Q:

What are the pros and cons of tankless water heaters?

Michael Ferguson, Atlanta, GA

A:

Mark Eatherton, a plumbing and heating contractor in Denver, Colorado, replies: The benefit of instantaneous, or tankless, water heaters is energy savings and floor space. Tankless heaters don’t have to maintain a tank full of water at 140°F all day, every day. There’s no bulky water tank and a virtually endless supply of hot water. However, these devices do have limitations, namely installation and output.

If the existing flue pipe in your house isn’t sized properly, a gas-powered instantaneous water heater will need to be vented directly through the wall. Electric instantaneous water heaters require a dedicated circuit with enough amperage to carry the load.

But as output, measured in gallons per minute (gpm), increases, water temperature decreases. For example, a moderately sized unit can deliver a hot shower at a flow rate of 1.3 gpm. But when you fill the dishwasher at 2.6 gpm, the instantaneous water heater can heat the water only half as much as it did for the shower. If the water going into the heater is 40°F, the dishwasher could be filled with 90°F, water which is pretty chilly for washing dishes.

You could buy a bigger heater, but you’ll need more amperage to run it, which can be a problem in older homes. If you have doubts about your home’s electrical service it’s best to consult a licensed electrician.