Modular boiler systems have been gaining in popularity since the energy crisis of the 1970s due to their compact design and energy efficiency. Combined with a microprocessor control unit, the system makes a perfect prime mover for your school or commercial or institutional building. Modular boiler systems are appropriate for either new construction or as a retrofit solution to that high- maintenance energy hog in the basement.

Boilers installed in commercial and institutional buildings from the 1930s to the 1970s were likely to be fire-tube packaged steam boilers with power burners rated at 4,000 to 8,000 lbs/hr or less. These boilers were sufficiently large that in many cases the building was erected around the boiler, making removal of the boiler difficult.

The easiest solution was to replace the single large boiler with several small boilers. The small boilers were of a sufficiently compact size that they could be brought in through the front door, carried in elevators or even moved down stairways if necessary. The units could be placed in any arrangement, making the best use of the space and often accommodating new equipment to boot. The process was time-consuming, but it didn’t cause the closure of the building. Business went on as usual.

Figure 1. The atmospheric boiler is simple and inexpensive.

Do Modular Boilers Save Energy?

There is no question that modular boiler installations save energy over existing old, inefficient boilers. But do they save energy over new models of the traditional fire-tube designs? The answer is a qualified —yes— the array saves energy by improving combustion efficiency and reducing jacket (skin) losses.

Although boiler heat transfer varies considerably for different designs, modular boilers generally provide higher combustion efficiency through closer control of the flame/coil interface. In addition, modular boilers reduce jacket losses because each one loses heat through the jacket only when it fires, and each one fires only as it is required.

Therefore, jacket losses are approximately proportional to load. For a large boiler, jacket losses are nearly constant regardless of load, and this reduces their low-load performance.

On the other hand, a single, larger boiler will have more heat transfer surface per BTU of load at low loads, as compared to the corresponding modular array. This helps its performance at low loads and somewhat offsets the increased skin losses.

To complicate matters further, either type of installation can be equipped with forced air (“power”) burners, modulating fuel controls, combustion air dampers, and a variety of other accessories that will also save energy.

Still, all else being equal, the average modular boiler system saves energy over the average single or dual boiler installation, although this savings is probably only a few percent. This means that although modular boilers may be an easy choice for restricted space retrofits, other factors need to be considered for new building applications.

Figure 2. The power burner adds efficiency and versatility to the system.

When Does A Modular System Make Sense?

First of all, is a boiler required at all? If it is possible to heat the building with gas-fired forced-air furnaces or gas-fired rooftop units, these alternatives might save first cost and operating cost. However, forced-air systems have definite limitations when it comes to building size, comfort, noise, and longevity. Most schools and other institutional buildings are built to provide consistent, quiet, and effective operation for many years. For this reason, hydronic (or steam) systems are frequently the best choice.

Steam heating systems are rarely used in new construction, but some buildings need steam. For example, medical facilities and laboratories need steam for autoclaves and other equipment. Any building requiring a 100% outside air system may need a steam coil to temper outside air. If a building has a large steam load, modular boilers are probably not the best choice. Although modular steam boilers can be piped together, this is unusual and exposes the owner to an increased danger of error in design and operation.

However, if a building has only a minor steam load, it may be possible to provide one or two modular steam boilers for the steam load and use several others for space heating. In this way spare parts can be minimized while reaping the energy savings of the modular system.

If a steam requirement for kitchen equipment is a roadblock, gas-fired equipment such as kettles and warming tables are usually available. It is generally not a good energy decision to install a steam boiler for a building when the only steam loads are kitchen equipment. A hydronic system will be easier to control, simpler to maintain, and will reduce energy bills as compared to a steam system.

Figure 3. Balanced piping provides even loading.

Selecting The Right Boiler For The Job

Small hydronic boilers suitable for modular installations come in a wide variety of shapes and sizes. They can be manufactured of different materials, fitted with a variety of burners, and configured to burn any fuel. Small boilers should be completely factory-assembled and -tested with no field assembly required.

This labor savings is an important factor in the payback of the system and vital to the reliability of the boiler array.

Most small boilers intended for hydronic heating systems are manufactured with heat transfer surfaces of either cast iron or copper finned tubing.

The heart of the cast iron boiler is the water box. Both sides of the box have an engineered heat transfer surface. The surface on the inside consists of ribs and posts that direct the water flow in a circuitous pathway from the inlet to the outlet. The airside contains posts to increase the surface area in contact with the hot gases and create turbulent flow.

Copper fin tube boilers employ a heat transfer surface of specially designed copper tubes with copper fins joined to the outside (the fire side). These tubes may have specially formed or machined inner surfaces to enhance waterside heat transfer efficiency. The copper tubes may employ cast iron headers on both the inlet and outlet sides.

The difference in performance between cast iron and copper tube boilers relates to their mass. The cast iron water box is massive and holds more water than its copper tube counterpart. As a result, the response time of the cast iron boiler is the longer of the two. Although this is not a big advantage in space heating, it provides domestic water faster for building showers and kitchens. The larger mass of the cast iron boiler works to its advantage in hydronic heating, helping to stabilize temperature. This helps valves and sensors downstream to control their loads in a more stable fashion.

Although copper tube boilers are available in very energy-efficient designs (up to 97% AFUE-Adjusted Fuel Utilization Efficiency), they are not always more efficient than their cast iron counterparts. Both types get more complicated as they get more efficient. All modular boilers with efficiencies above 88% to 90% are condensing boilers and use special materials to cope with the attendant threat of corrosion. Given the fact that the lowest efficiency of any boiler in this article is about 81%, the span of highest to lowest efficiency is 81% to 88% AFUE for noncondensing designs.

Cast iron boilers are generally considered more durable than fin- tube types, although the design and installation can make or break either boiler. Either boiler will corrode if exposed to condensing conditions on the airside and this must be avoided (see the manufacturer’s information on low temperature systems).

Either boiler may leak if exposed to thermal shock. Both types of boilers will suffer a decrease in efficiency if their waterside surfaces become coated with lime or other minerals in the water.

Figure 4. Secondary circulators ensure the best performance.

Atmospheric And Power Burners

Small boilers suitable for modular configurations may be equipped with either atmospheric or power burners. Atmospheric burners draw combustion air past the burners by natural convection. This provides adequate mixing and efficient combustion for either natural gas or propane, but not oil. Power burners fire natural gas, propane, and several types of fuel oil.

“Duel fuel” burners are actually two burners combined into one, such as an oil burner combined with a gas burner. The resulting flame control problems make this type of burner inappropriate for small boilers. If a facility needs to burn oil as well as gas, the problem is solved by an array of modular boilers that includes both gas-fired and oil-fired units. The gas-fired units could use either atmospheric or power burners and the oil-fired units would be equipped with power burners. For example, a facility that normally burned gas but used oil as an emergency back-up fuel could use a combination of burner types on the boiler array.

Atmospheric burners are the simpler of the two types, having no moving parts downstream of the gas train (Figure 1). They burn in the same manner as a gas burner on a kitchen range. Power burners use a blower to force combustion air into the boiler and provide better mixing with the fuel (Figure 2). Boilers equipped with power burners are capable of higher efficiencies than atmospheric boilers. Noncondensing boilers equipped with power burners range to about 88% efficiency and those with atmospheric burners top out at about 84%.

If the 4% increase in efficiency were the only driving factor, it would be hard to justify the increased first cost and maintenance of the power burner. But the power burner provides more than efficiency; it provides a more positive and reliable combustion system through a broader range of temperatures, wind conditions, and grades of fuel.

Atmospheric boilers can also be equipped with a fan to “power vent” the products of combustion to the vent stack. The power venting atmospheric boiler cannot burn oil but does provide a significant improvement in the reliability and flexibility of the boiler installation by allowing sidewall venting and better combustion control.

Figure 5. First-on, last-off boiler sequencing.

Which Fuel Is Best?

Natural gas is the best fuel for modular boiler systems. It burns clean and is available at competitive rates from abundant supplies. It is lighter than air and will not pool in basements or sumps.

If natural gas is not available, or if a backup fuel is required, propane or #2 fuel oil are the next best choices. Propane has a definite first cost advantage because many small boiler burners can be easily converted to burn it. It is a good choice for a backup fuel for facilities trimming their gas costs through an interruptible supply contract. For facilities using interruptible gas, the normal 24-hour notice of curtailment should be ample time to convert several burners to propane service.

The downside of propane is that some locales consider it dangerous to store the gas in large quantities inside the city limits. Burying the tank may ease concerns and this can usually be done without danger to the environment. If a facility is considering using propane as a fuel, local fire authorities must be involved in the design from the beginning. Also make sure that the risk management office is involved and the insurance company has a chance to review the plans.

Fuel oil is also a viable fuel for small boilers, although it has declined in popularity as natural gas distribution systems have grown. Underground oil tanks are regulated by environmental codes and require corrosion protection as well as leak monitoring. Aboveground oil tanks are unsightly and malodorous but may be the only feasible alternative. A power burner must be used to burn oil and this adds complexity to the system.

Figure 6. First-on, first-off boiler sequencing.

Piping Up The Team

Having selected the best boiler for the application, the next step is to design the hydronic loop. Most modern hydronic installations use primary/secondary piping arrangements to some degree. This provides considerable flexibility and reliability over a simple series piping arrangement. The owner and the design team need to make the decision as to which piping method is best. That is, the boilers may be simply piped in parallel or they may be piped with a secondary (pumped) loop at each boiler. Many modular boilers are factory-equipped with small circulators for exactly this purpose.

Figure 3 shows a piping diagram for an array of four modular boilers piped in parallel to a simple primary circuit. The pressure differential provided by the primary circulator provides circulation through each boiler. Each boiler is equipped with a balance valve and a motorized (or solenoid) shut-off valve to isolate the unit when it is not firing. This piping arrangement is called a balanced arrangement because the length of piping and number of fittings across each boiler is equal. This provides a fairly uniform flow to each unit, allowing the balance valves to be used for fine-tuning.

Figure 4 shows a piping diagram for the same boilers piped with secondary boiler circulation loops. Although this adds first cost as well as some pump energy and maintenance, this arrangement is preferred over that shown in Figure 3 for the following reasons:

  • The individual boiler (secondary) pumps circulate independently of the primary pump (the two pumps are “decoupled”). The primary pump can be sized and controlled to deal only with the building heating loads, without the variations that will be imposed by various boiler combinations. Any number of secondary pumps can be operating without having a significant effect on the primary circulator.
  • Each boiler’s secondary pump ensures a constant and reliable flow through the boiler that is not affected by the primary pump or the overall building load. This ensures optimal heat transfer efficiency from each boiler and reduces the chance of boiler “hot spots” and mineral deposits due to poor circulation. The motorized valve at each boiler operates in conjunction with the secondary circulator to shut off water flow through the boiler when it is not firing. <
  • The secondary pumps can be used to provide some freeze protection for the boilers even if the main circulating pump is shut down (for instance, during a night or weekend setback period). The best freeze protection is obtained by using inhibited glycol antifreeze in the hydronic loop. But pure water systems provide lower first cost and better heat transfer efficiency. Pure water hydronic loops are feasible in mild climates with secondary pumps and freeze protection thermostats used to prevent coil freeze-up.

Whichever pumping arrangement is chosen, time delay relays should be wired in series with the secondary circulators and automatic shut-off valves. Delaying the isolation of the boiler allows the residual heat in the unit to be dissipated into the hydronic loop. If the boiler is isolated, the residual heat will be lost up the stack and may even cause the temperature high-limit switch to trip. If the high-limit trips, this generally means a special trip to the boiler room to manually reset the safety.

Secondary circulators complete with time-delay controls are available factory-installed on many makes of boilers. This is an option that should be seriously considered.

Control Is Everything

Modular boiler arrangements are generally controlled by a microprocessor-based control unit that acts like a mini-building automation system (bas). This central controller mounts on a wall of the machine room and is equipped with an LED readout and programming keypad. Units are available that are interoperable with many bas controllers and provide complete information on every boiler from the central bas workstation. Through Internet servers, boiler operating information can be accessed from any Internet port in the world.

Some boiler manufacturers offer proprietary controllers and will provide the controller as a warranted part of the boiler package. Other controllers are available from third-party vendors specializing in controls manufacturing. Only the owner and design team can decide which is best for a given installation. Regardless of the model chosen, it should have at least the following capabilities:

  • First-on/last-off sequencing, and first-on/first-off sequencing;
  • Unoccupied setback;
  • Outside air reset;
  • Outside air lock-out;
  • Minimum cycle time;
  • Adjustable dead band; and
  • Extra stages for future boilers.

And, if applicable:

  • Combustion air damper control;
  • Low-fire/high-fire staging;
  • Auto-dial modem; and
  • RS 232, BacNet and/or LonTalk port.

In the case of boilers controlled by return water temperature, as the building load increases, the return water temperature drops and the boilers fire in the programmed order. Additional boilers continue to come on-line (with a programmed time lag between boilers) until the return water setpoint is satisfied. The boilers then shut down in a prescribed order and the process repeats.

The supply water temperature should be reset with regard to outside air temperature. As the outside air gets colder, the hydronic loop should get hotter. This is a simple means of providing the best control for the building in a wide variety of conditions.

First-on/last-off sequencing provides a mode of operation wherein one boiler stays the first-on or “lead” boiler until the program is manually changed. The rest of the boilers also fire in the same order until changed. This concentrates the load on the first boiler and to a lesser extent on the second, third, etc. units.

Figure 5 is a diagrammatic example of this mode of operation. The building runs during the night in a setback mode with only boiler number one operating. As the load picks up in the morning, boilers one through four fire. As the building warms up only three boilers fire at noon and two at 3:00 p.m. By 6:00 p.m. the building is once again in night setback and only a single boiler carries the load.

Notice that the lead boiler stays on most of the time. This type of sequencing is best when one or two boilers are preferred over the rest. For example, if two boilers had recently been replaced and are the most efficient and reliable, it might be preferable to use them the most.

In first-on/first-off sequencing, the group of boilers in use rotates through the group so that each boiler gets the same average use. As shown in Figure 6, only boiler number one is required during the night setback mode. By 8:00 a.m. four boilers are in operation. When the load decreases at noon the original “lead” boiler shuts off. At the next decrease in load, boilers two and three drop off-line. This continues until boiler number four is the only boiler firing during night setback.

The following morning, when the building is once again in full load, boiler four will become the “lead” boiler. On an increase in load, the three boilers after boiler number four will fire (boilers five, six, and one). As the cycle repeats, all boilers are used in rotation and the usage time averages the same for each unit. This is the mode of operation that is usually preferred for modular boiler arrangements in new construction.

And Don’t Forget...

No matter what boiler arrangement you use for your next hydronic project, remember that commissioning the project adds quality from start to finish. The commissioning authority (CA) should confirm that the owner and engineer understand the boiler application and have mutually agreed upon the approach. In checking the design the CA will further assure that the installation meets local and state code requirements and provides the O&M staff with the space they need to maintain the boilers.

The CA will confirm that the correct control sequence has been programmed and the boilers work as required. Finally, commissioning sees that the O&M staff is trained to operate and understand the boilers. This ensures a long and efficient life for your modular boiler system. ES