I'm considering purchasing one of those on-demand water heaters, but have heard mixed things about them. Do they really work?

Yes, on-demand water heaters-also called tankless or instantaneous water heaters-really work. The trick is to figure out how much water you're going to be using at one time and make your purchase accordingly.

On-demand water heaters do not have a tank; they are activated only when a hot water valve is open. The water is then heated on-the-spot as it makes its way to your faucet, showerhead, washing machine or wherever. This on-demand system saves a lot of energy over traditional water heaters, which typically keep their tank-full of hot water constantly heated, regardless of whether or not hot water is being used.

Complaints about on-demand heating systems generally come from people who want to use more hot water at once than their heater can produce. The cheapest heating models will only cover a single low-flow shower, while no other water is being used. If you want to run load of laundry while filling your Jacuzzi, you will have to pay a little more and get a higher-end system.

Before choosing a specific on-demand water heater model, figure out how many gallons of water you going to use at once. This list may be helpful:

Faucets: 0.75 to 2.5 gallons per minute
Low-flow showerheads: 1.2 to 2 gallons per minute
"Standard" showerheads: 2.5 to 3.5 gallons per minute
Washing machines: 1 to 2 gallons per minute
Dishwasher s: 1 to 2 gallons per minute

Decide how many of these activities you want to be able to do at the same time, add up the hot water flow you will need, and then select a water heater based on the maximum number you realistically come up with.

There is also one other thing to consider: if you have a dishwasher and it is not a self-heating model, you will want your water heater to heat water to 140 degrees. Dishwashers require hotter water than other appliances. The standard temperature for most other uses is 120 degrees.

You can find more information about on-demand water heaters from the Energy Efficiency and Renewable Energy Network at:
http://www.eren.doe.gov/consumerinfo/refbriefs/bc1.html

Remember that you can always ask for referrals if you're concerned about a particular brand. Feel free to also ask for a trial period money-back-guarantee. At the very least, explain your concerns to the person you would be purchasing a heater from, and ask for his or her recommendation.

See past questions here.

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