Sure, modulating boilers save energy. But consistent temperature control also means more comfort.

My friend Dennis Bellanti loves to try out new heating systems in his home. Most recently he’s installed a modulating condensing boiler. He swears that of all the things he’s tried over the years, this has done the most to reduce his fuel bill and to increase his heating comfort. And he gives the modulating gas valve the most credit for his great energy conservation results.

The important part of modulation is that if you need just a little heat, you burn just a little gas. If you need a lot of heat, you burn a lot of gas. And the valve will vary the amount of gas for any demand in between. Modulating can go down to as little as 25 percent of full firing rate in American boilers, and as little as 1 percent in European boilers.

This is quite different from a traditional valve where you either burn enough gas for the coldest night or you burn none at all.

Of course, the step-opening valve has been around for a long time. But it’s quite similar to a standard valve with an intermediate position called “low fire” before going to fully open “high fire.”

Modulation’s ability to deliver just a little heat means energy savings. But my favorite part - and the part that is forgotten by lots of folks who like to measure things and collect data - is that Dennis says his house is also more comfortable than it’s ever been. The temperature is constant because the boiler can deliver a little heat all the time. There’s no temperature swing that comes from traditional on-off heating.

Conventional Valves

It’s easier to understand what’s special about modulating gas valves if we first look at how a conventional valve works. A conventional valve is either on or off. That means negative consequences for both fuel economy and comfort.

It’s like driving your car either with the accelerator at full throttle or standing on the brake. It’s not only going to be a rough ride, but it will cost extra fuel.

The way the conventional gas valve works is that when there’s a call for heat, the gas valve fully opens (provided the situation has been proven safe) and fills the burner with gas, which is then lit by a pilot flame, spark or hot surface ignitor. The valve stays fully open and the burner operates until the call for heat has been satisfied. Then the valve closes and the burner flame disappears until next time. In the old standing pilot systems, the small pilot flame is always burning, heat needed or not.

With a good thermostat, this system can keep the room temperature within plus or minus 1 degree from thermostat set point. That’s within the definition of comfort. But still, it uses the gas full-on, full-off.


Most of us driving on a highway don’t even bother with the accelerator or brake. Past a certain speed, we can easily set our cruise control.

Likewise, a modulating heating system does the same thing. We don’t have to think about anything - the controls just do it. The result is a smoother ride (more comfort) and greater fuel efficiency (saves money).

In addition, modulation provides what is called “full stoichiometric” combustion. That means all of the fuel is burned completely, and that of course means highest fuel efficiency.

I asked my friend John Cueva, combustion engineer at Honeywell, just how modulation works. This is amazing - modulation is controlled entirely by air, even on a boiler! As the temperature of the air returning to the boiler varies, so does air flow from the blower. The more air, the more gas, and that varies the firing rate of the burner. Pretty simple.

The modulating valve has a constant 24V signal, so unlike a conventional gas valve, it is constantly powered, not switched on and off to open and close based on availability of 24V. The valve is modulated by the amount of air from the blower so that the valve maintains a 1:1 ratio of gas to air. If there’s more air, then the valve will allow more gas. If there’s more gas, then more Btus come from the burner. All of the air and gas is mixed before the mixture goes to the burner. There’s no secondary air source.

A venturi tube is the mechanism for mixing gas and air. A venturi tube is two pipes, one within another. Air is drawn down the center pipe. That happens because a pressure differential is created by a restriction in the pipe. If there is a 2-inch pipe, and we put in a washer to restrict it to 1-inch, pressure builds on the back and creates high pressure. As the air comes out of the restricting washer, there’s a negative pressure, creating a space that wants to be filled.

At the same time, gas goes through the pipe that surrounds the air pipe. There are holes in the air pipe downstream from the restricting washer. The air’s negative pressure sucks gas into the air stream.

The amount of air going into the venturi tube is controlled by a blower. As the demand for heat increases, the blower runs faster and makes more air available. The venturi tube is set up to mix a 1:1 gas-to-air ratio, regardless of the amount of air. So as the amount of air increases, so does the gas. The result is that increasing and decreasing the fan speed controls the amount of gas for the flame, which determines how much heat comes from the burner. An integrated controller monitors inlet and outlet temperatures.

The result is a wide range of firing rates and quick response to heating demands. In addition to heating, the boiler can provide domestic hot water quickly with no storage tank.

For the guys who like to noodle with the details, there’s a lot of debate about what part of a condensing boiler makes it efficient. Is it the condensing, the outdoor reset, or the modulating gas valve? Dennis is sure it’s the modulating valve, which lets his heating system constantly deliver heat - as much or as little as is needed right then.