Plumbing 2, Tankless Water Heaters

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Supply Meets Demand

The water heater in an average house has a storage tank that holds between 30 and 50 gallons of water. When cold water enters the house, from a municipal supply or a well, it is split in two parts. One goes directly to the cold taps in the plumbing fixtures, plus any toilets. The other goes to the water heater, which in turn supplies hot water to the house. A few dishwashers and clothes washers can use just cold water because they have their own heating elements, but all fixtures with hot water inlets draw from the water heater.

Saving Energy

Whether it uses a gas flame or an electric element for heating, a water heater with a tank consumes energy even when the hot water is not running. That’s because the water in the tank eventually cools down, and needs to be reheated so it’s ready to use. Insulation, both built-in and added on, can help keep its contents warm. Even so, this arrangement means a water heater with a tank is using energy all the time, whether or not you need it to.

One other large item in your house consumes energy all the time, even more often than your space heating and cooling systems: the refrigerator. Water heaters and refrigerators both use large amounts of energy. After space conditioning, they are the largest energy consumers in most households.

The difference in energy needs between a refrigerator and a water heater is that one has to run all the time in order to preserve food and

drinks for freshness and safety. The contents of your refrigerator have to be kept cool to avoid spoiling, while the contents of your water heater do not. When you need hot water, it doesn’t matter whether it was heated last week or just a minute ago.

Viewed another way, the practice of keeping a tank full of hot water makes little sense. You may run the hot water for an hour a day to shower, bathe, wash hands, and wash dishes. On laundry day, you use more hot water, but still only a few hours out of the day. Yet the
tank on your water heater consumes energy 24 hours a day, just to be ready for the few hours of supply you actually use.

By operating only when it’s needed, a tankless water heater saves all the energy that is otherwise consumed keeping a reserve supply warm. That can be as much as half of the energy a water heater uses. Tankless water heaters also make more efficient use of fuel.


Energy Factor

One way to compare the performance of water heaters is to check their energy factor (EF) ratings. Energy factor ranks energy efficiency by the average amount of hot water produced for each unit of fuel consumed. This number is made up of cycling losses (loss of heat as
water circulates through the tank and pipes), recovery efficiency (how efficiently heat is transferred to the water), and standby losses (the amount of heat lost each hour from a storage tank, compared to the heat content of the water).

Higher EF numbers translate to greater energy efficiency. When you are considering several choices, such as different fuel sources and tank options, the EF is a useful guide. It won’t tell you how much a water heater will cost to operate, but it does provide a common reference. Comparing models that use natural gas or liquid propane, for example, water heaters with tanks range from EF 0.59 to 0.65, while tankless heaters get 0.69 to 0.86.

In general, gas-fired tankless water heaters provide more heat faster than electric models. Because the use of natural gas or propane involves combustion, though, you will need to meet code requirements for venting, combustion air, and gas lines. Most electric models use
110 to 120 volt or 220 to 240 volt current. If possible, an electric water heater should have its own circuit breaker on the service panel.

Saving Time

When the water in a storage tank cools down, you need to run the tap until the cool water is flushed from the pipes. And once you use as much water as the tank had stored, you’re out of luck until it heats another tankful. By contrast, a tankless water heater has a startup time of just a few seconds, so barely any water passes through the system before hot water is available. If you have chosen the right size tankless heater for your needs, it will keep heating water on demand, as long as you want.

Newer models of tankless water heaters have internal thermostats and flow meters. They adjust the heat according to the volume of water being used, which provides consistent temperatures and uses only as much energy as required. When you turn off the hot water at
the fixture, the tankless heater switches off automatically.

Tankless water heaters can save time over the long term as well. Water heaters with tanks are only expected to work for about 10 years. Only the most expensive models are warranted to last longer. The leading tankless water heaters available today promise useful life spans of 20 years or more. Once you reach the payback period, which will depend on the efficiency of your current water heater and the tankless model you choose, the remaining service time is pure savings.

Saving Space

Most of the bulk in a conventional water heater is the tank itself. Without a large storage tank, all you need is a heating element with a pipe running through it. Of course, tankless heaters also contain valves, control circuits, safety guards, and so on, but even the largest
tankless heater is much more compact than the smallest model with a tank. Most can be mounted on walls, freeing up the floor space required by a heater with a tank.

Another side effect of storing water is that mineral deposits can form in the tank. Over the lifespan of the tank, these deposits can build up to critical levels, then reduce the energy efficiency of the heater, or even cause the tank to corrode and leak.

Choosing the Right Model

If you choose a water heater with a tank, you should probably buy the largest one available. That sounds like strange advice, but it makes sense. Because larger tanks hold more water, the greater mass of the tank’s contents will help maintain the temperature. More importantly, larger tanks often have thicker insulation and longer warranties.

For a tankless model, consider how much hot water you need each day, how much you need at any one time, and whether the water heater is for part of the house or all of it. According to manufacturers, tankless water heaters are about 30 percent more efficient than water heaters with tanks in households that use no more than 40 gallons of hot water each day. Households that use twice as much water will get about half as much improvement.