Tankless hot water systems – also known as “in-line”, “demand” and “flash” hot water heaters -- do not store hot water; rather, they heat the water intensely as it passes through a heating coil whenever the hot water tap attached to the heater is turned on.

Tankless hot water heaters are rated by the maximum temperature rise they deliver for a given flow rate, so the key to getting the amount of hot water that satisfies you is to very carefully calculate your flow rate. The desired flow rate is calculated by adding up the flow rates – gallons per minute—of every fixture that you want to be attached to a particular heater. Next, determine the temperature rise you need by subtracting the temperature of the water that would flow into the tankless heater (use 50 degrees unless you know the temperature to be something different) from the desired hot water output temperature -- 120 degrees should be adequate for sinks and showers, 140 degree for dishwashers that do not internally heat the water. The typical temperature rise capability of heaters currently on the market is about 70 degrees, but gas units will render this temperature for higher flow rates (5 gallons per minute) than electric units (2 gallons per minute). Some units have thermostats and can vary the temperature rise according to the temperature of the incoming water and the flow rate.

Tankless heaters also have minimum flow rates, important because the heater only produces heat/hot water when water is flowing through it. If the fixture to which the heater is attached is only used sporadically and thus does not meet minimum flow requirements for the unit, the unit won’t have time to operate long enough to heat the water, and the user’s experience will be that the heater does not work or does not deliver water when needed. Minimum flow rates can range from ½ gallon to 25 gallons per minute.

Because water must be running through the tank in order to produce hot water, it is inevitable that some cold water must run through the heater to activate the thermostats to start heating, thus resulting in cold water flowing through the tap for some period of time and attendant water waste, and the user’s experience being that there is a delay in getting hot water. This glitch, however, can be addressed in a few ways, such as preheating the water through a solar hot water system so that the water flowing through the tankless heater is always somewhat heated, or by keeping a small (5-10 gallon) storage tank of hot water (obtained from the tankless heater) to dilute the cold water coming out of the heater before it gets to the faucet during the period between when the faucet is turned on and when the heater produces hot water, or installing a drain water heat recovery system ,or a hot water recirculation system.

Typically gas units have greater capacity and, as of the time of this writing, are cheaper to run – that, of course, is a function of your relative electric and gas utility rates – but they must be vented to the outside which could pose an issue for retrofitting into bathrooms located in the center of a brownstone as it would require long runs of ducting and the opening of walls. They also require a large supply of fresh air, so the heater may not be able to be installed in a tight space posing perhaps another barrier in sometimes tight brownstone bathrooms. Electric units, however, require large electrical circuits because it takes a lot of electrical current to heat water quickly, and it can be expensive to run large-capacity electrical cables long distances; installing smaller units at each fixture or on a per-room basis may alleviate the drain on the capacity of your household electrical supply and may result in less water waste, because the unit is closer to its use point and will not lose as much heat during the journey, and less water sits in pipes that will eventually cool.

Check the ACEEE (American Council for an Energy-Efficient Economy) website for a list of tankless water heater manufacturers.