Electric Heat Tracing: The smart performance alternative to complex recirculation systems
By Brian Larkin
As a result of the decrease in single-family home construction, an increase in high-rise residential building projects is underway. High-rise buildings present many critical design and performance challenges. One of these challenges is the limitations of a conventionally-installed hot water recirculation system and its limited ability to adjust for rapid changes in building occupancy. This article addresses design and performance of recirculation systems and an alternative approach using electrical heat tracing.
The primary goal of an efficient hot water distribution system is to reliably deliver hot water at the lowest installed cost, with minimal water waste and optimized energy usage. For purposes of discussion, we will use 24 stories as the definition of high-rise construction, where, typically, three multiple pressure zones would be required. There are two approaches to deliver hot water to the tap; recirculation systems and electrical heat tracing systems.
The challenge with recirculation systems in high-rise construction is the increased head pressure, friction losses and varied flow rates. To compensate for this, the building is divided into multiple pressure zones in groups of 8 to 12 stories, with pressure relief valves to limit the head pressure at each zone. There are two basic recirculation system designs, a multiple water heater system and a central water heater system.
The most common recirculation method is the multiple water heater approach in which pairs of heaters (for redundancy) are used for each pressure zone, in a mechanical space on the roof, in the cellar or both. This approach quickly becomes expensive, since each water heater must be vented and can take up significant floor space inside of the building.
Engineers have developed a more economical approach, using large, centrally-located water heaters to supply the entire building. Backup water heaters (for redundancy) and a storage tank are placed in a single location, typically on rooftops where venting is less costly. A single recirculation loop is run for the height of the building, with pressure relief valves compensating for the water pressure in each zone. The building is still divided into pressure zones, and each level has an associated recirculation system. These pressure zone recirculation loops are isolated from the full head pressure of the main recirculation loop by utilizing heat exchangers to heat the zone.
The advantages of a central water heater design over the multiple heater design include eliminating the cost of the additional water heaters and venting. Also, in a central heater approach the size of the boiler can be reduced compared to the combined volume of the multiple boilers, again lowering installed cost.
Recirculation systems are frequently installed with the risers gathered into a single recirculation loop to save on pipe, installation cost and shaft space. Risers should be individually recirculated and balanced. Gathering the risers into a single recirculation loop makes balancing the recirculation system extremely difficult to achieve.
Electrical heat tracing systems
Many buildings in the largest cities in the U.S. (New York, Miami, Chicago, etc.) have turned to a more reliable method of providing code-compliant hot water distribution. A heat traced hot water system eliminates the complexity and unreliable performance associated with recirculation systems. The system includes an HWAT self-regulating heating cable and a control and monitoring system.
Most plumbing engineers have used electrical heat tracing for pipe freeze protection and other applications. Using heat tracing for hot water temperature maintenance is just another application where heat tracing can be successfully used. Installation and design is straightforward and reliable. The heating cable is taped to the pipe, covered with thermal insulation, and controlled by an advanced control and monitoring system (ACS-30).
Heat trace systems are installed on the outside of the pipe, so they are isolated from the water pressure; however, the building still has to be divided into pressure zones, each with a dedicated pressure relief valve to regulate the pressure at the fixtures. The central water heater, which is typically located on the roof, feeds the main supply line. The supply line runs the height of the building, with the distribution piping feeding the different pressure zones. The heat tracing is installed on each riser, branch line and as close to each fixture as required to satisfy water delivery time code.
With a heat tracing system there is no need for balancing valves or recirculation pumps, resulting in a lower cost, maintenance-free system. Since the hot water is not recirculated and balancing is not required, flow rates are not a factor in HWAT systems. Hot water is delivered to each residence regardless of whether the building occupancy changes. Since HWAT systems do not require heat exchangers, return lines or balancing valves, the owner and engineer can enjoy material reduction and improve space utilization.
HWAT heat traced hot water systems have many energy consumption advantages over recirculation:
• HWAT systems maintain the hot water pipes at a uniform temperature range throughout the building. Recirculation systems typically need to heat the water above the designed delivery temperature to overcome natural cooling in the system and to ensure that the furthest fixture has the correct water temperature. This overheating of water increases energy usage in recirculation systems.
• HWAT systems only consume the amount of energy that is lost through the insulation on the supply pipe. Recirculation systems have to replace the heat lost through the supply and return pipes.
• HWAT systems do not require recirculation pumps, eliminating their draw of energy consumption.
• Recirculation systems return cooler water to the storage tank, which must be reheated, essentially heating the water twice.
In high-rise residential construction, customer satisfaction is critical. To satisfy ASPE-preferred requirements, hot water must be delivered in less than 10 seconds upon turning on the tap. Since HWAT systems can be installed all the way to the fixture, they can provide hot water nearly instantaneously. Recirculation systems are not easily, or cost effectively, recirculated all the way to the fixture or even to the branch line. These uncirculated lengths of pipe will cool down; this cool water must be discharged before hot water can reach the fixture. Table 1 shows the water delivery time for different pipe sizes and fixture flow rates. Assuming an average 25 feet from the fixture to the recirculation line and 0.5 gpm fixtures, the resident can wait 60 seconds or more for hot water to be delivered.
Water conservation is a growing concern. When users have to wait for hot water to get to the faucet, water is wasted. Many cities now require hot and cold water metering to help offset the building owner’s cost. Recirculation systems flow the water through the water meter, which is reflected as hot water usage. An HWAT system does not have this issue, since water only flows when the fixture is opened.
Heat tracing systems provide integrated monitoring through the ACS-30 controller, so that building facilities personnel gets an early warning of any problem. Monitoring the hot water system (risers, branch lines, water heater and mixing valves) provides feedback of proper system performance. Recirculation systems do not typically have this ability.
The ACS-30 controller is a distributed control system with a central user interface terminal. The user interface is connected via communication wiring to heating cable control panels, which can be placed throughout the installation at convenient power drop locations. The ACS-30 can be integrated into the Building Management System (BMS) through BacNet, Lonworks, Metasys or Modbus RTU protocols, allowing you to easily monitor and track energy consumption and system performance of the hot water delivery system.
So, why is an electric heat tracing system the smart performance alternative to a recirculation system?
• Material cost: Beyond the return piping, recirculation systems require balancing valves, recirculation pumps, heat exchangers or dedicated water heating plants per pressure zone.
• Balancing: Heat tracing systems do not require any balancing valves. In recirculation systems, every riser, in each of the pressure zones, requires balancing valves to ensure uniform flow to each residence. This is problematic, since the flow rate at each level and each riser is not uniform and changes as the building occupancy evolves. The different pressure zones increase the difficulties of balancing between zones.
• Less maintenance: Heat tracing systems are significantly less complex, with no pumps or balancing that require periodic maintenance.
• Lower installed cost: Since heat tracing systems require significantly fewer parts, the installed cost savings can be significant.
• Space utilization: Heat tracing systems take up less floor space.
• Energy savings: Without return lines, water overheating and reheating return water, heat tracing systems can save significant energy over recirculation.
• Water savings: Since heat trace systems can be installed up the fixture, there is virtually no water wasted while waiting for hot water.
• Building owner satisfaction: Since heat tracing systems do not require flow balancing, residents will consistently receive hot water regardless of building occupancy changes; thus, reducing callbacks and adjustments.
• Resident satisfaction: Heat tracing systems can deliver hot water to the fixture almost instantaneously, so residents do not have to wait. Also, as the trend increases in metered hot water, residents only have to pay for what they use.
Brian Larkin is product manager at Tyco Thermal Controls.