This month I would like to discuss on-demand (also known as instantaneous or tankless) water heaters.

There are basically two types of water heaters, storage type and tankless. Storage water heaters are still the most popular in the United States due to the higher initial cost of tankless and the flow demand limitations of some units.

Storage type heaters keep water at a 120-140 degree constant temperature ready to use. This is a waste of fuel and can represent 10-20% of energy costs. In a tankless heater water is only heated when there is demand (flow). Once activated the unit continues to heat water as long as there is demand. The elimination of continuous storage of heated water can substantially reduce standby heat loss thus saving energy. Since demand water heating required more BTU's when operating than standby you will notice units designed to replace a 40-gallon storage type heater (usually about 40-50K BTU) will be about 125,000 BTU's.

Tankless heaters are easily installed and occupy much less space than a storage type. In high flow conditions (multiple bathrooms) several units may be necessary strategically placed near bathroom groups or centrally located and connected in series to provide high volume heated water.

Nearly every one of these units I have seen (whether electric or fuel gas) is clearly labeled with minimum clearances to combustibles. Of course the clearances are much less for electric models.

Many of these units are factory supplied with overheat sensors to auto shutoff. However, the IRC requires any equipment used for water heating or hot water storage to be equipped with pressure and temperature relief. Most units have tappings specific for this valve. Some units have anti-freeze protection wherein the unit turns on intermittently if the temperature drops below freezing.

I have included an illustration of a typical fuel gas fired tankless water heater for further demonstration.

We'll talk next month,

Michael Casey
Kaplan Professional Schools
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