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As is well advertised, tankless water heaters save
energy. The savings come from the fact that the tankless water heaters
do not have a tank full of water slowly leaking out heat all day and all
night. Another factor for gas water heaters is the tankless units don't
having standing pilot lights. Actually some do, but most don't. Even
so, we are not talking about a very large amount of energy.
One of the advertised benefits is endless hot
water. Take as long a shower as you like, you will never run out.
However, there is a price to be paid. Suddenly you are not actually
saving energy, but using more energy with that longer shower. Human
nature being what it is, many families actually spend more money on
heating water after switching to a tankless water heater.
As for those folks interested in conserving water, tankless water heaters are not a friend. It takes longer to get hot water from a tankless unit than a storage type water heater because when you turn on the tap with a storage heater the hot water immediately flows into the pipe and off to the tap.
But with the tankless unit, there is no hot water ready and waiting to flow off to the tap, all the water in the system is cold. So first the water must be heated by the tankless heater and then make it to the tap. Not only does it take considerably longer to get hot water to the sink, but it warms up much more gradually than with a storage type water heater.
This means you
are running more water down the drain while you are waiting for the hot
water to reach the fixture, and finally stabilize at the desired
temperature. Here is more information about tankless water heaters and
wasting water. Tankless Water Heaters Test Results
There is a fix; a demand type hot water pump would solve the wasted water problem. With a demand hot water system you don't run any water down the drain while you are waiting, and you don't wait as long either.
The circulating system must be a "demand" type system. A traditional full time hot water circulating system with a return line from the last fixture back to the water heater will not work. It would either keep the heater on all the time, or it wouldn't pump enough water to turn the heater on at all, depending upon the size of the pump. A traditional circulating system will void the warranty of a tankless unit, or greatly reduce it.
By combining a tankless water heater with a demand type hot water circulating system you get the best of both worlds. You save energy and you save water. Perhaps best of all you save time since you don't have to wait as long for your hot water.