I'm looking for advice on a whole house tankless water heater. Every review or consumer report I can find is several years old (2005) and I would think that there have been innovations in the past few years in that area making those reports obsolete. We really need the space that the tank takes up so we would like to find out if there is a good tankless solution.
It very much depends on where you live, how you use hot water, what your expectations are, and how much you want to pay. If you expect to fill a large soaking tub while someone else is showering, or the washing machine is filling, you will likely be very disappointed. If your incoming winter water temperature is much below 50-degrees (mine is close to freezing - literally), you won't like it.
Think passing your hand across a flame...the water must move through a heated space fairly quickly. To make the water hot enough for you, it needs a pretty large supply. The more water you pass through it, the colder it is on the output, so the temperature can vary based on volume. There are ways to resolve this, multiple units either in series, or for dedicated spaces (like one for the master bath, one for the rest of the house, or even more than that). You will likley need to expand the gas or electric service to supply one, and if you don't have soft water, you'll need to make provisions to delime it periodically (think of your teapot - eventually, it gets a big deposit of minerals).
So, they have their place. A good tank type heater doesn't have to be a big energy hog, and it has a big quantity of hot water that can be dumped at one time. It then slowly recovers and waits. With a tankless, it has to have enough volume to satisfy you, and what that is, depends. You may love it, you may hate it, but in either case, it will likley cost you more since they cost more to install and purchase than multiple tank replacements.
Use the search function...there are various reports and thoughts...no need to repeat them ad nausium.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013