Hot Water Heating Systems

Conventional Hot Water Tank Hot water heating systems in most homes are major energy users, second only to the heating and cooling system.  At today's fuel prices, a gas hot water heater is generally less expensive to operate than an electric hot water heater.   However, installation can be trickier and more expensive, especially in confined spaces due to the need for ventilation and a flue.

Residential water heating utilizes conventional element heated tanks or tankless, on-demand hot water heating systems which heat the water as it flows.   Instant hot water systems use a small pump to circulate hot water through the plumbing so it is always available saving energy and reducing wasted water.   Heat pump water heaters apply refrigeration technology to heat the water and provide extra cooling while using less energy.

Timers  l   Tank Insulation  l   Pipe Insulation  l   Heat Traps  l   Drain Water Heat Recovery

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If your home has a conventional gas or electric hot water heater tank there are a number of simple, low cost accessories you can install which will aid in reducing your hot water heating system cost:

Before making any changes or additions to your hot water heating system, we recommend using your home energy monitor to establish a baseline which can be documented in the Power Panel Profiler, our home energy audit tool.   This way, any changes you make the system can be measured and compared to the baseline.

Electric hot water heaters in the U.S normally run on split-phase 240 volt circuits. The energy can be measured with load profiling  if using a TED 5000 from Energy, Inc.   Sub-metering also works well with any multi-point or circuit level energy monitor.  Plan for an additional set of 30 amp current transformers or CT's and a double channel to record the two hot legs of the 240 volt appliance.

Set Up a Timer
Electric Hot Water Heater Timer Intermatic WH21 Electric
Water Heater Timer
One of the simplest ways to lower energy cost is to install an electric water heater timer to turn an electric hot water heater off at night.  Set the timer to cut power at the time you normally retire and restore power about 15 minutes before rising.

If you retire late, the hot water remaining in the tank should meet demand unless someone takes a long shower.  In the morning, 15 minutes should give the tank adequate time to restore the temperature.  240 volt timers are mechanical so remember to adjust the dial when daylight savings time goes into effect.

Insulate the Hot Water Tank

Another simple energy saving technique for hot water heating systems is to place an insulation jacket around the hot water tank.  Look for a minimum insulation value of R-10. Be sure measure the circumference and the height of your hot water tank before ordering.  Insulation jackets and pre-cut kits are available at most home centers or you can click here to order one on line.

When you trim the blanket be sure to leave the thermostat access panel uncovered and keep the thermostat set below 130ยบ F. to prevent the wiring from overheating.   When installing or replacing a hot water tank consider placing a piece of rigid insulation or foam board under the tank to prevent heat loss into the floor.

Electric Hot Water Heater Insulation Blanket
Thermwell SP57-11C
Water Heater Blanket

If insulating a gas hot water tank additional precautions should be taken.  Keep the insulation clear of the combustion intake provide a cutout for the thermostat access panel as with an electric unit.  Do not insulate the top of the tank or let any insulation get near the flue due to fire hazard.  Follow instructions carefully or have a qualified HVAC contractor do the work for you.

Verify savings with your home energy monitor using the same techniques as suggested for installing a timer.  The $15 to $30 investment you make in an insulation jacket should be recovered within a year or so in most areas.

Insulate Hot Water Pipes

Insulated Hot WaterPipe Ideally, all hot water feeder pipes in the home should be insulated with pre-formed foam insulation, especially if you have a circulating instant hot water system.  The riser pipes do not need to be insulated as they only hold a very small amount of water as shown here.

Complete insulation coverage can only be done during new construction or when remodeling, before the drywall is installed.  If your home is already built, the next best option is to insulate all hot water pipes accessible in your crawl space or basement.

A third option is to insulate pipes within just six to eight feet of the hot water tank provided you do not have an instant hot water system installed.  This is where the greatest heat loss occurs in standard hot water heating systems.  If you have a heat trap on the cold water inlet, just insulate a foot or two beyond the heat trap.  Insulate the hot water outlet line as far as you can go for better results.

Heat Traps

Hot Water Tank Heat Traps Heat traps are simply check valves mounted in the inlet and outlet lines of a hot water tank that help to keep heat "trapped" in the tank.  The check valves prevent hot water from leaving the tank via convection but do not restrict water flow when a faucet is turned on.

If not integral to the hot water tank they can be added with flexible copper connection pipe kits that contain them.  The heat traps operate with either a flexible flapper valve or a check ball that is held in place with a light spring.

Drain Water Heat Recovery

For those hot water heating systems that use a lot of water one may wish to consider recapturing some of that lost heat by installing a drain water heat recovery system.  This system uses a heat exchanger to recapture some of the heat that is discarded in the warm drain water. The reclaimed heat is used as a thermal booster to pre-heat water entering the hot water tank.

The heat exchanger is essentially a copper coil that is wrapped around a vertical section of the main drain line that is fed by the primary hot water users; shower(s), dishwasher and clothes washer.  As warm waste water flows down a vertical pipe it tends to cling to the inner circumference of the pipe wall.  This "thin film" distribution of waste water around the entire inside surface provides for better heat transfer than if the pipe was positioned horizontally on a mild slope.   Six to eight feet of vertical drop is needed for effective heat transfer.

Cold incoming supply water is circulated through the copper coil heat exchanger wrapped around the drain line.  As it circulates it absorbs some of the heat from the warm waste water and is plumbed to replenish the hot water being drawn from the hot water tank.   This pre-heating saves energy by warming the supply water so the hot water heater does not have to add as many Btu's of energy to reach the desired hot water temperature.

If the pre-heated water is injected directly into the cold water supply for the entire house can reduce cold water settings during showers.  However, this injection will increase the cold water temperature at the faucet which may not be desirable when brushing teeth or drinking water.

There are two methods to measure the output of a drain water heat recovery system.  The first way is to continue sub-metering the hot water heating system with your home energy monitor and compare the results with the baseline you set up in the Power Panel Profiler.

The second way is to use a Btu meter to measure the change in temperature and the flow rate of the water as it passes through the heat exchanger.  Once Btu output per hour is known it can be converted to kilowatt-hours using the following:

1 Kilowatt-hour = 3,413 Btuh

Multiply the equivalent kilowatt-hours by your electric bill rate to determine the energy savings.   Divide that savings per year into the system cost to determine the pay back.  Drain water heat recovery systems will generally pay for themselves in two to five years if you can keep the total installed cost under $500 to $600.

For additional ideas on how to reduce the energy costs of hot water heating systems please visit any of the following:

Instant Hot Water System   l    Tankless Hot Water Heaters   l    Heat Pump Water Heaters

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