By Stephen Varone, AIA and Peter Varsalona, PE
I’m on the board of a 51-unit cooperative in Manhattan. Our boiler is on its last legs (it’s 32 years old) and will need replacement. Our current system uses steam heat with an oil burner that’s approximately 25 years old. What factors do we need to consider when choosing a replacement boiler? Can we just replace the boiler alone, or should we also replace the burner at the same time? What about the rest of the heating system?
Few issues generate as much concern as a building’s ability to produce sufficient heat and hot water. Every 30 years or so, property owners are faced with the major task of undertaking a comprehensive upgrade of their existing heating plant. All too often, however, building owners neglect their heating plant until the system shuts down, forcing them to rush into a large-scale replacement program without having carefully considered the factors involved in such a critical project, such as which kind of boiler to install and the type of fuel to burn.
Boiler Types
The first consideration in any effective heating plant upgrade project is the type of boiler to install. Because you will be replacing a unit that’s more than 30 years old, you cannot simply assume that the new boiler will be the same type as the old one. For the vast majority of mid- to large-size apartment buildings, a steel Scotch-marine unit will be the boiler of choice. Steel Scotch-marine boilers, which typically last between 25 to 30 years, are fire-tube boilers: In a steam-heat system, hot flue gases, which are the byproducts of burnt fuel, pass through sections of tubing before they are emitted out of a chimney stack. Water in the boiler is heated by the tubes, producing steam, which is piped throughout the building to heating units such as radiator or baseboards in individual apartments.
For most mid- to large-size buildings, a steel Scotch-marine unit is the
boiler of choice.
The building’s domestic (potable) hot water runs on a separate line from the water used for heating, but it also can use the heat produced from the boiler, via an integral hot water coil, to heat the water that supplies kitchen and bathroom faucets, toilets, dishwashers, washing machines, hot water heaters, and the like.
Two other types of boilers, sectional cast iron and modular cast iron, are used far less frequently as replacement boilers in older, large-sized apartment buildings; they’re more common in new construction and low-rise developments. Iron boilers are much heavier than steel ones (they can weigh up to 800 pounds a section) and more difficult and expensive to replace. In addition, iron boilers usually have a lifespan of less than 25 years and are typically limited to smaller buildings because of their limited power generation (175 horsepower compared to up to 700 HP for steel Scotch-marine). On the plus side, cast iron boilers do not require as much space as steel boilers and therefore are frequently found in buildings with small basement areas.
While it’s less inexpensive to install one large boiler equipped with an integral coil for domestic hot water production, this setup leaves the building vulnerable to a complete loss of heat and hot water if the boiler breaks down. Ideally, as a backup measure, if budget allows, building owners would replace one large boiler (even if equipped with a separate hot water heater for use during the warm weather months) with two slightly smaller boilers (say, 75 percent capacity each). With this configuration, the building can alternate between the two boilers, which improves heating efficiency and extends the life of the boilers, in addition to providing redundancy protection.
Fuels to Burn
The next factor to consider is which type of fuel to burn in the heating plant. For systems burning only oil, there are three choices: #2, #4, and #6 oil. Number 2 oil, which is essentially diesel oil with additives, is the cleanest oil to burn, but it is also the most expensive. Number 6 oil is the least expensive to burn, but because of its thick viscosity it has to be pre-heated before burning, and it tends to foul up systems, leading to higher maintenance and equipment costs. Number 4 oil, produced from mixing #2 and #6, has qualities in between the other two types.
For heating plants that burn #2 or #4 oil, additional equipment such as a sidearm pre-heater and air compressor would have to be installed to run #6 oil. Since the relative costs of the different fuel types may change over time, project specifications for the burner replacement should be written so that the units can burn any of the various oil types—or gas.
Gas is the cleanest fuel to burn, but it also tends to be more expensive than running solely on oil. One option for boards planning to replace their burners, which typically last between 15 and 20 years, is to switch to what’s called an interruptible system—one that burns both gas and oil. In an interruptible system, a dual-fuel burner burns gas most of the time and switches to oil only when the temperature dips below 18°F. Utility companies sometimes offer a more attractive rate for interruptible service because it enables them to reduce their required gas load during the cold weather. (The size of the discount varies based on the size and anticipated usage of a particular building.)
If your board opts for an interruptible gas system, you will need to install additional equipment. A new dual-fuel (gas-oil) burner will be necessary, as well as a new gas train, which is a pipe that runs from the utility’s gas main (the line coming into the building) to the burner. If the new burner requires high-pressure gas, you may also need to install a gas booster pump. For large buildings with heavy energy usage, the utility may offer a rebate toward the purchase of this new equipment.
Heat Distribution
A crucial component all too often overlooked in a heating plant upgrade is the distribution system. Replacing a boiler does not automatically improve heat distribution to individual spaces throughout the building. In any heating plant upgrade, the risers, branch lines, heating units (radiators, baseboard heaters, etc.), and air-release valves should be evaluated and replaced where necessary. Corrosion, leaks, or other defects will affect the ability of the new boiler to send heat to individual apartments and spaces. In addition, boiler water must be treated with anti-rust chemicals to prevent deterioration to the boiler and distribution system.
The best time to replace a boiler is during the warm weather months.
Because today’s boilers are much more efficient than units installed 20 or 30 years ago, a new unit may produce more steam and hot water than the existing heating system is able to effectively handle. A new boiler, therefore, may also require the installation of a boiler feed-water tank to handle the excess condensate that results from using a better-operating unit. The feed-water tank regulates the return condensate back to the boiler to prevent system surges and overflows.
Finally, two other important factors should not be overlooked when planning a heating plant upgrade. First, oil tanks typically last 35 to 40 years, but problems such as leaks caused by corrosion can develop over time and need to be looked at as part of the overall upgrade project. Second, asbestos-containing materials are often found in insulation around water pipes and other heating components, so their removal will entail an asbestos abatement as part of the heating plant upgrade, which can significantly increase the cost and time spent on a heating plant upgrade. Failing to consider such “incidental” items can lead to budget “surprises.”
The best time for replacing a boiler is during the warm weather months, when the building’s heating needs will not be as pressing as during the winter. During the eight to 10 weeks on average that it takes to replace the existing unit, the building will need a temporary boiler. (Even during the summer, a temporary boiler will be needed for hot water.) Temporary boilers usually are placed in a trailer close to the building, and the steam or hot water is piped in through a hose connection. Permits from the New York City Department of Transportation and the Department of Buildings will be needed if the trailer is parked on the street. (This is separate from the Buildings Department permit needed to ultimately replace the boiler.)
Given the considerable costs involved in a heating plant upgrade, it’s best to start with a detailed feasibility study that addresses all relevant factors, such as boiler size, fuel type, distribution components, additional equipment, and the like so an appropriate and cost-effective plan of action can be taken. With proper planning and consideration of the various upgrade options, boards can save a great deal of money and grief over the long haul and never be left in the cold.
Stephen Varone, AIA is president and Peter Varsalona, PE is principal of RAND Engineering & Architecture, DPC. This column was originally published in the March 2004 issue of Habitat Magazine.