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Is Your Commercial Boiler System Oversized?

Hint:  The answer is Yes!


Well, I spilled the beans in the title, didn’t I? How am I able to make such a blanket statement? I am a mechanical engineer, I know mechanical engineers, I’ve been a partnering consultant for engineers for over 15 years for boiler systems, and I’ve worked with some of the best boiler manufacturers in the industry. I have learned that no one wants to take unnecessary risks.


It was pointed out to me by one engineer that he had never been called by a customer complaining about too much hot water or heat. If they call and say they don't have enough heat, then that is usually a sign of an expensive mistake. By the nature of our business, no one wants to risk under-sizing a boiler system by design. Let's agree that it would be impossible to size a system perfectly; there are just too many variables involved. So, if it is not under-sized, and it is not perfect, then by simple deduction your boiler system must be oversized. 


It is oversized, but to what extent? Whether or not it is a good thing is up for a separate discussion.  Certainly, we cannot blame designers for making the conscious decision to err on the side of caution.  Plus, oversizing is inherent in the design and selection of the boiler system. So, how is a boiler system sized and equipment selected?  First, the engineer determines the total heating load (or heat energy needed) for the building. In this case, the load is calculated according to the worst possible climate condition expected in the installation area.  (By the way, in most cases the worst conditions are only ever realized for less than 12 hours a year!) 


The load also assumes no internal heat gains, meaning there is nothing else in the building that would give off heat that would lower the heating responsibility of the boiler system (such as the building being unoccupied and all equipment like desk heaters or computers are turned off). When determining the total heating load, a diversity factor may be employed. This means that a portion of the total heating load may be removed from the equipment sizing considerations. There may be circumstances where the engineer knows that the total load calculated will never be needed due to how the building is used. In my experience, diversity is rarely considered for heating. Once the total heating load for the building is determined, then equipment may be selected. As an example, let's say the total heating load calculated is 12,690-MBH.  Typically, commercial heating boilers are built for round number capacities, such as 1500 MB, 2000 MB, 300 MB, 400 MB, and 6000 MB. It should be obvious that no combination of these numbers will add up to 12,690. In order to achieve that capacity, multiple boilers with larger capacities are combined. There are times when excess capacity can provide the boiler system with a standby boiler for much of the year. There is nothing wrong with oversizing, and in most cases, it can be beneficial. However, if not managed properly, grossly oversized boiler plants could present operational problems. The coldest day of the year is the best time to inspect your system. How close is it to running at 100%? What does this mean for your system? 


Be sure to reach out to your boiler system designer or boiler equipment supplier for help. Understanding how your boiler system operates and performs on a design day may aid you in resolving issues and preparing you for future decisions.

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