Weil McLain

What is a Multiple Boiler System (MBS)?

A MBS heating system is one that uses:
Two or more smaller boilers in place of a single large boiler
A Boiler Control Panel (BCP) to turn boilers on or off as needed for maximum efficiency
Specially-designed piping

How do MBS work?

The Boiler Control Panel (BCP):
Looks at inputs (i.e. temperature readings and remote signals) and calculates how much heat is needed
Turns on (or off) just enough boilers to handle changes in demand using its knowledge of how the system responds

Why use MBS?

In large homes or commercial applications, you save money. Weil-McLain MBS use less fuel to meet system heating needs - by eliminating unnecessary boiler cycling.  Other systems or single-boiler systems can waste 25% or more fuel due to cyclic losses.  Also, you know you'll have heat - even if a boiler is off line for service, the rest of the system keeps running.

How are lower operating costs possible?

The answer is simple: There are three different kinds of boiler efficiency:
Combustion efficiency - how efficiently the boiler burns the fuel
Steady-state efficiency - how efficiently the boiler uses the heat from combustion when operating under full load
Seasonal efficiency - how efficiently the boiler uses fuel over the entire heating season

While each type is important, seasonal efficiency is the most important because it determines how much you will pay for fuel over the course of the heating season

An easy way to understand the difference between steady-state and seasonal efficiency is to picture an automobile. On the expressway, going 60 mph at 27 mpg, the car is using gas very efficiently.  This is similar to steady-state efficiency, which assumes that the boiler always operates under full load.  But as soon as the car reaches the city, the average mileage will drop to 15 mpg due to starting and stopping.   Now the car is no longer using gas as efficiently.  This is similar to seasonal efficiency, which will not be as high as steady-state efficiency because of the start-and-stop cycling of the boiler during the heating season.

High seasonal efficiency requires good steady-state efficiency as well as good combustion efficiency.  But a boiler with a high steady-state efficiency - say, 80 percent - can easily have a seasonal efficiency of only 65 percent!  It all depends on downtime losses - the losses that occur when the boiler isn't operating.  Downtime losses depend on boiler construction, type of application, and design of the system.  

When a boiler shuts off, the heat in the boiler continues to radiate through the jacket.  This is lost energy.  In addition, boiler-room air continues to flow through many boilers after the burner shuts off.  This air is drawn up by chimney draft, cooling off the boiler.  When the boiler turns on again, it must reheat the boiler water back to operating temperature.  Further, it must also reheat the chimney to produce proper draft for efficient combustion.  The more often the boiler cycles, the greater the downtime losses and the lower the seasonal efficiency

The longer a boiler operates, the higher the seasonal efficiency.  Therefore, a boiler that is smaller than required will more closely match the heating load of the building for a larger part of the season because of fewer on-and-off cycles.  When the first boiler can no longer keep up with the heat loss, a second boiler picks up the extra load, and then a third boiler, if necessary.

However, the key to maximizing efficiency with two or three boilers is to be sure that each boiler is completely isolated from the others so that non-operating boilers will not be hot with system water.  This will eliminate most of the jacket heat losses from the non-operating boilers.

MBS Commercial Solutions

One of the largest school architectural and engineering firms in the U.S. recommends Weil-McLain boilers.  "School boiler systems must be efficient," said Charles McCoy, senior associate, mechanical designer, of Fanning/Howey Associates, "but they also must be dependable and easy to maintain.  We have seen Weil-McLain systems reduce heating costs from $90,000 to $12,000 annually.  That's really remarkable in an area of the country where we have so many days when the temperatures fall below 65 degrees F.  These boilers can be serviced by local heating contractors.  That can mean a tremendous cost savings for the school."  

For smaller commercial buildings (up to 3 million BTU/Hr. heat loss), there is an even better method for increasing seasonal efficiency: multiple boiler systems using two or more high-efficiency residential boilers.

Residential cast iron boilers are available with efficiencies as high as 87 percent, whereas commercial boilers operate at about 80-percent efficiency.  The steady-state efficiencies and seasonal efficiencies of the new high-efficiency residential boilers are about the same whether these boilers are used individually or in multiple systems.

In a multiple boiler system, the lead boiler (the first boiler fired) will handle the load on the first cool autumn day.  As the outdoor temperature drops, the second boiler will fire, and then the third, and so on.

As with two or more commercial boilers, the key to maximizing efficiency is isolation of the non-operating boilers.  Many manufacturers' multiple boiler systems do not - or cannot - isolate each boiler, which essentially defeats the efficiency gains possible with multiple boiler systems.

There are several advantages of multiple boiler systems.  The system will still function even if one of the boilers requires service.  The piping system brings the coolest return water equally to each operating boiler, thereby increasing efficiency.   And one or more of the boilers can be used for high-efficiency domestic hot water heating without detracting from comfort heating.

Primary-secondary piping.

The key to maximizing efficiency with two or more boilers - either residential or commercial - is the Weil-McLain primary-secondary piping system.  With this system, the primary piping circuit runs continuously to supply the radiation; each boiler has its own secondary circuit to add incremental amounts of  heat into the primary piping.

The Weil-McLain primary-secondary piping system:
Assures precise water temperature control
Isolates non-operating boilers to eliminate wasteful jacket heat losses
Returns the coolest water to each operating boiler to maximize efficiency
Reduces installation costs through the use of Weil-McLain's Easy-FitTM Manifolds

Energy management control systems (EMCS).

Weil-McLain offers a wide selection of energy management control systems to automatically sequence the boilers to match heating load requirements for increased efficiency and reduced fuel usage.  Control packages and prewired panels are available for both commercial boiler systems and multiple boiler systems.  In addition, there are control systems for zoning with zone valves or circulators, as well as for combined space heating and service water heating.

Increasing efficiency of an existing boiler.

When an old, inefficient boiler still has many years of service left, and the building owner does not want to invest in a complete replacement, two or three high-efficiency residential boilers can be installed to supplement the existing boiler.  Using primary-secondary piping, the new boilers handle the load in milder weather at seasonal efficiencies as high as 87 percent; the old boiler fires in only the coldest weather.   Even in a northern city as New York, the average winter temperature is only 43°F, so the small boilers can handle the demand during most of the heating season.

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