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Hot water heating renewal series: plant sizing for best value

If you’ve been around the heating business for any length of time, you’ll come to the realization that heating plants are almost always oversized. It is common for the heating capacity of a boiler plant to be twice as much as is needed on the coldest day of the year. Why is this? Is it necessary for them to be oversized? When replacing, would it make sense to install boilers that more closely match the heating requirements of the building?


September 14, 2009
By Scott Martin

As a building owner/manager, you usually leave these decisions up to the engineer or the contractor.  So how do you know that they are making the best decisions on your behalf? The easiest thing for them to do is to install the same capacity as was there before. This “Re and Re” (Remove and Replace) method takes no analysis (time) with zero risk for the designer (if there is one). They know that if 12,000 MBH was enough before, than it will be enough now.

But as the building owner/manager, doesn’t it make sense to explore the options? Your staff may tell you that only half of the boilers ever operate at any one time. Why do you need 100% redundancy? It is common knowledge in project management circles that time spent at the planning/design stage pays huge dividends at the implementation stage. The proper analysis of your actual heating requirements will save you a significant amount on the installation cost and operating cost of your new heating plant.

Reasons for over sizing
At this point, you may be asking “Why the initial engineer oversized the heating plant in the first place?”  Most of today’s buildings were built in the 1960s and ‘70s. The engineers of the day did not have the computing power that we have today. They calculated the heating requirements by hand, which was no small feat. A small change in one number would mean recalculating everything. Therefore, the engineers would be very conservative in their numbers to avoid having to recalculate everything if the architect made a small change to a window size, or the material make-up of a wall. Also, it was common practice (and still is) to assume that the heating plant is the only source of heat. Internal heat gains from lighting, appliances and people are not accounted for.  In reality, during unoccupied periods the heating requirement drops, as supply and exhaust fans are turned off. 

Another reason for over sizing the heating plant is for redundancy. An “N+1” rule is often used, where “N” is the number of boilers required to satisfy the heating requirements. For example, if the calculated heating load is 8,000 MBH, the engineer may have elected to install 3 x 4,000 MBH boilers. In this scenario, if the designer overestimated the heating load by a mere 2,000 MBH, the heating plant would be twice as large as required.

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And can you blame the original designer for being conservative? There are no prototypes or “concept” buildings to test the design. There are no scaled models to measure the actual heat loss under different conditions. The design has to work the first time.

Capital cost benefits of right sizing
Retrofitting an existing plant gives the owner a unique opportunity to properly size the heating plant. The retrofit engineers have a huge advantage over the original design engineers, because they can see how the building reacts to various outdoor air temperatures. They also know from building operators, trend logs and/or observation exactly how many existing boilers are required to operate the plant.

Newer modular plants allow the retrofit engineer to right size the plant while maintaining “N+1” redundancy. Say only 6,000 MBH of the original 12,000 MBH is required. At peak heating requirements, only 3x2000MBH boilers are needed. For N+1 redundancy, a 4th boiler would be added.  Therefore, right sizing the plant would result in 4 new boilers instead of 6. At about $60K per installed boiler, right sizing the plant would save $120K, well worth the added cost of a proper engineering analysis.

Operating cost benefits of right sizing
A typical “Re and Re” (remove and replace) will result in little or no operating savings. The boiler piping and controls generally stays unchanged, resulting in high standby losses and low seasonal efficiencies.

By right sizing the plant, it is not uncommon to see a 20% to 30% reduction in fuel usage. A properly sized boiler plant with modular boilers, modern controls and an efficient piping arrangement can greatly reduce standby losses and increase part-load efficiencies.

Taking a little extra time and effort at the planning/design stage of your boiler plant retrofit is a sound business decision that will pay dividends for years to come. The added cost of design and analysis are miniscule compared to the capital cost savings and operating cost savings.

Scott Martin, P.Eng. (smartin@ee-solutions.com), a partner with Efficiency Engineering Inc. in Cambridge, specializes in energy efficiency and control of central plants.