I'm looking to install a GAS tankless water heater and I read that you need an area of 750 sq ft of combustable air for this unit I want to install. My question is; Does that mean that this has to be a wide open area because my basement is 1000 sq ft but it is framed out into 3 rooms. All rooms have doors with a 1.5 in clearence on the bottom. Is this enough for that Tankless water heater to breath? The room where the heater is going to be is approx 122 sq ft.

I think you should be fine. You might consider adding an outside vent to the room with the water heater. The 650 sq. ft. is based on the idea that enough outside air will leak into a room that big to provide all the combustion air needed. It still means pulling in outside air to replace the conditioned air the heater uses. A more direct vent will reduce your heating and cooling costs.

Watch your clearances. The unit may be small, but can't be close to anything. I looked at installing one in my crawl space, but didn't have room. I also concluded we don't use enough hot water to ever pay for it. Look at the minimum flow too. You may have to open the tap wide open to get hot water.

Thanks for the feedback! Is it pretty expensive to operate? Does it suck up the NG just to heat the water?

Supposedly you make up their stiff initial cost through savings on operating cost. They use as much or more gas as a furnace when they are running, but only when you are using hot water. In the summer time, when we only use a conventional hot water tank and dryer, our gass usage is much lower than the figures they use to prove the tankless saves money. Due to the pricing structure, we wouldn't pay much less for gas if we didn't use any at all in the summer. I don't think we use near as much hot water as the estimates say. Of course, the gas needed to maintain a tank of hot water will be about the same no matter how much you use.

The real way to save hot water is to locate the heater near the point of use. Tank or tankless, the water in the line cools off between uses, and goes down the drain before you get hot water. My tank is in the front of the garage now, and most of the water is used in the middle of the back of the house. I was hoping I could fit a tankless heater under the bathroom, but don't have the clearance needed. If you can, put the tank as close to the usage as possible, specifically the sink most used for hand washing.

Thanks again for all your help. I think I'm going to stick with my TANK, it just seems like it's too much trouble to reconfigure my pipes for no real saving on the NG bill.

There are electric tankless point of use heaters that are very efficient. I installed one unit to handle 6 hand wash sinks and it was .6 gal per minute. Hasn't failed to provide hot water in two years of service. Unit cost about $120. 120v 15 amp