Small Gas Boilers

If you are considering the purchase of a gas boiler, add residential boilers (Figure 1) to your list of options. Even though they are designed for a different market, they can play an effective role in commercial buildings as well. Residential boilers are manufactured in larger quantities than commercial units, which makes them relatively inexpensive and readily available with a wide variety of options. Their small size enables them to be easily moved into and out of buildings, and the most efficient residential units exhibit efficiencies similar to those of their best commercial counterparts. In addition, multiple small boilers can be staged to provide high efficiency over a wide range of heating loads.

Figure 1: Residential boiler
Boilers come in a variety of configurations using cast-iron, steel, or copper heat exchangers with different types of burners and venting systems. In the example shown, cast-iron “wet walls” connected by copper finned tubes contain hot water that has been heated by a gas flame. The system is vented by a natural draft chimney.
What are the options?
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Size. The size of a gas boiler is given in terms of its heating capacity, in British thermal units per hour (Btu/h) 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 are rated in the range of 40,000 to 300,000 Btu/h.

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 their annual fuel utilization efficiency (AFUE). AFUE accounts for the effect of part-load efficiency and cyclic losses that occur in actual installations; a single number represents performance under a specific set of conditions. The conditions, chosen to represent operation in an average climate with a certain usage pattern, include flue and infiltration losses during on and off cycles.

AFUE serves well for comparing two boilers under the same test conditions, but it is less useful for predicting annual fuel use in the field, where local conditions may not match the AFUE test conditions and calculation assumptions. Additionally, AFUE is based on a residential load profile, which may be quite different than the load profile of a commercial building.

Since 1992, the U.S. Department of Energy (DOE), under the National Appliance Energy Conservation Act, has required that small gas boilers have an AFUE of at least 80 percent. In November 2007, the DOE established a revised minimum efficiency standard of 82 percent for residential boilers, which will take effect in November 2015. The Energy Star Program, which is run by the DOE and the U.S. Environmental Protection Agency, awards an Energy Star label to boilers with an AFUE of 85 percent or better. The most efficient boilers on the market boast an AFUE of around 96 percent (see Figure 2).

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

To find the ratings for a particular boiler, review the EnergyGuide label found on the unit, check the Consumers’ Directory of Certified Efficiency Ratings for Heating and Water Heating Equipment, available at the Air-Conditioning, Heating, and Refrigeration Institute’s Directory of Certified Product Performance, or consult manufacturer literature.

Sealed combustion. Boilers draw the air they need for combustion either 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 is closed to occupied areas. This configuration virtually eliminates any risk that combustion gases could leak into occupied space. It does, however, require some complicated installation techniques, so check the manufacturer’s installation instructions carefully.

Load matching/multistage/modulating burners. Most boilers are two-position devices: They are either running at maximum output or they are off. 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 percent.

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 of varying levels of sophistication can be purchased with new boilers or retrofitted to existing boilers and 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 post-purge, 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 a user-determined priority order. These controls can increase the efficiency of noncondensing boilers by 10 percent and reduce idle losses to 0.3 percent.

The purchase 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 percent. 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 percent.

How to make the best choice
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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 that approach is that an oversized boiler is noisier, less efficient, and more expensive than an accurately sized unit. 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 web sites.

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. Table 1 presents an example of a simple cost-effectiveness calculation.

Table 1: Cost-effectiveness calculation
This example assumes that two 100,000-Btu-per-hour boilers service an identical load. 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.

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, 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.

In addition, 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 are 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 is circulated 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 they 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 virtually unknown a decade ago. Installing these systems properly is tricky. An improperly installed venting system may result in a boiler that cannot function reliably and requires costly repairs. Furthermore, premium-efficiency boilers condense water out of combustion gases, and that condensate stream must be properly 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?
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Increasing demands for efficiency and low emissions are pushing manufacturers to develop more efficient and clean combustion technology for boilers. More sophisticated controls that offer increasingly precise load-matching capabilities are being explored. One company is developing new digital controls that would increase the precision of burner modulation to reach a high turndown ratio—the ratio of maximum to minimum firing rate. The company is testing a microprocessor-control system that could be installed on multiposition valves to regulate pressure by minute percentage changes, creating precise gas-flow rates.

Who are the manufacturers?
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A list of Energy Star–certified boilers is available from the Energy Star web site. 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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