A tankless water heater is a copper-tube boiler with a high pressure drop and no circulator. They are manufactured for domestic water heating. http://www.gaswaterheaters.com/flash/gasheaters.htm http://www.sets-systems.com/ http://www.northdoorway.com/water/mora.htm http://www.globaltownewarehouse.com/HomeProducts/aquastar/aquastargastanklesswaterheaters.htm http://www.theplumber.com/faq.html
If your hot water use is for showers and tubs of water, and you heat your tea and small amounts in a pot, and you will have factory representatives in to completely clean and repair your instantaneous heater every year as done in Europe or Japan, the unit should suit your needs.
An instantaneous gas heater uses the same blowtorch to heat a cup of water as it does your shower water, so you will have a water heater that has a gas flame that is larger than the one used to heat your house. Most have difficulty heating less than a 1/2 gallon per minute flow rate of hot water, so they tend to waver in temperature at that slow rate, though they become steady at greater flow rates.
Your old water heater lasted many years, so the water is not hard; this is important, because many instant water heaters use small tubes that can lime up and burn out in hard water areas.
Otherwise they do save money by not losing heat as does a 40-gallon vessel sitting all day throwing heat away. As the gas bill for heating water for a family is about $240 per year, you could save about $60 per year, This would amortise a $1000 installation in about 16 years.
Most People think of the Instantaneous as the unit that Halves the Pressure of the hot water and uses lots of gas and only have one tap on ( and you can still buy those units ). Where as the Continuous flow units are designed to allow hot water to be drawn of at several points at one time (or single point ) without affecting the other points, they are Adjustable from 37 to 55 in temp ( from a key pad at point of use), and they are more efficient today in gas consumption. and the Infinity has a Three year Warranty.