Tankless water heaters have been on the market for awhile and provide an option for homeowners who want to recover some floor space, are looking for something more efficient, or who are space-limited. Manufacturers make both electric and gas-fired tankless water heaters. You can also find propane-fired tankless systems.
The up-front cost of a tankless system is generally higher than a conventional hot water tank, but tankless systems also last about two to three times as long as a conventional tank system. Over time, you’ll actually save money on a tankless water system through lower operating costs and a longer replacement cycle.
Tankless hot water systems don’t do a very good job of providing hot water at a low flow rate. Generally, a tankless system will heat water if it detects a flow rate that exceeds about ½-gallon per minute. If you turn the hot water tap on to get just a small stream of hot water, the flow rate may not be sufficient to trigger the water heater. Likewise, there is a delay in getting hot water from the system, as it takes a brief period of time to get the water heated to the desired temperature.
Tankless systems free up floor space, but they’re wall-mounted, so you’re giving up wall space to get floor space. Based on the unit’s size, you’ll need about 2 square feet of wall space plus some new piping, and since the units can weigh about 75 pounds or more, they’ll need to be anchored securely to the wall. They can be mounted to either an interior or exterior wall. Annual maintenance on the tankless coil involves cleaning and/or replacing a sediment filter, and deliming the system to discourage lime and scale buildups.
Natural gas-fired tankless water heaters
Gas-fired tankless systems can be designated as either “condensing” or “non-condensing.” Condensing tankless water heaters can achieve efficiencies of up to 98%, while non-condensing water heaters have a maximum efficiency of about 80%. Since your current water heater has an efficiency rating in the mid-50% range, either type of tankless water heater offers a major efficiency improvement over what you have now. An important design variation between condensing and non-condensing water heaters accounts for differences in both up-front and operating costs.
How does a tankless water heater work? Unheated water in a tankless system flows across a heat exchanger, which is itself heated by combustion. Heat is transferred to the water as the water flows across it. One of the natural by-products of combustion is steam. Because the combustion process produces other noxious gases, the exhaust is vented to the outside.
A condensing tankless water system extracts additional heat from the steamy exhaust and returns it to the water in the system. This technique reduces the corrosiveness of the exhaust gases and increases the efficiency of the system. Condensing tankless water heaters are somewhat more expensive to manufacture, but they can use common venting materials like PVC. Throughout 2015, Gas Networks customers can claim a rebate of $800 on gas-fired condensing tankless water heaters with an efficiency rating of .94 or better.
A non-condensing tankless water heater vents all of the corrosive by-products of combustion, including the super-hot, toxic steam to the outside. Because the steam is not recaptured and reused, a non-condensing system is less efficient and requires higher quality venting materials. Typically, non-condensing water heaters require stainless steel venting, which – as you might guess – is expensive. On the plus side, non-condensing systems cost less to produce and buy. Throughout 2015, Gas Networks customers can claim a rebate of $500 on gas-fired non-condensing tankless water heaters with an efficiency rating of .82 or better.
Gas-fired tankless systems normally require a ¾” or 1-inch gas line, which is larger than the line your conventional tank takes, so you will need to re-plumb your gas service at the time of installation. Tankless systems also require an electrical service, since they feature an electronic pilotless ignition. (This means you won’t have hot water during a power outage.) Tankless hot water systems work best when they’re located close to the fixtures and appliances that draw hot water. Long hot water pipe runs will cause the water temperature to cool, so you’ll definitely want to insulate your hot water pipes if you go tankless.
Electric tankless water heaters
If you don’t have natural gas service, you can consider electric tankless water heaters. Electric tankless systems need to be sized carefully to ensure that they provide sufficient hot water. Electric tankless water heaters require a minimum household electrical service of 200-300 amps, 240 volts, and a dedicated, double-pole, high-amperage breaker for the unit itself. If you are considering an electric tankless water heater, consult with an electrician to determine how your current electrical service will need to be improved to accommodate your new water heater.
Electric tankless water heaters are priced similarly to natural gas tankless water heaters. They are also comparably efficient to condensing tankless water heaters, and do not have the same venting issues that gas water heaters do. Even though an electric tankless water heater is highly efficient, the operating costs for a tankless water heater are significantly higher because electricity typically costs about 2.5 times more than natural gas does to produce the same amount of heat. Electric tankless water heaters are typically used only where natural gas service is not available.
Many electric tankless systems are designated as “point-of-use,” which means that they provide hot water for a single application, such as an individual sink, shower or appliance. Point-of-use units typically do not supply hot water for an entire building, and are commonly used in additions and remodeling.
If you’re considering the installation of a tankless water heater, please contact us at Boston Standard Plumbing & Heating at (617) 288-2911 anytime. We can explain the benefits and costs associated with tankless water heaters, and help you make the best choice for your home.