Figure 1. Relationship between heat input and load for (a) two-boiler system with on/off burners, (b) four-boiler system with on/off burners.
Multiple boiler systems have been around for several decades. The underlying concept is simple: Divide the overall heating load among two or more independently controlled boilers, and operate only the boiler(s) necessary to meet the load at a given time. Each boiler constitutes a "stage" of heat input. An intelligent controller continually monitors system supply temperature and determines what boiler stages to operate to keep that temperature at or very close to a target temperature.

The benefits offered by multiple boiler systems include:

  • Increased seasonal efficiency relative to that of a single large boiler.
  • Partial heat delivery if one boiler is down for servicing. This is especially important in locations where extreme temperatures could quickly freeze up nonoperational systems.
  • Easier installation than with a single larger unit, especially on retrofit jobs. Repair parts are often more readily available for smaller boilers.
  • The ability to provide elevated output during periods of high domestic hot water demand or snow melting, while still retaining high efficiency under lower space heating loads.

Figure 1 shows the operating characteristics of two different multiple boiler systems relative to a hypothetical load profile. The first system uses two on/off boiler stages, each capable of supplying 50% of the design load. The other has four on/off boiler stages, each capable of supplying 25% of design load. Notice how the four-boiler system is better able to track the load profile relative to a two-boiler system.

Based on this, one might conclude that the more stages the boiler system has the better its ability to match the heating load, especially under very low load requirements. However, as the number of boiler stages increases so does the ratio of boiler jacket surface area to total heat output. Additional heat loss from the boiler jackets eventually offsets any incremental gain in combustion efficiency. For this reason, it seldom makes sense to use more than eight boiler stages.

An inherent limitation of multiple boiler systems using on/off burners is that capacity must be controlled in steps, rather than as a continuously adjustable process. Although more stages reduce the height of the stepped heat input, the ultimate form of capacity control is a fully modulating boiler plant.

The Next Step

The newest concept in multiple boiler systems is multiple modulating boilers. Rather than using a simple "on/off" burner, modulating boilers vary their rate of fuel input and thus control their heat output over a wide range. The small modulating boilers currently available on the North American market can reduce their firing rate down to 20% to 30% of full output rating.

The ability of a modulating boiler to reduce its firing rate is often called its "turn-down ratio" (e.g. the boiler's full heat output rate divided by its minimum stable heat output rate). For example, a boiler with a peak output of 150,000 Btuh and a minimum stable firing rate of 30,000 Btuh would have a turn-down ratio of 150,000/30,000, or 5:1.

When modulating boilers are combined into a multiple boiler system, the overall turndown ratio of that system is significantly better than that of an individual boiler. If four of the boilers cited in the above example were used in a system, the overall capacity control range would be from a peak of 600,000 Btuh all the way down to 30,000 Btuh. Hence the system's turndown ratio would be 20:1.

At low heat production rates, the heat exchanger of a modulating boiler operates at relatively low combustion-side surface temperatures. When such a boiler is matched up with low temperature distribution systems such as for slab-type floor heating, it is common for the boiler to operate with sustained flue gas condensation. Modern modulating boilers have heat exchangers constructed of stainless steel or cast aluminum to withstand the effects of such condensation. When operating in such low temperature modes these boilers are also capable of efficiencies in the 95%-plus range.

Small modulating boilers are designed as sealed-combustion devices. Most are sidewall vented, using PVC, CPVC, or other polymer tubing. Combustion air is ducted directly from outside to the boiler.

Figure 2. Piping of four modulating boilers. Note parallel reverse return arrangement and individual circulators on inlet side of each boiler. A boiler circulator operates only when its associated boiler is active. Closely spaced tees prevent interference between boiler circulators and system pump.

Piping Pointers

There are three crucial details when combining two or more small modulating boilers into a multiple boiler system.

First, the piping and control configuration should allow flow through each boiler to be independently controlled. When a boiler is not firing, there should not be flow through it. Doing so simply uses the boiler's heat exchanger and jacket as a heat-dissipating device. This is also an important detail when conventional on/off boilers are combined into a multiple boiler system.

Independent flow control is achieved by using individual circulators for each boiler as shown in Figure 2. Since the boiler circulators are in parallel, it is necessary to install a flow check downstream of each circulator to prevent reverse flow through inactive boilers. One excellent hardware option for such situations is a wet rotor circulator with integral check valves.

The second important detail is to pipe all boilers in parallel (Figure 1). This allows each boiler to receive the same inlet water temperature. Keeping the inlet water temperature as low as possible favors condensing mode operation and boosts boiler efficiency.

Some controllers for multiple modulating boilers can be configured to turn on the circulator for a given boiler shortly before the burner is fired. They can also be set to keep the circulator running for a time interval after the burner stops. The former allows heated system water to warm the heat exchanger of the boiler to reduce thermal stress (and flue gas condensation for conventional boilers) when the burner fires. The latter allows residual heat to be purged from the boiler's heat exchanger rather than escape through the venting system during the off cycle.

Note that the boiler circulators in Figure 2 are pumping into their respective boilers. This allows the pressure in the boiler heat exchanger to increase when the boiler is firing, which decreases the chance of cavitation or steam flashing.

Finally, it is important that the boiler circulators provide consistent and adequate flow through operating boilers regardless of the system flow rate. The closely spaced tees in Figure 2 or low-loss header shown in Figure 3 uncouple the boiler flow from the system flow to ensure adequate boiler flow regardless of flow variations in the distribution system.

Modulating Logic

The staging controller for multiple on/off boilers typically uses a PID control loop that continually measures system supply temperature, compares it to a calculated target valve, then determines what burner should be on or off to eliminate any deviation between these temperatures as quickly as possible.

The control strategy changes when modulating boilers are used. In most applications, the operating logic is based on maximizing operating time in the lower output range of the boilers. Although exact operating logic varies among manufacturers, a common approach is to fire one boiler then modulate that boiler up to some relatively high percentage of full output as necessary to meet the current load. If additional heat input is needed, the firing rate of the first boiler is significantly reduced, the second boiler is fired, and then the two boilers modulate up together to match the load.

This strategy favors operation of two boilers at lower percentages of their individual outputs rather than operating a single boiler at a high firing rate. This increases the ratio of boiler heat exchanger area to heat output and thus increases the potential for flue gas condensation and higher efficiency.

Assuming the load continues to increase beyond what two boilers can provide at a relatively high percentage of output, the second boiler reduces its firing rate, the third boiler fires, and the second and third boiler modulate up in parallel. In some systems, the first boiler may also reduce its firing rate when another stage is activated to allow all three boilers to modulate up in parallel.

Operating Modes

Most controllers for multiple modulating boiler systems also provide at least two operating modes.

In the space-heating mode, the target value for the system supply temperature is based on outdoor reset control. The lower the outside temperature, the higher the target supply temperature. This logic eliminates the need for mixing controls between the boiler plant and space heating loads. Full outdoor reset control including adjustments for temperature setbacks and internal heat gains are provided.

In setpoint mode, the target value for system supply temperature is not dependent on outdoor temperature. It is based on a fixed setpoint temperature. This mode is often used to provide higher temperature water to load supplied through heat exchangers, such as domestic water heating, or snow melting. Boiler modulation reduces or eliminates the need for a differential temperature range centered on the target temperature. The latter is necessary to prevent short cycling with on/off boilers.

Some controllers also provide control for the main system pump and interfacing with the DDC BAS.

Figure 3. Three modulating boilers piped in parallel through a low-loss header. The low-loss header prevents interference between boiler circulators and system pump.

Tiny, Quiet, and Powerful

The physical size of several currently available modulating boilers relative to their heating output is indeed remarkable. For example, one manufacturer offers a system expansion capability of up to eight modulating boilers with an overall heat output range of approximately 100,000 Btuh to 3,200,000 Btuh. That represents an overall "turn-down ratio" of about 32:1. Equally noteworthy is that such a system may only require a mechanical room footprint of about 50 sq ft.

The low operating noise of such boilers is equally remarkable. It's often necessary to put an ear on the jacket to hear any sound from the boilers.

The current generation of small modulating boilers offers space savings, high efficiency, quiet operation, and reliability. They thrive on low-temperature operation and are ideally suited to radiant heating, snowmelting, pool heating, DHW or process water heating, and water source heat pump systems. Individually, these boilers will surely carve out their niche in residential heating systems. As part of a multiple boiler system, they will also establish a new paradigm for commercial boiler plants. Give them a good look for your next hydronic heating project. ES