Favorite Multifamily Retrofits
Which retrofit is better: changing the boiler or changing the lighting? There are so many possibilities for multifamily buildings that choosing the "best" retrofit depends a lot on your definition. Are you after the most cost-effective option? Then low-cost or no-cost fixes like adjusting equipment settings will probably win. Are you interested in massive energy savings? Then a deep energy retrofit involving everything from envelope to equipment is probably needed.
At Steven Winter Associates, we've been recommending energy saving retrofits for multifamily clients for many years. Some have been small and inexpensive, while others constituted overhauls of major systems. Choosing the "best" retrofits among these is pretty difficult, but over the years we've come to appreciate a few more than others. In this and a future issue we'll describe six of SWA's favorite retrofits, some of which may have application to a building near you.
Supply and Return Temperatures from One Boiler in a Two-Boiler System
Table 1. Boiler Venting Categories
Atmospheric versus Sealed-Combustion Boiler Venting
Advanced Control for Electrically Heated Buildings
Electrically heated buildings are common throughout the United States. In areas of the country with high heating loads and high electric rates, some common forms of electric heat may translate to a major expense because electricity can cost much more per Btu delivered to the building than gas or oil. In addition, many cheaply-built multifamily electric buildings suffer from an unfortunate but common combination—a miserably poor envelope and a poorly controlled and inefficient heating system.
Retrofitting these buildings for major energy savings can be a challenge, as converting the building to other forms of heating like hydronic or heat pumps requires a major capital expense. Still, effective retrofits for electrically-heated buildings can follow the familiar script we all know: reduce the heating and cooling loads by treating the envelope, and then strive for better control.
Whole-building electric heating controls have been around for quite a long time, but recently they have gotten a boost with the advent of lower-cost, more sophisticated wireless technologies that drop installation costs and open up new possibilities for control. There is great cause for hope for the many electrically heated multifamily buildings across the United States.
Roosevelt Landings, a multifamily complex in New York City, is a case study in this strategy. This million square foot-plus complex built in the 1970s houses over 1,000 apartments and is heated by electric baseboards. Like many from its era, it is masonry construction but uninsulated, and electricity is master-metered. Residents had little incentive to conserve heat since they weren’t paying for it directly, so open windows were a common sight, and overheating and underheating was rampant. This project used a multipronged strategy to cut their electric costs.
First, the owners hired an experienced air-sealing company to seal as many holes and cracks in apartments as possible: electrical outlets, missing seals on windowsills, air conditioner sleeves, and so on. Electric baseboards were removed for service or replacement, and gaps in the drywall behind them were sealed. Next, uninsulated surfaces were treated. Many of the lower floors had exposed concrete slabs which were a cause of cold complaints and energy loss. High-density polyurethane foam was applied to the underside of these slabs. These measures reduced heating loads and, according to resident feedback, improved comfort.
The next challenge was improving the rudimentary control of the electric baseboards. A common type of baseboard has a simple bimetal coil thermostat integrated in the unit. This thermostat doesn't sense the room temperature as a wall-mounted thermostat would—especially if furniture is obstructing the heater—and regardless, it's hard for a resident to tune for precise temperature control. Often this means cranking up the baseboard heat and mitigating overheating by opening windows.
To combat this unfortunate situation, the electric heat retrofit included installing new wall-mounted thermostats in each room to control the baseboard heaters. The thermostats selected for this project are wireless; they communicate with a small relay box appended to each baseboard unit, and that controls the current to the heater. In addition, in the window frames, a small magnetic switch was installed that senses when a window is open. This relays back to the thermostat, which limits the maximum temperature in the room when the window is open.
A main goal of the retrofit was to save the building owner energy costs while minimizing the disruption to residents. The window control was acceptable to tenants because it doesn't shut off the heat entirely; it limits the temperature so that residents are encouraged to keep the windows closed.
Here's where it gets interesting. The wireless thermostats are able to communicate not only with the local baseboard heaters but also with each other. They are connected in a "mesh network" in which each device is both a receiver and transmitter of information, and commands from a central control can be carried out across the whole system to some or all apartments. It all adds up to a building-level control where there was only simple control before. The central control can perform more advanced functions remotely, such as night setback, vacant apartment control, and temperature limits.
The cost for the retrofit as a whole was about $1,300 per unit. Preliminary data is showing that the improvements are yielding more than 20% savings, exceeding expectations, and netting over $500,000 a year in electricity cost savings, or about $500 per unit.
Combined Heat and Power
Combined heat and power (CHP) is on-site generation of electricity combined with recovery of waste heat for a building’s use. For multifamily buildings, this often takes the form of electricity production for a building’s base load and recovered heat for domestic hot water production. The technology is mature, as it has been used in all types of buildings for decades, and in some buildings it can be a very worthwhile investment that surpasses several others in cost-effectiveness. In buildings where solar power may be difficult to install because of a lack of suitable roof space, this is an attractive option.
Combined heat and power makes the most economic sense in areas of the country where electricity is expensive and other fuels less so, because then it may become cheaper to produce electricity on-site than to buy it from the electric grid. In some markets, electricity may be three times or more expensive than natural gas on a per-Btu basis. This difference in utility costs, sometimes referred to as the "spark spread," is a main driver of CHP economics.
In general, CHP may be a more attractive investment for larger buildings. These have greater electrical and thermal loads that can be concentrated at a single point and assigned to a CHP plant. The economies of scale are important for the best return on investment. Smaller-capacity units can be used in smaller buildings, but because of relatively inflexible equipment and design costs, larger units in larger buildings are often more cost-effective. There are also many design considerations that affect CHP economics.
A CHP unit is usually designed to meet either a thermal load or an electric load, depending on the utility rates and the owner's goals for the system. For multifamily buildings, it is common to design the system to meet a portion of a building's base electric load, such as 24/7 hallway and stairwell lights, with a steady stream of electricity. The waste heat is commonly used to supplement the building's domestic hot water production.
Specifics of the site can influence costs significantly. For example, there must be sufficient space to house the unit and its accompanying hot water storage tank or tanks. The storage tanks act as buffers, discharging heat during periods of high domestic hot water demand and accumulating heat at other times. The tanks can be very large and, particularly in a retrofit situation, finding room for thousands of gallons of hot water storage can be a challenge.
What happens if a CHP unit produces more heat than the building needs? Care must be taken not to oversize the plant, because if excess heating capacity can’t be used, the waste heat must be disposed of. CHP units can overheat, so they are designed to keep cool. Some are air-cooled and so must have plenty of venting, while others are water-cooled and have radiators to "dump" excess heat outside. Some buildings can utilize waste heat in more creative ways, such as for supplementing hydronic heat in the winter, but any heat dumped outside is wasted energy, reducing efficiency and harming CHP economics. Common designs for CHP include reciprocating engines (like a car engine) and microturbines. There are units that can burn a variety of fuels, including natural gas, propane, biogas, and landfill gas. Since they are combustion appliances, they are often treated very much like boilers by code.
As with any combustion appliance, venting the exhaust is a primary concern for both safety and cost. Many CHP units burn natural gas or propane, and their exhaust gases can be corrosive. When a chimney must be lined or created anew, the cost of a new stainless steel chimney liner for a medium-sized CHP can be as much as $10,000 per floor in urban areas. It is worth noting that CHP units can be common-vented with boilers in some applications, but this must be determined on a case-by-case basis.
For every installation there are certain fixed costs. Since a CHP unit produces electricity, service disconnections and utility inspections are required, just as they would be for a solar PV installation. In addition, some local utility incentive programs require that the system be capable of starting itself ("black start capability") in the event of a power outage. This means the unit must have advanced controls and grid interconnection hardware and oversight, significantly increasing cost.
CHP may be used as partial or even full backup power, though many other design considerations must be taken into account. Comparing a CHP unit to a conventional emergency generator, the generator costs much less but commonly generates no income and must be maintained at a cost. Combined heat and power is able to produce electricity year-round and utilize the waste heat in an effective way. For many multifamily buildings, this may be a very attractive upgrade.
CHP is becoming a mainstream technology; Department of Energy data shows 517 CHP plants in operation in New York State alone as of 2013. More mechanical, electrical, and plumbing (MEP) engineers now have experience designing and integrating these systems with base building systems, improving the efficiency of their operations. Off-the-shelf CHP units are available from numerous vendors in size ranges that are suitable for a variety of multifamily applications. Like any other technology, close attention must be paid to analyzing the target thermal and electrical loads to get the best return on investment.
Upgrading Atmospheric Boilers
There is seldom a good reason to install a new atmospheric boiler in a multifamily building. In fact, in some cases it makes sense to pull out the existing boiler, regardless of its age, and replace it with a sealed combustion unit. Looking at the nameplate efficiencies of two types of boilers, you might wonder if it's worth it. If an atmospheric boiler has a rated thermal efficiency of 80% and a sealed combustion boiler is 86%, is this reason to replace the whole boiler? Possibly. There are big differences in the design and operation of sealed combustion boilers, and these factors need to be considered before switching to the newer technology.
Atmospheric boilers operate on the same principle as a pot over a campfire. Hot combustion gases, carried by their own buoyancy, rise past a network of water tubes and warm them. Fresh air rushes in freely at the bottom of the boiler to provide for more combustion. The buoyancy of the hot exhaust gases carries them up the chimney.
The regulation of draft through the boiler—that is, the amount of air that passes through for a given amount of fuel—is largely dependent on the weather. The hot combustion gases have more buoyancy during colder weather, so draft is effectively stronger then. Too much draft means excess air sweeping through the boiler and diluting the hot gases with cool air. The fact that the draft is so variable means that maximizing the combustion efficiency of the boiler is more challenging. Barometric dampers found on oil systems are meant to assist with regulating draft but they are notoriously difficult to adjust correctly for variable weather conditions.
Another reason that atmospheric boiler efficiency suffers is that the draft cannot be stopped when the boiler is off. Any water in the tubes is cooled by air moving freely through the boiler when it is off. This has implications for multi-stage boilers. Many atmospheric boilers are marketed as "modular," and individual units may be piped together in a row to operate as stages of a larger system. Because of the way they are generally piped, water flows through all units all the time, whether they are firing or not. The water in inactive boilers cools off due to the constant draft—the inactive boiler is now like a radiator.
Figure 1 illustrates one-half day of entering and leaving water temperatures from two atmospheric boilers in a two-boiler system and illustrates a few points. Data from boiler 1 is on the left and data from boiler 2 is on the right. Because the entering and leaving temperatures of boiler 1 are always very close, we know that this boiler never fires (adds heat to the supply), and that boiler 2 is capable of rapidly meeting the load by itself. Firing of boiler 2 causes spikes in water temperature of both boilers. But looking closely, the water entering boiler #1 is also always slightly warmer than the water leaving the boiler, which means the boiler itself is cooling the water off! In fact, the average temperature loss just through boiler 1 is 0.5°F. When these boilers are not heating the water up, they are cooling it down.
Boiler efficiency ratings are measured in laboratories under controlled conditions so that two boilers may be evenly compared. However, these tests cannot take into account field conditions such as chimney height, weather, or use of multiple-stage configurations, all important factors in the seasonal efficiency of the system. The real-world efficiency of boilers may be far less than their thermal efficiency rating.
Atmospheric boilers do have a definite installed-cost advantage, since the units are considerably simpler than sealed-combustion or forced-draft boilers. Since atmospheric boilers lack fans for combustion, the main challenge is designing the chimney to provide proper draft—not too much and not too little (publications like NFPA 54 are helpful). Nevertheless, high operating costs over the life of atmospheric units make them prime targets for replacement.
Retrofitting an atmospheric boiler with something better is not as simple as pulling out the old boiler and putting in a better one. There are many considerations, starting with venting, which can be complicated. Understanding ANSI venting categories will help you predict what is entailed in retrofitting an atmospheric boiler (see Table 1). The passive design of atmospheric boilers contains no combustion air fans to push air through the boiler. The boiler and chimney are designed for negative pressure, and the hot exhaust gases do not normally condense, making atmospheric boilers Category I appliances.
One possibility for replacement is a sealed-combustion boiler (Category III) (see Figure 2). With this type, the chimney becomes pressurized by the boiler's fan. An existing chimney may often need to be lined to accept a Category III appliance so that positive pressure in the chimney cannot push flue gases into the living space. If the boiler is in the basement, lining a chimney all the way to the roof may be very expensive and a large component of the project cost.
One option with some sealed-combustion units is to vent out the side wall of a building; however, in urban settings, finding a place to safely exhaust out the side of the building can be difficult. In general, there are more chimney configuration options with sealed combustion boilers than atmospheric. For very long vent lengths, a variable-speed draft controller may be used to ensure constant, proper draft. Condensing boilers must be vented using corrosion-resistant material, such as PVC or stainless steel. In urban settings, stainless steel chimneys may cost as much as $10,000 per floor of the building, easily eclipsing the cost of the boiler itself.
Making the case for the retrofit simply based on the rated efficiency of the boilers will not yield an attractive payback, but it probably underestimates the potential for savings. Make sure to look at the control of the boiler. Is water pumped through the boiler even when it is off? Do "modular" sections serve to cool off the water when some stages are inactive? Is the chimney very tall, contributing to a difficult-to-control draft?
Learn more about electrically heated building retrofits (PDF).
Read more on CHP (PDF).
To learn more about atmospheric boiler retrofits, see the Williamsburg Court profile here.
When considering a retrofit, take into account the following factors: Is the existing boiler nearing the end of its life? Is it being used for heating as well as domestic hot water production? What are the venting options for a possible replacement? Looking past the nameplate efficiency rating will give you an understanding of whether or not atmospheric boilers are really hurting your building’s efficiency.
While some of these retrofits obviously won’t have application in every building, they are common enough that in most areas, you can find an application for them. They have proven themselves to be not only cost-effective but impactful. We believe they should be scaled up on a national level. In the next article, we will cover three more retrofits from Steven Winter Associates' list of favorites.
Marc Zuluaga, PE, is vice president at Steven Winter Associates and director of its multifamily energy services division. During 13 years with the company, he has directed audits of over 20 million square feet of multifamily buildings.
Sean Maxwell was senior energy consultant at Steven Winter Associates and now resides in Australia. His nearly seven years of work at SWA ranged from dozens of multifamily energy audits to research for DOE’s Building America program.
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