Q:
Two people live in my town house, which has one bathroom, a dishwasher and a clothes washer. I’m thinking of having a tankless water heater installed. What are the pros and cons of this type of system?
Michael Ferguson, Atlanta, GA
A:
Mark Eatherton, a plumbing and heating contractor in Denver, Colorado, responds: The benefit of an instantaneous, or tankless, water heater is that all standby energy losses associated with maintaining a tank full of 140ºF water all day long, 365 days a year, are eliminated. I have a gas-fired instantaneous water heater, and for the most part, I love it. Tankless water heaters, both electric and gas, are wonderful devices that can let you get rid of a bulky tank-type water heater, allowing more floor space. As manufacturers claim, tankless water heaters give you a virtually endless supply of hot water. However, these devices do have limitations.
The initial limitation to gas-fired instantaneous water heaters is the size of the existing flue pipe, which can be overcome with the installation of a through-wall vented type of heater. The availability of a circuit with the necessary amperage to carry the load limits the installation of electric instantaneous water heaters. Beyond that, the limitations of both styles are their gallon-per-minute (gpm) outputs. Let’s look at the electric units as an example.
A typical 19kw unit is capable of raising the water temperature 100°F at a flow rate of 1.3 gpm, an adequate amount for one person showering with a low-flow shower head. The electrical requirement for a 19kw unit is approximately 79 amps, not normally a problem for most home electrical services, but usually necessitating a dedicated circuit to be installed for the water heater.
Bear in mind that with this scenario, only one person or device can use hot water at a time. And as the flow rate increases, the temperature rise of the water decreases. So when you’re trying to fill the dishwasher at 2.6 gpm, the water temperature is raised only 50°F. If the incoming winter water temperature is 40°F, the dishwasher would be filled with 90°F water, pretty chilly for cleaning purposes. To get hotter water, the flow rate would need to be restricted.
A 28kw water heater is capable of raising the temperature 100ºF at 1.9 gpm, which again surely would satisfy the needs of a low-flow shower head, but with only that shower head or a similar device flowing at one time. A 28kw load would require 116 amps, which could be a problem for the electrical service in an older home. If you have doubts about the size of your home’s electrical service, it’s best to consult a licensed electrician.
Another word of caution: Getting enough hot water for slowly filling a large soaking tub for a decent bath may take 15 or 20 minutes. As long as you think ahead, the system works great.
Under-the-sink instantaneous water heaters are essentially the same type of device except on a much smaller scale with a much lower flow rate. These models are great for a good cup of hot water for making tea or soup, but probably wouldn’t be much help for doing the dishes.
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