The first thing you want to do is perform a heat loss calculation. By establishing the true load on the building, the replacement boiler or boilers will not be oversized. However, the conditions we use to establish the heat loss of the building are assumed to be at design conditions, which means the coldest day of the year. If you want this capability, then you need to size the boiler or boilers for this maximum load. But, there are other considerations.
With multiple smaller boilers, when the load on the building is light, only one of the boilers may be required to heat the space. This smaller boiler will operate for a long run cycle, increasing its operating efficiency, while the other boilers remain "off," keeping the building owner happy because he is not wasting fuel.
There are several methods available to prevent this unwanted flow from occurring through the off boilers, but the best method is to pipe them by using the technique called primary/secondary pumping, whereby each boiler has its own circulator which is sized just for the flow rate and pressure drop of its boiler.
The boilers are piped into a manifold, which is connected to the primary loop through a set of closely spaced tees. Piped this way, the primary loop circulator will not create flow through any of the boilers. The circulator on the individual boiler will cause the only flow that occurs.
Built-in redundancy. Because design conditions typically exist for only 3-5 percent of the heating season, if one of the smaller, multiple boilers goes down, the remaining boilers will meet the building's heating load.
Off-the-shelf high-efficiency boilers can be used to satisfy the heat load in light commercial applications. The boiler plant thus operates at higher efficiencies and replacement parts are easily accessible.
Smaller packaged residential boilers may be used in some commercial buildings, instead of bringing in commercial sections and constructing the boiler in place.
Large domestic hot water loads in commercial and large residential applications can be met with multiple boilers. The staging control will fire the appropriate number of boilers to satisfy both the heating and domestic water load, and then shut them off as the domestic load is satisfied.
Summer operation efficiencies can be realized in some commercial applications where the only load is domestic hot water. Here, a staging control will fire only the appropriate number of boilers to satisfy the domestic water load, no matter how heavy or light.
Reprinted with permission from the Bell & Gossett newsletter CounterPointâ„¢, volume 11, issue 1, June 2004. For more information, visit the Bell & Gossett Web site at www.bellgossett.com.
Publication date: 03/21/2005