Tankless hot water heaters – pros and cons from the experts at This Old House

First there needs to be a definition of terms. There are heating boilers that have an internal or external heating coil, called a tankless, for making hot water. I am not a very big fan of these systems, which require you to leave your heating boiler up to full temperature for the whole year.

A newer tankless water heater is a wall-mounted unit that only comes on to heat the water when a faucet opens looking for hot water. Because you are heating water as you need it, it is "tankless," also called instantaneous. They work beautifully and are made of stainless steel, so they last. The up side is that you could shower for 24 hours straight. The down side is that it makes a limited amount of water per minute, so two major uses of hot water cannot happen at the same time. They are perfect in smaller use applications, such as vacation homes, campers, or boats. They are very popular in Asia and parts of Europe.

The amount of water we can get out of these units depends on temperature rise, or how cold the water coming into the house is versus how hot you want the water to be at your faucet. The greater the temperature rise, the lower the flow rate. Maximum flow rate on most units is 2.5 gallons per minute. That's more than enough in real life, but less than Americans are used to. You can get water hot enough for dishwashers.

They are available in gas or electric, but I would only recommend gas at this point. The electric would need at least 14 to 15 KW (a lot of power), and the electric elements can scale in hard water.