Q. My question is on instantaneous water heaters or tankless water heaters. My current water heater is about 12 years old, so it is getting near the end of its life cycle. I have seen and researched tankless water heaters. They appear to be very effici ent and would save space but still I'm not certain. What are the pros and cons such as costs, ability to keep up with demand, maintenance, gas versus electric models etc. My intention is for this to be a whole house unit as a replacement for our existin g tank unit. It’s just two people in the house with dishwasher and washing machine as major appliances. Is this feasible?

A. Your question is one I hear often these days but the answer unfortunately isn’t simple. Folks who have traveled, especially to Europe, have encountered them and see the benefits. I was stationed with troops from an unnamed European country some years ago and I can tell you their association with hot water was nowhere near ours.

Are they more efficient than tank-type water heaters? You bet, but only in a narrow sense. You see, tankless water heating appliances are called “on-demand” water heaters and, as the name suggests, only heat water when you call for it and only heat water that you use. Sounds neat and tidy.

But a unit of energy is a unit of energy and to elevate the temperature of a given amount water from say the 55ºF that it is as it comes into the house to 120ºF for hot water use will take exactly the same amount of energy whether it’s in your tankless u nit or taken from the big old round, five foot tall water heater in the basement. The difference is the older water heater technology heats water up slowly and stores it for use in a vessel much like a big thermos bottle. The tankless unit uses a great amount of energy all at once to rocket the temperature up for your use. The energy needed to keep the stand-by water at temperature until you need it is considered wasted by those who calculate such things. More about that in a minute.

If you have gas to the house then a gas unit is what you want. The electric models require a huge amount of power to operate them and if your house is older you may even need a “heavy-up” of your electrical capacity at the main panel box to supply the a dded draw. In today’s dollars that can run $1,200. and up. Also the location of the on-demand water heater relative to the panel box comes into play as very heavy cable will have to be used to wire it. If it’s gas you need to position it on an outside wall so the flue gasses can vent out of the house.

My experience is that the manufacturers of these tankless heaters are quite optimistic about their output rates of hot water. Your needs might be calculated at 2.5 gallons of water at 120ºF. per minute. If so, then I would recommend that you up-size th e unit one magnitude of output greater to be sure.

Tankless water heating that most folks see overseas are not only on-demand but also are point of use; that is, they hang on the wall above the sink or tub. Most American families run 26 to 29 gallons of water a day down the drain at sinks, showers and t ubs waiting for hot water. So if you use a central tankless water heater you should think about the hot water you are going to let cool in the pipes when you’re done and the jolt of power that it took to heat it in the first place. Traditional water heat er jackets are getting much better and the Btu leakage tends to be from the conductive metals of the pipes going in and out of them.

If your interest is in thrift and you squared all the costs associated with the buying, installing and operating a tankless water heater against the cost to buy, install and use a conventional tank-type water heater, you’d soon see that any break-even po int and savings would probably exceed your lifespan. If your interest is along the lines of thinking globally but acting locally, you’d be better off putting solar panels on the roof and running that heated water into the cold water side of your convent ional tank type water heater.