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A combination heating and domestic-hot-water plant solved Lincoln Towers’ recurring rooftop-equipment failures while fitting in only 600 square feet of an underground parking garage. The 2002 retrofit serving the 720-apartment, twin-tower complex in Arlington, Virginia, replaced exposed rooftop heaters and boilers with ten 2-million-BTU/hour boilers, shared distribution piping, local heat-pump loops, and separate storage tanks and heat exchangers for domestic hot water.
The Buildings case study reports an 11–12% increase in plant operating efficiency and a lower construction budget, but those figures are project claims rather than independently verified results. The useful lesson is not that every apartment building can cut capacity or fuel use by the same percentage. It is that plant location, coincident heating and hot-water loads, return-water temperature, distribution cost, controls, and redundancy must be designed together.
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What problem was Lincoln Towers trying to solve?
Lincoln Towers opened in 1990 with two 22-story residential towers and 720 apartments. According to the Buildings case study, each tower had five 1.76-million-BTU/hour hot-water heaters and three 1.76-million-BTU/hour boilers on its roof. Across both towers, the reported installed capacity exceeded 28 million BTU/hour.
The rooftop location created a recurring winter-maintenance problem. The case study says moisture froze on the equipment overnight, then thawed during warmer afternoons. That freeze-thaw cycle was associated with repeated heating and domestic-hot-water breakdowns. Residents experienced failures in the services that a multifamily building cannot easily take offline: space heating in winter and hot water throughout the year.
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In 2002, the owner engaged Consolidated Engineering Services Inc. with Holman Boiler Repair to relocate the plant. The project was not a simple equipment replacement. The new plant had to fit in five parking spaces, or about 600 square feet, in an underground garage, and it was more than 150 feet from one of the towers. Existing mechanical rooms inside the towers were retained, so the design had to connect a remote central plant to equipment and loops that already served each building.
The project engineer, Andrew Huck, summarized the constraint in the case study: “Given the small space we had to work with, a combination system using high-efficiency boilers to support both space-heating and domestic hot water requirements from a single plant was the best solution.” The combination approach made the limited plant footprint useful for both loads instead of requiring separate boiler systems for space heating and domestic hot water.
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How was the combination plant arranged?
The reported arrangement had four functional layers: a central boiler plant, supply piping to each tower, existing water-source heat-pump loops for space heating, and a domestic-hot-water heat exchanger and storage tank in each building.
Central boilers
The underground plant used ten AERCO Benchmark boilers rated at 2 million BTU/hour each, for a reported total of 20 million BTU/hour. That nameplate total was lower than the more-than-28-million-BTU/hour capacity reported for the original rooftop equipment. The difference should not be treated as proof that a 20-million-BTU/hour plant will serve another 720-unit property. Boiler selection depends on calculated design heat loss, domestic-hot-water recovery demand, diversity, sequencing, standby requirements, and the actual condition of the distribution system.
The case study also reports a 20:1 burner turndown for the Benchmark equipment used in that historical installation. Turndown describes how far a burner can modulate below its maximum firing rate. A high ratio can reduce short cycling during mild weather or low domestic-hot-water demand, but the practical result depends on minimum firing rate, control logic, water volume, sensor placement, and whether the connected loads can absorb heat continuously. The 20:1 figure belongs to the equipment claim in the 2006 account; current models must be checked against their own manuals and listings.
Shared distribution to the towers
One set of pipes carried boiler water from the garage to the existing mechanical room in each tower. The reported boiler-loop supply temperature was 160°F. In each building, sensors and modulating two-way valves admitted boiler water as the local loop cooled. The existing water-source heat-pump loop was designed around 70°F water, so the central plant did not simply send 160°F water directly to every terminal unit. Local controls mixed and regulated the heat delivered to the building loop.
On warmer days, when the loop had more heat than the apartments required, excess heat was routed to a rooftop cooling tower. That arrangement allowed the system to reject unwanted loop heat instead of forcing the boilers or pumps to operate against a load that was not present. It also shows why a central plant cannot be evaluated in isolation: the tower piping, valves, pumps, heat rejection, sensors, and existing heat-pump loop all affect operation.
Domestic-hot-water production
Each tower retained a 200-gallon domestic-hot-water storage tank and received heat through a double-wall plate-and-frame heat exchanger. Boiler water passed through a three-way control valve and the heat exchanger. Incoming city water, reported at 40°F, was heated to a 140°F storage setpoint.
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When a tank fell to 120°F, a pump recirculated stored water through the heat exchanger until the 140°F setpoint was restored. The storage-and-recovery sequence separates burner operation from every individual fixture draw. That is important in an apartment building, where many showers, kitchens, laundry rooms, and janitorial fixtures can create short, uneven demand peaks.
A double-wall exchanger also provides a physical separation between the boiler circuit and potable water. The exact code, leak-detection, temperature-control, scald-protection, and Legionella-management requirements for a new project must be established by the engineer of record and the authority having jurisdiction; the 2006 account does not provide a current code analysis.
Why did the designers combine space heating and domestic hot water?
Combining the loads allowed one boiler plant, one fuel supply, one flue strategy, and one control platform to serve both major thermal demands. In a constrained garage, eliminating duplicate equipment and room requirements can be more valuable than maximizing the theoretical efficiency of separate systems.
The loads are not identical. Space-heating demand is seasonal and weather-driven, while domestic hot water is present year-round and can peak sharply at particular times. A properly sequenced plant can use domestic-hot-water recovery to keep some boilers operating during mild weather, then stage additional boilers as space-heating demand rises. The design must still account for the possibility that space heating and hot-water recovery peak together. Without an hourly load analysis, the historical 20-million-BTU/hour total cannot be used as a sizing shortcut.
Huck also said the approach “reduced the construction budget and offered the customer long-term fuel savings,” as quoted in the case study. Those are attributed statements from the project team, not an independent financial or energy audit. A present-day owner should request the underlying load calculations, fuel records, maintenance history, and construction-cost basis before using the claim in a capital plan.
How did the design pursue condensing efficiency?
Condensing boilers recover additional heat when water vapor in combustion gases condenses on the heat exchanger surfaces. The opportunity increases as return-water temperature falls below the relevant flue-gas dew-point range. A system that always returns very hot water will operate more like a noncondensing plant, even if the boiler is marketed as condensing equipment.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsThe Lincoln Towers design used a reported 160°F supply strategy and relied on the building loops, valves, heat exchangers, and load diversity to produce lower return temperatures during portions of operation. The case study attributes an 11–12% increase in plant operating efficiency to that condensing-oriented design. Because the article does not describe a third-party test, baseline measurement method, weather normalization, or later operating record, the percentage should be read as a historical project claim, not as a guaranteed saving for another property.
For a new retrofit, engineers should model return temperatures by operating mode rather than use a single annual efficiency number. Useful questions include:
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- What return temperature reaches the boiler plant at design heating conditions, part load, and domestic-hot-water recovery?
- How many hours can the plant remain below the condensing threshold without compromising apartment comfort or hot-water recovery?
- Will three-way valves, bypasses, minimum-flow requirements, or closely spaced tees raise return temperature?
- Can controls stage boilers so the active units operate at an efficient firing rate instead of cycling?
- How will heat rejection operate when the water-source heat-pump loop has excess heat?
Why was the plant capacity lower than the original rooftop total?
The reported replacement total was 20 million BTU/hour versus more than 28 million BTU/hour of original nameplate capacity. That comparison is informative, but it does not show that the old boilers were correctly sized or that the new plant had less usable service capacity.
Nameplate capacity is not the same as coincident building load. The original equipment may have been selected with large safety factors, separate equipment may have been counted independently even when loads did not peak together, and the replacement design may have used modulation, storage, diversity, and improved controls. Conversely, a lower total can create unacceptable risk if a building loses a boiler during a design-cold event or if domestic-hot-water recovery is underestimated.
A current design should document at least three capacities: calculated space-heating load at the local winter design condition, domestic-hot-water recovery load for the required draw profile, and the capacity available with the largest boiler or heat exchanger out of service. Owners should ask whether the engineer is designing for N+1 redundancy, a documented temporary-load strategy, or another resilience standard. The answer affects the number and size of boilers more than a simple comparison of old and new nameplates.
What distribution trade-off shaped the project?
The team considered a more elaborate swing-boiler concept with separate distribution loops to each tower. The case study says the additional piping cost would have erased the equipment savings, so the team selected one shared set of pipes serving the existing tower mechanical rooms.
This is a useful retrofit lesson: equipment efficiency does not determine project economics by itself. A separate loop may improve hydraulic control, temperature reset, or redundancy, but it can require long mains, insulation, pumps, valves, structural penetrations, fire stopping, and access that overwhelm the boiler savings. A shared loop can reduce construction cost and preserve existing rooms, but it places greater demands on balancing, controls, pump selection, and fault isolation.
Before choosing a topology, compare the following items on a common basis:
| Decision area | Questions to answer |
|---|---|
| Load coincidence | Do space heating and domestic-hot-water recovery peak together, and what diversity assumptions are documented? |
| Routing | How long are the supply and return mains, where can they run, and what penetrations or fire-rated assemblies are required? |
| Hydraulics | Can pumps maintain flow at every tower, and can each building be isolated without shutting down the other? |
| Temperature | What supply and return temperatures are required by the existing heat-pump loops, heat exchangers, and storage tanks? |
| Redundancy | What capacity remains during boiler, pump, control, heat-exchanger, or electrical failure? |
| Access | Can replacement equipment enter the garage, and can failed components be removed without dismantling the building? |
| Operating cost | What are the local fuel, electricity, water, emissions, and maintenance costs under realistic operating schedules? |
How should an owner evaluate a similar apartment-building retrofit?
The Lincoln Towers case is most useful as a sequence of design questions. The following process adapts those lessons without assuming that another property has the same loads or equipment.
- Document the failure pattern. Separate rooftop freeze damage, burner or heat-exchanger failures, control faults, power interruptions, water-quality problems, and distribution failures. A central plant will not solve a problem that originates in neglected piping or controls.
- Survey the existing system. Record boiler and heater nameplates, pumps, valves, heat-pump loops, domestic-hot-water tanks, heat exchangers, flues, gas service, electrical capacity, and access paths. Confirm which tower mechanical-room components can remain and which are at the end of their service life.
- Build a measured load profile. Use utility bills, weather data, domestic-hot-water draw records, apartment occupancy, and plant run-time logs where available. Distinguish calculated design load from actual peak demand; neither one alone is sufficient for final selection.
- Test the distribution concept hydraulically. Model pressure drop and flow through the longest run, tower branches, heat exchangers, control valves, and minimum-flow paths. Check whether one shared loop can be balanced and isolated without unacceptable temperature swings.
- Set temperature objectives. Establish the supply temperature required by the existing terminal systems, the domestic-hot-water storage setpoint, the recirculation trigger, and the lowest practical boiler return temperature. Confirm that condensing operation is achievable in real modes, not only at mild outdoor temperatures.
- Size for resilience. Decide how many boilers may be unavailable while maintaining heat and hot water. Document the standby philosophy, automatic staging, manual override, emergency operation, and spare-parts strategy.
- Coordinate controls. Define outdoor-air reset, loop-temperature sensors, modulating valves, pump control, domestic-hot-water priority, boiler sequencing, alarm points, and integration with the building-automation system. Assign responsibility for sensor calibration and trend-log review.
- Plan construction logistics. Confirm garage clearances, ventilation, combustion air, drainage, flue routing, gas-meter capacity, lifting points, noise and vibration control, fire protection, and resident access. A 600-square-foot allocation is meaningful only after these clearances are drawn.
- Commission and verify. Balance each tower, verify setpoints under heating and hot-water demand, confirm low-temperature operation, test lead-lag rotation and failure alarms, and trend supply and return temperatures. Establish a baseline for future fuel and maintenance comparisons.
Which controls and operating details matter most?
Controls determine whether the theoretical benefits of a combination plant appear in operation. Boiler sequencing should bring on only the capacity required by the active loads, rotate lead units to equalize run hours, and prevent rapid cycling. The control sequence should also define what happens when a tower sensor fails, a pump loses flow, a heat exchanger reaches a high limit, or a boiler locks out.
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Temperature reset is equally important. A fixed 160°F supply can satisfy a design condition, but a lower reset temperature during mild weather can improve condensing opportunity if the connected loops and domestic-hot-water equipment permit it. The reset curve must not compromise apartment comfort, heat-pump-loop requirements, storage recovery, or freeze protection.
Domestic-hot-water controls need clear priority rules. If a tank falls to its recovery threshold while the building is also calling for space heat, the plant should stage capacity predictably rather than starve one load or force all boilers to full fire. The 200-gallon tanks reported for each tower provide thermal storage, but tank volume, recovery rate, recirculation losses, and fixture demand must be recalculated for current occupancy and use.
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Trend logs should include boiler enable status, firing rate, supply and return temperatures, loop differential pressure, tank temperature, heat-exchanger valve position, pump speed, cooling-tower operation, and alarms. These records help distinguish a genuine capacity problem from a sensor, valve, balancing, or sequencing fault.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What current boiler information is relevant to this historical case?
Current manufacturer information provides context, not proof that the 2002 equipment matches today’s models. AERCO’s current Benchmark product page lists commercial condensing boilers in capacities from 750 to 6,000 MBH and describes integrated boiler sequencing and an option to pair Benchmark equipment with SmartPlate EV for a combination plant.
AERCO’s Benchmark NX page describes a 399–500 MBH gas-fired boiler for light-commercial, multifamily, and institutional applications. It lists up to 97.4% efficiency, up to 10:1 turndown, and sequencing for up to 16 boilers. Those are current manufacturer specifications; they do not establish the performance, controls, pricing, code compliance, or product equivalence of the boilers installed at Lincoln Towers.
For a current project, compare any proposed model using certified ratings, minimum firing rate, approved venting, water-quality requirements, allowable flow range, controls compatibility, service coverage, and parts availability. The number of boilers required should follow the project load and redundancy analysis, not the number used in the historical case.
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The case provides a clear account of a constrained retrofit: rooftop failures were addressed by moving the plant underground; space heating and domestic hot water were combined; existing tower mechanical rooms were retained; and a shared distribution arrangement connected the new plant to both buildings. It also reports a smaller total boiler nameplate, an 11–12% operating-efficiency increase, and a construction-budget reduction attributed to the project team.
It does not provide a measurement protocol, a third-party evaluation, a weather-normalized annual fuel comparison, a detailed maintenance record, or a later operating history. It does not show whether the reported efficiency percentage included pumps, cooling-tower energy, domestic-hot-water losses, or only boiler operation. Nor does it establish that the same percentage, capacity reduction, or budget result applies to a different property.
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Use the project as a design precedent rather than a guarantee. The strongest transferable idea is systems integration: a compact central plant can be effective when load calculations, distribution routing, return temperatures, storage, controls, and maintenance access are considered together.
What is the practical takeaway for multifamily owners?
A combination plant is most compelling when a building has a difficult or unreliable equipment location, enough load diversity to share boilers, a viable route to existing distribution, and return temperatures that allow condensing equipment to operate efficiently. It is less compelling when long piping runs, incompatible temperature requirements, inadequate redundancy, or poor access outweigh the savings from consolidating equipment.
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Frequently Asked Questions
Can one central boiler plant serve multiple apartment towers?
Yes, if the mains, pumps, controls, and isolation valves are designed for the distance and pressure drop between buildings. The engineer should also show how each tower can maintain safe heat and hot water when the other tower, a branch pump, or a control component is isolated.
What records are most useful before sizing a replacement plant?
Collect at least two years of fuel and electricity bills, outdoor temperatures, boiler run-time and alarm logs, domestic-hot-water complaints, occupancy information, and any balancing reports. Interval data and temporary temperature or flow logging can reveal peaks that monthly utility bills hide.
How can a lower-temperature system improve condensing-boiler performance?
Lower return water keeps more combustion-gas vapor below its condensation point, allowing the heat exchanger to recover latent heat. The effect disappears if bypasses, high-temperature terminal requirements, poor balancing, or domestic-hot-water controls keep return temperatures high.
What redundancy should a multifamily combination plant have?
There is no universal boiler count; the owner must define the acceptable service level and the capacity required with the largest unit, pump, heat exchanger, or control panel unavailable. Many projects document an N+1 or equivalent strategy, but the final requirement depends on climate, building risk tolerance, staffing, and code.
