler is being transferred into the system on demand instead of 'leaking' out of the jackets when the unit is inactive.”

Condensation also plays a part in gaining efficiency. Modulating boilers are capable of operating at lower temperatures. The lower the temperature of the flue gases, the better likelihood of condensation, which means the boiler is pulling off more heat into the system, explains Al Mangus, Weil-McLain product marketing manager.

To prevent corrosion, these boilers use parts made of stainless steel and aluminum. But they still should be monitored at least once a year, maybe twice a year for aluminum, Fallon says, as material will eventually build up inside the units and may cause problems.

To Modulate Or Not To Modulate

This emerging niche of the heating market has its roots in Europe, but the technology is making its way into North American markets. And more and more contractors are installing modulating boilers in today's homes. Buffer tanks and boiler oversizing are virtually eliminated, and the units are generally lightweight and easy to install.

“Changes in codes over the years, which now require chimney lining, as well as outdoor air requirements, have made it more difficult to install a conventional chimney-vented boiler,” notes Peter Desens, corporate technical service manager at ECR International. “The direct venting of a modulating boiler addresses these issues.”

Some contractors might shy away from these high-efficiency boilers because they are a little more high-tech than traditional boilers. They are controlled differently, McKeegan says; they require analog as well as digital control signals.

Most units have some type of module to control fuel input and fan speed. “You need the right combination of gas and air for proper combustion as well as for achieving the correct fuel-air ratio,” Mangus says.

Components may include a fan or draft inducer, gas valve, a control that can modulate, temperature sensors to relay information back to the control, and a burner that is able to operate over a wide range of heating rates, Desens says. Traditional boilers use components that are designed to deliver one set rate at all times.

For homeowners, the deciding factor is often cost savings. Desens says that, while modulating/condensing boilers may initially cost more, the high efficiency “guarantees lower fuel bills and quicker payback” for the homeowner.

“Anytime energy efficiency is a deciding factor in boiler selection, modulating boilers should be considered,” he adds.

Comfort level is also important to homeowners, and modulating boilers can deliver that as they are responsive to varying heat demands, says Nordstrom. Since the units are not constantly cycling on and off, the room temperature remains more consistent.

Products on the market today include:

  • ECR International (Dunkirk and Utica models) - Monoblock design, wide waterways for low pressure drop, no flow switches, isolation ball valves on circulator, UL-listed low water cut-off device. (Note: Boilers are still in development stage.) Web sites:,

  • Patterson-Kelley Mach series - Cast aluminum heat exchanger, no push nipples or gaskets, 5:1 turndown ratio, low NOx emissions, multiple venting arrangements, cast metal sectional construction, variable speed fan, zero-offset fuel-air ratio controller, Honeywell MCBA controls, small footprint, lightweight, silent operation, five-year warranty on heat exchanger, 92 percent to 99 percent efficiency. Web site:

  • Peerless Pinnacle - Stainless-steel burner and heat exchanger, Honeywell controls, spark ignition, Honeywell gas valve - venturi system, pre-mix blower, direct vent, lightweight, can be shipped UPS, low emissions, 92 percent efficiency, 12-year warranty on heat exchanger. Web site:

  • Viessmann Vitodens 200 - Wall-mounted, modular design, stainless-steel burner, combines space heating and domestic hot water heating, modulates both the water temperature and burner input, stainless-steel heat exchanger, ASME constructed, variable speed DC fan, multistage heating circuit pump, up to 94.2 percent efficiency, low emissions. Web site:

  • Weil-McLain Ultra - Stainless-steel burner, 5:1 turndown ratio, cast aluminum heat exchanger, electronic control module, negative regulated gas valve, variable speed motor, venturi mixing body, floor or wall-hung applications, PVC venting, low NOx emissions, 92 percent to 98 percent efficiency, free five-year protection plan on parts and labor. Web site:

  • Heat Transfer Products' Munchkin - Stainless-steel construction, compact and lightweight design, can be shipped UPS, direct vent with standard Schedule 40 PVC pipe, quiet operation, self-diagnostic microprocessor control system, Honeywell gas valve venturi system, low emissions, 12-year warranty, 1 1/4-inch NPT return and supply connections, 96 percent efficiency. Web site: