Tenants value hot water that is reliable and arrives quickly. Many hot water systems in multifamily buildings suffer from temperature fluctuations and from slow delivery times. These problems lead directly to tenant complaints and maintenance costs, as well as wasted energy and wasted water.
Basics
In multifamily buildings, domestic hot water (DHW) can be supplied either by a central system that serves many dwelling units, or by individual water heaters in each unit.
Both central and individual systems can be made compliant with Title 24—there are prescriptive standards for both types of system, and cost-effective, energy-efficient equipment is available for both types of system.
Central Systems
Central systems are usually the best option in multifamily buildings, for several reasons:
One large heater or boiler is cheaper to purchase and install than many small ones, and increases the available space in each dwelling unit.
Larger heaters or boilers are more energy-efficient than smaller ones
Recirculation loops give tenants quick access to hot water at a constant temperature
Maintenance is easier and more convenient
Systems can be located in a basement or other location that minimizes the risk of water damage in the event of a leak3
Hot Water Boiler Storage Water Tank Co ld W a te r Ma k e -U pRepresentative Setup for Boiler with Storage Tank
Hot Water Supply
Hot Water Return Semi-conditioned Conditioned Unconditioned Demand Flow Sensor RECIRC Pump Bo il e r R o o m RECIRC Pump
Figure 6: Representative Graphic of a Central DHW System Including Components
Recirculation Loops
Central systems take two forms—trunk-and-branch systems, and recirculation systems. Trunk- and-branch systems carry hot water from the source to each dwelling unit, along pipes that fan out through the building. Recirculation loop systems have one central, large-diameter insulated pipe that serves many units, and constantly circulates water from the storage tank. From this recirculation loop, small “runout” pipes typically 6-12’ in length tee off to supply each fixture. In larger buildings, recirculation loops are typically used because they provide hot water to tenants more quickly than trunk and branch, because they are cheaper to install, and because they are more energy-efficient. This is for two main reasons:
The total length of hot water pipe is reduced, so heat loss through the walls of the pipe is reduced
Because cold water in the pipes does not need to be replaced with hot water each time there is demand, the peak load on the DHW system is reduced. This allows heaters or boilers to be reduced in size.Optimizing the layout of recirculation loops can significantly reduce their cost and improve their performance. By planning the dwelling units so that faucets are close to the loop, the length of the loop as well as the length of the final delivery pipes can be reduced. The loop can pass vertically through two or three stories without incurring pressure balancing problems.
If trunk and branch is used (typically in smaller buildings), “parallel piping” can be used to reduce energy use. Parallel piping simply involves minimizing the length of pipe between the source and each end use (rather than minimizing the total length of pipe). Parallel piping requires more copper, and so has a higher initial cost.
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Heater and Boilers
Central systems can be powered by heaters or by boilers. These two technologies function in slightly different ways, but the end result is the same, and either technology is suitable for most buildings.
Boilers usually provide a longer service life, and parts are more easily replaced. Since boilers allow the storage tank to be more fully insulated, they often have a lower standby heat loss than heaters do. Conversely, heaters are often cheaper than boilers, and in larger buildings many heaters can be used in parallel to provide a backup if one heater fails.
Solar
Solar water heating is a mature technology that is almost always cost-effective in sunny climates, and carries the additional benefit of reducing cooling load by shading the roof from hot summer sun. Solar systems are especially cost-effective for larger buildings.
A great variety of solar systems are available, and most can easily be integrated with a gas-fired central system so that peak loads are met even when there’s no sun. Title 24 calculations can now include savings from solar systems, so those savings can be fully accounted.
Individual Water Heaters
Water heaters are available in two basic forms: with and without storage tanks. Storage tanks are used to meet peak demand (such as for showers in the morning) by providing a store of hot water. Tankless (“instantaneous”) heaters must be able to meet this demand without the benefit of storage, and therefore have larger burners.
Tankless heaters currently have a higher initial cost, but have a number of advantages over storage water heaters: