Thanks to everyone who helped out with the direct vent/power vent water
heater and boiler question a few days ago.

Now I have another question. I found many sites with tankless water heaters
which have very high efficiency. But I am worrying whether this will cause
unstable water temperature in practice. Could you point me to some reviews,
or share your own experiences with me?

Also, I am not clear what is the difference between a hot water boiler and a
water heater. I know that a boiler has a lot more heat capacity (in
BTU/hr). Howeve, I don't know whether it has a water tank as well, and
whether the size of the tank is important (it is not listed in many
specifications). I also appreciate help on that issue.