There are 143 results from "tankless" on just the Plumbing Forum. Reading those past threads should educate you enough to make your decision.

The summary would be that it is an expensive choice for retrofitting compared to just replacing a conventional water heater. You have to be willing to live in the home 30+ years to see any payback. It's a great choice for a new build, where you can build in the different infrastructure that tankless needs in order to be successful in your climate.

Here is a link that might be useful: ''Tankless'' search results

First, I'm going to assume that we are talking about domestic hot water and not used for heat.

It could make sense to use two tankless units - one on each end of the house.

Ignore any claims of "high efficiency" when comparing gas water heaters; Use actual numbers. A "high efficiency" standard gas water heater is about 67% - a non condensing tankless is 85-87% and a condensing tankless is 90-95%.

Tankless water heaters are more expensive than tank heaters, but you really need to know the details: If you have to install new gas lines, forget it. If not, it can be reasonable as long as your installer doesn't inflate the cost.

Self installed, mine was about $1,100 with the vent kit and valves. Payback was 6 years (over standard tank)and I have continuous hot water. Your mileage may vary.

Regarding the sizing: That needs to be determined with actual numbers.

And don't assume that since you have an existing gas furnace that you have enough excess capacity from the meter to fire up a gas tankless heater. This has to be checked and confirmed. Running a new line and installing a meter with greater capacity would be costly, especially after the fact.

My choice setup is the whole-house tankless heater installed as close as possible to the bathrooms, and a small point of use heater under the kitchen sink. Tankless heaters can have low-flow issues with the dishwasher, or kitchen sink use. This is easily corrected by putting a 2 gallon electric heater under the kitchen sink, and feeding it from the tankless unit. It doesn't have to do much heating. It just mostly stores hot water and keeps it hot, ready for washing dishes and hands.

Thanks for the advice everyone. The contractors said my house set up doesn't allow tankless. Plus your precious input, I guess I stay with tank gas now. But here is another problem. Going with 50 gal .62 pilot fire model, we would survive power outage with hot water supply. But it's not Energy Star qualified. Hence no rebate. If we go with .67, not only the installation cost goes up as much as $900, but also we would end up with no hw during power outage. How different is .62 vs. .67?
We have two quotes in now, each ask for > $2000 cost for .67 tank gas wh. With .62, it's about $1200.

I'm a big advocate for energy conservation, but this is a difference that doesn't matter.

Go with the .62 unless the rebate covers the difference.

Add pipe insulation if you want to reduce heat loss.

Thanks! good advice appreciate it. So for tank wh, does anyone know how Rheem compares with Bradford White? It seems different company sell different products. Is there any specific features/spec's I should pay attention to?

Bradford White limits their product to plumbers, and some people swear by them. They are typically more expensive. It could just be that because of that limitation, they are more likely to be professionally installed.

Marathon uses fiberglass tanks.

Other than that, I'm not sure there are a lot of differences between brands.

My opinion is that you go with the moderate priced unit - and know that you are going to replace it in 10-15 years.
That estimated time line seems to hold true if it's an expensive tank or not.