Heating water accounts for up to 30 percent of the average home energy budget. The most common water heater is the 30-50 gallon insulated storage tanks that use either electricity or natural gas to heat the water inside where it sits and waits for us to turn on a tap. Recently, tankless water heaters, also known as on-demand water heaters, have come on the market to provide hot water only as it is needed thus eliminating the storage tank. This is how they work:
1. Study the Issue
There have been many government and private studies comparing the various on-demand and conventional units available. The U.S. Department of Energy (DoE) identified a hot water daily usage of 41 gallons as typical. However, industry tests upped that to 78 gallons of daily hot water. That’s the equivalent of taking three showers, washing a load of laun¬dry, running the dishwasher, and turning on the faucet nine times, for a total of 19 draws which is representative of an average family’s hot water needs.
The industry study also ran more than 45,000 gallons of very hard water through tanked and tankless hot water systems to simulate about 11 years of regular use. Analyzing just the operating costs, the DoE found that gas-fired on-demand water heaters can be 18–34 percent more energy efficient than conventional storage tank water heaters depending on the usage and installation. The private study narrowed that down to 20 percent.
However, even the largest electric tankless heaters can’t deliver hot water fast enough to replace a conventional whole-house water heater if ground¬water is too cold or the demand too high. Here in Arizona where our groundwater is warm, most homeowners would need to upgrade their electrical service to power a whole-house tankless model.
2. Installation Costs
A conventional hot water heater, once installed, can easily be replaced by a handy homeowner. This is especially true if an electric water heater and the insulation around the storage tank has become so good that very little heat escapes during standby. Tankless water heaters can be installed either as a direct replacement of an existing conventional storage water heater, called a whole house model, or as several point-of-use devices that services individual lavatories, sinks, showers, and other special applications. In this article we will be talking about the whole house replacement units. Keep in mind, the initial installation of a point-of-use water heater requires running new electric or gas lines inside your home, driving the costs up.
While replacing a conventional electric storage tank system is a relatively simple DIY project, replacing it with an electric on-demand heater is not so easy. The conventional unit heats slowly over time whereas the on-demand model requires more power while it’s heating the water very quickly. That means you must hire an electrician to run the new high voltage line and hook it into your main panel. And if your main electrical panel is already maxed out, you will need to upgrade it to accommodate the heavier loads the on-demand system will need, sending installation costs skyrocketing.
Replacing a conventional gas system with a gas-fired on-demand model will also need professional installation. When they fire up, the on-demand heaters use 3-4 times the gas of a conventional gas water heater. Thus, the average homeowner will need a skilled plumber to verify their current gas line, size, and length can provide sufficient gas flow to the unit. Proper routing and testing the copper gas lines is critical. Plus, all gas burners require a dedicated vent and failure here can mean carbon monoxide poisoning.
It’s clear that neither gas nor electric installations of an on-demand water heater should be attempted by the average homeowner. Hire professional installers or your new water heater could cost you more than you bargained for.
3. How Much Hot Water Is Enough?
Another consideration is how much hot water you will need at any given time. Gas on-demand water heaters can deliver up to 5 gallons per minute (gpm) while the largest (and most expensive) electric models top out about 3 gpm. A typical shower uses about 2.5 gallons of hot water per minute so if you install an electric system, having more than one demand happening at the same time will have a detrimental effect.
Although gas-fired tankless water heaters have higher flow rates than electric models, they can waste energy if they have a constantly burning pilot light. This factor alone can offset the elimination of standby energy losses when compared to a storage water heater. In most gas-fired storage water heaters, the pilot light heats the water in the tank so the energy isn’t wasted. This is not feasible in an on-demand system. Although recent improvements in the direct-spark electronic ignitions are a good thing, they are just another device that needs an experienced professional perform maintenance on your unit on a regular basis.
4. Temperature Swings
Manufacturers of tankless water heaters are fond of bragging about their products’ ability to instantly provide an endless amount of hot water, but there are some issues they are glossing over.
• Tankless water heaters do not deliver hot water instantaneously. It takes time to heat the water to the target temperature, and just like conventional storage water heaters, any cold water in the pipes needs to be pushed out.
• When you repeatedly turn the hot faucet on and off then on again, tankless models feed in some cold water to gauge how big a temperature rise is needed. This means you’ll receive a momentary cold-water shock between the old and new hot water. Inconsistent water temperatures are typical complaints among those participating in the studies.
• A tankless water heater’s burner might not ignite when you try to get just a trickle of hot water for shaving. A typical on-demand system when it’s new has a minimum flow rate of about 0.5 gpm and as your unit ages this will increase. How much depends on how well you keep it maintained.
• Even the gas-fired on-demand models have electric controls and sensors meaning no hot water during a power outage. Here in Arizona that has not been a problem but it is something to consider when deciding if tankless is your next hot water heater purchase.
5. Tankless units need more care
During long-term testing, sensors on the tankless models repeatedly warned of scale buildup. Industry pros recommend that tankless models be serviced once a year by a qualified technician. Calcium buildup can decrease efficiency, restrict water flow, and damage the inner workings. Experts suggest installing a water softener if your water hardness is above 11 grains per gallon. Ignoring this advice can greatly shorten the life of the unit. All of these combine to create a high maintenance water heater.
While tankless water heaters eliminate the storage tank sitting in the corner of your garage and thus any standby heat loss associated with maintaining your hot water 24/7 until you are ready to use it, they also have a higher initial purchase price, much higher initial installation cost and continuing maintenance issues for the life of the unit.