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On-Demand Water Heater Basics

On-demand (‘tankless’ or ‘instant’) water heaters provide hot water more efficiently than conventional storage water heaters because they only heat water when it is called for at a faucet. Instead of making use of storage tanks for heating large quantities of water, when hot water is called for at a faucet it passes from the tap through a small on-demand unit where it is heated by burning natural gas or propane or by an electric heating element. One advantage of this type of system is that the amount of hot water it can provide is not limited by the amount of hot water in a storage tank. On the other hand, on-demand water heaters are limited by the amount of water they can heat at any one time.

Gas vs. Electric On-Demand Water Heaters

Gas on-demand water heaters can typically provide a greater flow of hot water—commonly about 5 gpm—than electric on-demand water heaters. Electric on-demand water heaters also require a significant amount of electricity to heat water so quickly. Some electrical panels may need to be upgraded to accommodate the high demand of electric units. The diameter of gas lines will often need to be increased for gas units as well. If electric on-demand water heaters become widespread it might also require electric utilities to add power capacity to deal with these increased loads.

Is an On-Demand Water Heater Right for Your Household?

It is critical when deciding if an on-demand water heater is right for you to calculate your maximum use at a given time in gallons per minute (the ‘flow rate’). For example, if you have a showerhead that emits 2 gpm and might run a kitchen faucet at 2 gpm at the same time, you’d need an on-demand water heater capable of delivering at least 4.0 gpm. Installing an aerator could decrease your flow rate for the kitchen faucet to 1 gpm, so hot water conservation is especially important when considering an on-demand system. Multiple on-demand units can be installed in a household if more hot water is desired than can be provided by a single unit or if one faucet is relatively far from the others.

The volume of water that can be produced by on-demand water heaters depends on the temperature of the incoming water from the tap. The warmer the water is (as in summer and in warmer climates), the greater the flow that can be delivered at the desired temperature. Most on-demand unit manufacturers can provide a graph that compares flow rates at different temperature rises above the incoming water. Because most new on-demand water heaters vary their application of heat according to the flow rate, households on well water may experience cold slugs as the pressure provided by the well pump changes. Solutions to this issue might include installing a water pressure regulator or adjusting the setting of your well pump switch and can be discussed with a qualified contractor.

Other Considerations

Other considerations for on-demand water heaters include their relatively high upfront cost and potentially high cost of repair (offset by lower energy costs over time); longer lifetime compared to storage water heaters (20 year expectancy); the creation of space in homes when replacing storage water heaters (most on-demand units are small and are wall-mounted in inconspicuous locations or are mounted at the point of use such as under sinks); and the requirement of a minimum flow rate for the burner to start in gas units (uses such as shaving that require small trickles of hot water may not meet this minimum). And unless they are optimized for elevation, on-demand water heaters will not perform as advertised at high altitudes. Some units may not be suitable at all for use at high elevations.

Last updated: October 18, 2017 at 10:50 am