Small Gas Boilers

Heating and cooling

If you’re in the market for a gas boiler, consider getting a residential boiler (figure 1). Even though they’re designed for a different market, they can work well in commercial buildings. Residential boilers are manufactured in larger quantities than commercial units, which makes them relatively inexpensive and readily available with a variety of options. Their small size means you can easily move them in and out of buildings, and the most-efficient residential boilers are as efficient as commercial boilers. You can stage multiple small boilers to provide high efficiency over a range of heating loads.

Figure 1: Diagram of a residential boiler

Boilers come in various configurations using cast-iron, steel, or copper heat exchangers with different types of burners and venting systems. In this example, cast-iron “wet walls” connected by finned copper tubes contain hot water that has been heated by a gas flame. A natural draft chimney vents the heating system to the outside air.
Figure 1: Residential boiler

What are the options?

Size The size of a gas boiler is given in terms of its heating capacity, in Btu/hour of gas input. A Btu is equal to the amount of energy it takes to raise 1 pound of water 1 degree Fahrenheit. In practical terms, 1 Btu is the heat given off by completely burning a single kitchen match. Most residential gas boilers have ratings in the range of 40,000 to 300,000 Btu/hour.

Efficiency Boiler efficiency ratings are designed to help you readily compare boiler energy performance and make selections. The most common rating for small boilers is the annual fuel utilization efficiency (AFUE). The AFUE represents performance under a specific set of conditions, accounting for the effect of part-load efficiency and cyclic losses. The conditions, chosen to represent operation in an average climate with a particular usage pattern, include flue and infiltration losses during on and off cycles.

The AFUE is helpful when comparing two boilers under the same test conditions, but it’s less useful for predicting a boiler’s annual fuel use in the field, where local conditions may not match the AFUE test conditions and calculation assumptions. Keep in mind that the AFUE is based on a residential load profile, which may be different from the load profile of a commercial building.

Since 1992, the US Department of Energy (DOE), under the National Appliance Energy Conservation Act, has required that small gas boilers have an AFUE of at least 80%. In November 2007, the DOE established a revised minimum efficiency standard of 82% for residential boilers, which took effect in November 2015. The Energy Star program awards an Energy Star label to boilers with an AFUE of 85% or better. The most-efficient boilers on the market boast an AFUE of around 96% (figure 2).

Figure 2: AFUE ratings distinguish three boiler types

Condensing boilers typically have AFUEs greater than 85%. Contemporary, high-efficiency models use fully modulating burners to reach AFUEs over 92%. Fully modulating boilers, the most-efficient boilers on the market, boast AFUEs near 96%.
Figure 2: AFUE ratings distinguish three boiler types

To find the ratings for a particular boiler, review the EnergyGuide label found on the unit, check the AHRI Directory of Certified Product Performance website, or consult manufacturer literature.

Sealed combustion Boilers draw the air they need for combustion from inside the heated space or directly from the outside. Drawing air directly from outside—typically through a plastic pipe that runs through an outside wall—is more efficient and safer. This method is usually referred to as sealed combustion because the gas is burned in a chamber that’s closed to occupied areas. This configuration virtually eliminates any risk that combustion gases could leak into occupied space. However, sealed combustion requires some complicated installation techniques, so check the manufacturer’s installation instructions carefully.

Load-matching/multistage/modulating burners Most boilers are two-position devices: They either run at maximum output or are in the off position. This on/off operation tends to send heat to the zones in pulses rather than as a steady flow. Contemporary, high-efficiency condensing boilers use modulating burners to reduce the number of on/off cycles (and cycling losses) and allow the boiler to operate for longer hours at lower firing rates, which improves efficiency. In 2005, researchers at the University of Dayton found that changing from on/off to modulation mode can improve average boiler efficiency by about 8%.

Controls Electronic controllers for boilers have rapidly improved in capability and reliability since the early 1990s, allowing them to maximize comfort and minimize energy use. Controls for new boilers (or retrofitted to existing boilers) come in varying levels of sophistication; the controls can increase equipment life, improve boiler efficiency, and enhance comfort.

Modern electronic controllers can reset boiler water temperature (especially important with condensing boilers), create time-delay relays, perform automatic postpurge, prevent warm-weather boiler operation, control the position of mixing valves, activate multiple boilers in stages, control pump speeds, and activate and deactivate boilers in user-determined priority order. These controls can increase the efficiency of noncondensing boilers by 10% and reduce idle losses to 0.3%.

The costs of controls vary dramatically depending on the size, age, type, plumbing configuration, and sophistication of the existing boiler. For example, add-on controls for conventional boilers typically cost anywhere from $150 to $1,000 and can reduce fuel use by up to 12%. Sophisticated controls come standard or as add-on options for condensing boilers and typically cost $500 to $1,000—but they can reduce fuel use by up to 20%.

How to make the best choice

Pick the right size Heating contractors sometimes use larger boilers than they need so they can spend less time selecting a model and still guarantee that it will maintain comfortable conditions. The problem with this approach is that an oversized boiler is noisier, less efficient, and more expensive than an accurately sized unit is. To determine the proper size, calculate the heat load served by the boiler by following the procedure explained in the ASHRAE Fundamentals Handbook. Many software products are also available that can guide you through the calculations, and several boiler manufacturers include sizing guidelines or software on their websites.

Compare the cost-effectiveness of boilers with different efficiency ratings High-efficiency boilers cost more than lower-efficiency models. To determine if a more efficient unit will be cost-effective, compare initial costs and annual energy costs to estimate the payback period. Figure 3 presents an example of a simple cost-effectiveness calculation.

Figure 3: Cost-effectiveness calculation

This example assumes that two 100,000-Btu/hour boilers service identical loads. We estimate the annual energy consumption by multiplying that input capacity by the annual operating hours (adjusted by the AFUEs). With an incremental first cost of $1,400, the high-efficiency boiler in this example yields a simple payback period of about three years.
Figure 3: Cost-effectiveness calculation

Consider installing multiple small boilers. If building loads are highly variable—and this is often the case in commercial buildings—multiple boilers are a good option. The best small boilers are more efficient than their larger counterparts are, and multiple-staged small boilers can operate more efficiently than a single large system. This is because each boiler can spend more time operating at or close to its full load—the most efficient operating point for a boiler.

Multiple-staged small boilers provide redundancy, which can reduce system downtime, and small boilers can reduce installation costs because they’re light enough to be handled without a crane. You can also team a small high-efficiency boiler with a large, inefficient, old boiler to improve overall efficiency—the small unit turns on whenever there’s a heating load, but the old unit only kicks in during periods of high load.

Select a boiler with modulating controls Modulating boilers use controls to regulate their heat output to more closely match loads. They’re similar to a staged multiple-boiler system in that they can reduce fuel use and ignition-related cycling losses.

For domestic water-heating applications in both commercial and residential applications, consider using an indirect tankless water heater A common way to produce domestic hot water with a boiler is to pair it with an indirect water heater. An indirect water heater uses a built-in heat exchanger that’s connected to the boiler like any other heating zone. Fluid circulates from the boiler to a double-wall heat exchanger to heat water for domestic use while keeping the boiler water separated from the potable water. These units can heat water on demand instead of storing hot water in a storage tank and provide excellent combustion efficiency. The absence of a storage tank eliminates standby losses and reduces space requirements. For more information, see the guide on Tankless Water Heaters.

Follow the manufacturer’s installation instructions To achieve high efficiencies, many boilers use venting systems that were almost unknown a decade ago. Correctly installing these systems is tricky. An improperly installed venting system may result in a boiler that can’t function reliably and requires costly repairs. Furthermore, premium-efficiency boilers condense water out of combustion gases, and that condensate stream must be appropriately disposed of. Before buying a boiler, check the manufacturer’s instructions to make sure it can be correctly installed in your building.

What’s on the horizon?

Increasing demands for efficiency and low emissions are pushing manufacturers to develop more-efficient and clean combustion technology for boilers. Some companies are exploring more-sophisticated controls that offer increasingly precise load-matching capabilities. Digital controls may increase the precision of burner modulation to reach a high turndown ratio—the ratio of maximum to minimum firing rate. In the future, microprocessor-control systems may operate on multiposition valves to regulate pressure by minute percentage changes, creating precise gas-flow rates.

Who are the manufacturers?

A list of Energy Star–certified boilers is available from Energy Star’s Boilers web page. The following is a partial list of manufacturers of small gas boilers:

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.

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