If you’ve been shopping for a new, high efficiency, condensing gas boiler, you may have seen “turndown ratio” mentioned in product literature. This can also be referred to as “firing ratio,” or simply “turndown.”
So, what is the turndown ratio of a boiler?
To understand the term, we first have to cover how a conventional, cast iron gas boiler operates. The gas burner on a conventional boiler comes on when there’s a call for heat and turns off when the thermostat tells the boiler that the call for heat is satisfied (that is, the room reaches the setpoint temperature of the thermostat). As far as a conventional boiler burner is concerned, there is not an in between; it is either on, or it is off.
Enter the modern “modulating” or “condensing” high efficiency boiler. Condensing boilers are also called modulating boilers because the unit’s burner and burner fan can change speeds (modulates). They modulate between on and off, similar to the way your vehicle’s throttle can modulate between off and full throttle.
Boiler turndown ratio defined
Boiler turndown is the ratio between a boiler’s maximum and minimum output. Depending on the burner’s design, it may have a turndown ratio between 5:1 and 10:1 or even higher. A 5:1 turndown means the boiler’s minimum operating load is 20% of the boiler’s full capacity (100% capacity divided by 5). A 10:1 turndown means the minimum operating load is 10% of the full load capacity (100% capacity divided by 10).
To get more technical, if a boiler with a 10:1 turndown is capable of a maximum input 100,000 BTU/H, it means that the unit can actually supply as little as 10,000 BTU/H. Of course, it can also supply 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, and what the industry calls “high-fire,” or the maximum 100,000 BTU/H. All of this takes place automatically. The boiler’s onboard control selects the right firing ratio based on system inputs.
Why is boiler turndown ratio important?
So, who cares about different boiler turndown ratios? Well, you might, when your gas bill arrives! When properly installed in a well-designed hydronic heating system, a modulating boiler can easily save the owner a great deal of energy.
Installing a modulating boiler means that the unit has the potential to run longer cycles. It’s better for a boiler (and better for your energy bill) to have the unit run on low fire for a long time than to switch on and off at high-fire. Turning on at full throttle for a few minutes, satisfying a call for heat, and shutting down again in a few minutes is called “short cycling.” This not only wastes fuel, but it is hard on system components. When properly installed, modulating boilers eliminate or greatly reduce short cycling.
Additional benefits of boiler turndown ratio
Using a modulating boiler also helps the installer select an appropriately sized unit for your property, especially if the home is small, very well insulated, or has many different heating zones. Imagine if the home has a 100,000 BTU/H boiler installed to heat the whole house, but only one small zone calls for heat. A modulating boiler will run on low fire to provide the small amount of heat needed, instead of firing to full capacity.
Modulating boilers are also great in small homes for supplying domestic hot water through the use of an indirect fired water heater, like the U.S. Boiler Company Alliance. Many small homes, or well insulated homes, need more heat to provide domestic hot water than they actually need for space heating. By using a modulating boiler, the unit can supply heat to the house while firing at a low input. When a call for domestic hot water comes, it can go to high-fire, rapidly heating the indirect tank up so that cold showers never happen.
Selecting the right turndown ratio boiler
As you shop for a new boiler, expect modulating boilers to cost more than conventional boiler of the same size. The controls are more capable, and the stainless-steel heat exchangers are more intricate than their cast iron counterparts. However, if a modulating boiler is installed by a licensed heating professional, expect the unit to provide energy savings, comfort and long service life!
Be sure to also browse our contractor locator tool to find a qualified heating contractor in your area. Your professional heating contractor will be able to determine what heating equipment will be best suited for your home heating system.