Designing Efficient Boiler Systems
Heat Load, Operating Temp, Piping Are Top Concerns
When it comes to designing and installing boiler systems that take advantage of the efficiency promised by newer condensing models, there are a lot of factors to take into account. According to the experts, the most common mistake made by designers and installers is failing to calculate the correct heat load. Other common errors occur when system operating temperature and piping are not properly considered.
Although it’s an often-repeated refrain in the HVAC world “bigger isn’t better,” experts say oversizing boilers is still one of the most common mistakes made by designers. Oversizing occurs when designers fail to obtain a proper heat load calculation, said Gerry Hogan, director of training and applied technology, PB Heat LLC, Bally, Pennsylvania. “A designer may assume that matching the capacity of the boiler being replaced is accurate enough,” Hogan said.
David Hansen, product marketing manager, U.S. Boiler Co., added that designers frequently oversize “based on existing radiation or boilers.”
But making these assumptions instead of properly calculating heat loss is the wrong way to start out. “Oversizing leads to short cycling, decreased efficiency, increased operating costs, and equipment wear,” said Don DeCarr, hydronic/warm air product manager, ECR Intl. “Swapping Btu for Btu is probably the worst way to size a boiler. It is not uncommon to find 20- to 30-year-old equipment that is 50 percent oversized. And, even if it wasn’t grossly oversized when installed, over the years building envelope improvements, such as insulation, windows, etc. may have drastically reduced the heat loss.”
Rather, DeCarr said, designers must start with an accurate Manual J calculation.
Once heat loss has been accurately calculated, Hansen warned designers should not “underemphasize” it. Instead, Hogan said designers must “match the boiler capacity to the load.” And, he noted, they should consider providing additional capacity for domestic hot water (DHW) production, if needed.
Design for Lower Temps
Hydronic designers must also have the right mindset when it comes to system operating temperature in order to take advantage of the efficiency offered by condensing boilers.
According to Matthew Freeman, boilers product manager, Bosch Thermotechnology Corp., “With a conventional boiler, the installer wants to be sure to protect the cast iron heat exchanger from thermal shock, which occurs when cold return water enters the hot cast iron boiler. Expanding or contracting too quickly may cause the cast iron sections to crack and leak. With condensing boilers, installers are encouraged to bring low return water temperature supply back to the boiler for condensing.”
And the reason for that encouragement is, of course, tied to the system’s efficiency. Freeman explained, “For every 3°F that supply temperature is reduced, there is a 1 percent savings in fuel. Utilizing 160° and 140° supply water encourages condensing and thus reduces energy consumption.”
Yet Joel Southwell, director of commercial heating, The Fulton Cos., noted, “Hydronic designers may not be selecting coils or heating elements that allow for lower operating temperatures of the system.”
He continued: “A slight increase in surface area can lower the hydronic loop temperature, thereby increasing the condensing boiler efficiency and lowering gas usage. Designing the hydronic system around a higher Delta T [30°-40°] decreases system pump kW requirements and allows colder return water temperatures to the condensing boiler.”
Read the Piping Directions
Another common source of errors is found in the near-boiler piping for condensing boilers. A lot of potential mistakes can be avoided by simply reading the manual.
For instance, Hogan has seen “near-boiler piping that is too small to carry the heat out of the boiler.”
In these cases, he said: “A designer may simply match the tappings on the boiler, especially on condensing boilers. When the piping is too small, the boiler reaches the target quickly and will short-cycle, with the burner starting and stopping frequently. This effect is often compared to ‘city driving’ of a car with a resulting lower efficiency.”
To prevent this problem, Hogan said near-boiler piping should be modified to ensure proper system water flow to the boiler.
Freeman stressed these kinds of mistakes occur when the manufacturer’s instructional literature is ignored. “Each manufacturer has its own heat exchanger design. With that comes different pressure drops and flow rate capabilities. It is important to consider this when selecting a piping arrangement,” he said. “Cast iron boilers typically were large-mass, high-volume vessels that could be piped in a supply/return header configuration. Condensing boilers have much less mass and volume, requiring primary secondary piping or the use of a low loss header.”
DeCarr agreed that many problems can be avoided by slowing down and looking at the manual first.
“The most common installation errors occur because the technician isn’t familiar with the equipment,” he said. “Particularly, if it is your first time with a brand or model, take some time to read through the installation manual and pay close attention to venting requirements, fuel supply pressure/line sizing, any special piping arrangements required or recommended, etc., your profits will go out the window if you have to redo the vent or re-pipe the boiler, and the customer won’t be too pleased you have to return to tear something out.”
Publication date: 10/27/2014