OT/Gas Tankless Water Heaters

Now we have a 70 gal tank heated by the boiler as a seperate forced hot water zone.

If I were starting from scratch I would look into the in-line type of electric on-demand water heaters that they use in hotels. Basically part of the plumbing in the walls, just before the faucet. Supposed to be very efficient (no loss of heat from pipes between when the hot water is used, and when it is used the next time - no waiting for hot water to reach the faucet) and you don't need to run separate hot-water lines through the house. Just right before the faucets.

http://www.tanklesswaterheaters.com/tanklessfaq.html

We have also used a Bosch Aquastar unit in our horse boarding barn where we have a need for highly variable amounts of warm water for the horse wash stalls. Again, in that use it has worked well.

It does not take a lot engergy to keep a tank of water hot - if it's well insulated. And, a coventional tank heater is cheap, easy to get parts for, and easy to fix.

Tankless? High cost to buy, high cost to fix, high cost to install (gas or electric) and parts, when needed, are often not in stock locally. They do NOT last forever as Paul Harvey states - I've fixed many. The one big advantage to a tankless - is if you are going to install it at a remote location from where the main water heater is - or was, and hot water use is not high. Many houses need the gas-plumbing redone just to feed enough gas to run a tankless. Also needs its own venting system. If electric - even a small spot heater needs a large, usually 220VAC circuit dedicated to it. Also, most tank heaters do not warm water to a predetermined temperature - they work on temperature rise. If your house runs on cold spring or deep-well water that is very cold - the tankless heater will have to be a huge one to get good flow at a high temp. That is, unless you install a pre-tank that lets the water reach room temperature first.