Issue #11
July 2008

In this issue...

Domestic Hot Water:  Arguments for a Central System

Things to Consider When Designing a DHW System

DHW Resources

Tankless Water Heaters

Solar Water Heating

Water-Saving Measures that Save Energy Too

AHEEA Handbook:  Your Guide to Energy Efficiency in Affordable Housing

Domestic Hot Water (DHW): Arguments for a Central System

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.

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. 

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 leak

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. The total length of hot water pipe is reduced, so heat loss through the walls of the pipe is reduced. Additionally, old 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.

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.  

Central systems can be powered by heaters or by boilers.  These two technologies function in slightly different ways, but 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.

Recirculation Loop Controls

Recirculation loops don’t need to run constantly to provide a high level of service to tenants.  Constant recirculation wastes a lot of energy and increases maintenance costs for the pump and copper piping.  Controls can be added that save more energy, improve the level of service to tenants, and provide feedback to the building manager.

Demand Controls regulate the recirculation pump so that it only circulates water in the loop when there is demand from tenants and the loop water is below a threshold temperature.  This minimizes the amount of time for which the pump is switched on, and also minimizes heat loss from water in the loop.  These systems are especially effective in small buildings where there are likely to be long periods with no hot water demand.

Temperature Modulation Controls modulate the temperature of the water in the storage tank so that the temperature is high during periods of peak demand, but is set back to a lower temperature during periods of low demand (for instance, overnight).  This type of system may be especially suited to the detection of faults such as leaks, crossover flows, and pump failure.

Things to Consider When Designing a DHW System

There are many technologies and methods that can be used to improve the level of service, reliability, and efficiency of DHW systems in a cost-effective way.

Condensing versus Non-Condensing

Natural gas contains a certain amount of water.  Conventional non-condensing heaters and boilers waste a lot of energy in converting this water into steam which is then allowed to escape with the flue gases.  Condensing heater and boilers recapture this lost energy by cooling the flue gases to the point where the steam recondenses and gives up its energy.  Condensing heaters can produce 20-30% more hot water for a given amount of natural gas and are especially efficient under part load, the most common operating condition for DHW in multifamily buildings.

Forced (or Induced) Venting versus Atmospheric Venting

More efficient heating and more reliable operation can be achieved by controlling the flow of air into and out of the combustion chamber.  In “forced” or “induced” draft heaters, air is either pushed (“forced”) or pulled (“induced”) through the flue by a fan.  These systems also reduce standby losses by reducing air flow through the flue.

Conventional heaters use “atmospheric ventilation”, i.e. there is no fan and they allow the hot, buoyant flue gases to rise through a vertical chimney, which draws fresh air into the combustion chamber.  Note that all condensing heater and boilers have forced or induced venting.

Pipe Insulation

Pipe insulation is a very low cost measure that provides hot water more quickly at a higher and more consistent temperature, and saves energy.  A length of uninsulated pipe can result in a plug of cold water being carried into a dwelling unit, resulting in tenant complaints and long wait-times for hot water.  It important to insulate the whole length of hot water piping in a building, whether it is served by a trunk-and-branch system or by a recirculation loop. 

Drain water Heat Recovery

Free heat can be recovered from hot water as it flows away down the drain.  This heat can then be used to pre-heat cold water before it flows into a boiler, laundry facility or shower.  Many drain water heat recovery systems are made only of copper pipe, have no moving parts, and require no maintenance.

Specifications

Different energy performance specifications are used for water heaters than for boilers.  This is because storage water heaters can be tested as a complete package, whereas central systems can mix and match components.  Water heaters are typically described by an energy factor (EF) while boilers are described by thermal efficiency or by annual fuel utilization efficiency (AFUE).  It is not possible to directly compare EF values with thermal efficiency or AFUE values, because they are calculated using different test procedures.  However, energy modeling software allows direct comparisons to be made, in terms of annual Btu consumption, for the purposes of code and program compliance modeling.  Figure 8 shows the energy factors and thermal efficiencies associated with various water heater and boiler types.

Energy Factor (EF) Ranges by Water Heater Type

* Thermal efficiency varies—condensing units are more efficient in low-temperature applications such as DHW.

DHW Resources

• Rocky Mountain Institute Home Energy Brief #5, Water Heating

www.rmi.org/images/other/Energy/E04-15_HEB5WaterHeat.pdf

• The Energy Guy

www.theenergyguy.com/Links_HeatCoolProducts.html#WaterHeating

• Energy Solutions Center

www.energysolutionscenter.org/

• ToolBase Services

www.toolbase.org

• American Council for an Energy-Efficient Economy (ACEEE)

www.sceee.org/consumerguide/topwater.htm

Tankless Water Heating

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.  They:

• Occupy less space than storage heaters

• Do not require periodic cleaning of sediment that settles to the bottom of tanks

• Provide unlimited hot water

• Are modular, so parts can be replaced more easily

• Typically have a higher energy factor because the heater supplies water only when needed.

Instantaneous heaters at the point of use are a good solution in locations that have intermittent hot water use and are a long way from the central heater or boiler.  A typical example is a bathroom in a common area of the building.  Instantaneous heaters reduce the length of the recirculation loop or can remove the need for a separate hot water branch, thereby saving installation cost as well as energy.

Learn more about tankless water heaters at www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820

Solar W ater Heating

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.

See how the CEC plans to promote solar hot water heating at www.gosolarcalifornia.org/csi/solar_thermal.html

Water-Saving Measures that Save Energy Too

Plumbing Fixtures

You may not think of sink aerators and low flow shower heads as energy-saving features, but they are.  These types of fixtures save energy by reduce the amount of water needing to be heated.  There are a number of products available today for reducing water use without sacrificing comfort.  California Water Service Co. is issues a variety of water saving fixtures to qualifying customers at no charge.  For more information visit www.calwater.com/conservation/plumbing-fixtures.php.

Landscaping

Landscape and irrigation is another topic in water conservation that can directly affect energy use.  Have you ever considered the energy needed to deliver water to your property?  What about electricity used to operate timers on sprinkler systems?  Using native plant species can conserve water, lower water bills, and reduce maintenance needs on your property.  For southern California landscaping tips, visit www.bewaterwise.com/knowledge01.html.

AHEEA Handbook:  Your Guide to Energy Efficiency in Affordable Housing

Download a free color copy today at www.h-m-g.com/multiafamily/aheea/newsletter, or send an e-mail request to aheea@h-m-g.com for a black and white hard copy.

Este programa es financiado por los usuarios de las compañías de servicios públicos de California y es administrado por Southern California Edison Company bajo los auspicios de la Comisión de Servicios Públicos de California mediante un contrato con Heschong Mahone Group. Los contribuyentes de California que decidan participar en este programa no están obligados a comprar ningún servicio adicional ofrecido por el contratista. Las marcas registradas usadas aquí son propiedad de sus dueños respectivos. SCE se reserva el derecho a modificar o interrumpir este programa a su criterio o a pedido de la CPUC.

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