Hello,
Can anyone tell me why you need to use stainless steel pipe for venting on a tankless gas water heater. What would happen if you connected to existing galvanized steel? Any info would be appreciated.

Probably it has to do with the short cycles. The heater doesn't run long enough some cycles to heat up the pipe. Water would condense along with acidic components of the flue gas, and corrode galvanized more quick.

Have you made careful calculations on any savings from the system? How does your current gas usage in the summer compare with the claimed stand by loss? The pay back could be even longer if you have to rip out a galvanized vent and replace with stainless steel. If you locate a tankless heater where the old tank was, you will still have the same wait while running cooled off hot water down the drain. When you shut the water off, the pipe will be full of hot water, which will cool off, just as water out of a tank does. A slightly better insulated tank might be a much better investment.

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Originally Posted by Conniefixit

Hello,
Can anyone tell me why you need to use stainless steel pipe for venting on a tankless gas water heater. What would happen if you connected to existing galvanized steel? Any info would be appreciated.

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Good question Connie, simple answer.
Galvanized will rust out in time while stainless won't. Regards, Tom

Thanks for the answer. I thought it had to do with condensation and rust but I could never get a straight answer from the tankless manufacturer.
In reply to your question Labman, I installed a tankless system for my friends almost 2 years ago and their gas usage has been cut in half.
But we're in warm and dry So. California. Even though their old 50 gal. tank was insulated and located in a closet in a heated part of the home, it took almost 6 min. of continuously running the water before hot water got to the 3rd floor bath. Now they get hot water in 2 minutes. They couldn't be happier with the new system. Although the tankless was 3 times the cost of a replacement 50 gal, it was worth it for another reason besides the savings in gas and water-- they now no longer have to worry about the tank rusting out and leaking all over their new floor, which is why the old tank was replaced. Our water is so hard here in L.A. that unless the homeowner is dilligent about flushing out their tank every year, they are not going to get too many years out of it before rust out and leakage. .