Options to replace 40Gal water heater with on-demand type

I am remodeling my basement and part of the project was to relocate my gas 40 gallon HW heater over about 3 feet to free up some space. The existing water heater will fit into the new space, but I came across these "on-demand" HW heaters that look appealing. I have a few questions.
1) what would be a unit that is equivelant to a 40-50 Gal HW heater?
2) Is this something I can install myself? I have done HW heaters before.
3) This is on an outside basement wall. Can I use the existing flue pipe that was on the old HW heater? Any other installation requirements?
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google tankless heaters on this newsgroup. lots of discussions mostly on the downsides of yur planned upgrade.......
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Mikepier wrote the following:

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Bill
In Hamptonburgh, NY
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It's good you have gas because the eletric ones need a huge circuit. They have their drawbacks. You need to size it based on fixtures and usage rather than just comparing it to your tank one. If you're in the more northern parts of the continent you may find it struggles to deliver hot water in the winter. The colder entry water temp means they have to work harder to get the water hot. I'd oversize a bit if I lived north of the mason dixon. And prepare to shell out major bucks for it.
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jamesgangnc wrote:

They usually have a set 'rise' in temperature at so many gallons a minute, say a 50 degree rise at 2 gallons a minute. If the water is 50 you get 100 degree water but if the water is 40 you only get 90 degree water unless you use it slower, then it gets hotter up to the cutoff temp.
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LSMFT

Simple job, assist the assistant of the physicist.
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you will need large natural gas line, and perhaps a new meter too. there will be a delay between water on and water hot, time lag for ignition and heating. you wouldnt save much energy, standby losses are actually pretty low and help to heat the home if heater is in conditioned space.
flue may not be big enough most water heaters 35K BTU, tankless over a 100K BTU... Tankless must do all the work heating a standard tank does, on a flash basis in a tankless.
Check for local affordable service, newer tankless are high tech and may need qualified service tech, and perodic cleaning heat exchanger with acid, espically in hard water areas
heres a good experiment turn your current heater to vacation or off, go out for day. you will find temp doesnt change much...
low flow situations may be below trip level for burner on, so no hot water.
on many tankless and all power vent tankless no electric means no heat at all. power is necessary for unit to function
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We all know you get a knot in your shorts every time someone brings up tankless. But the "conditioned space" baloney has been shot down so many times, yet you keep throwing it out there. Most people not only use heat in their conditioned space, but also AC. And during AC season, that heat loss from the tank heater ADDS to the cooling requirements. Yet, you don't list that as a negative for tanks.
Whether a tankless is good for him or not depends on many factors. I guess I'm surprised that he would even being asking, given that he's a regular here and it's been thoroughly debated here many, many times.
As for the main issues as I see it:
Sizing: depends on incoming worst case water temp and worse case demand. Tankless have charts showing what they will do.
Check gas line size requirements and assess based on all other gas appliances in the house, sharing those lines, etc.
Either get a model that will fire without AC or you have no standby hot water when power goes out, like you would with tank type
Check manufacturer for install requirements regarding chimney, but I would expect that it can't be vented into the existing chimney. I think they are designed to be direct vent.

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The gas line may be fine, a new meter is unlikely and the delay adds about 4 seconds to the 30 second delay from removing cold water already in my pipes. And of course you dont own one so you are not saving money, so you just put them down like an ass.
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There's nothing particularly bad about them but the savings do not make up for the higher initial cost for many people. It would be different if the price comes down some more.
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so 4 seconds of wasted water and sewer times every time you need hot water.
the gas line may be fine, may not be. at minimum probably a new gas line directly from meter to your tankless, pro testing needed. and no hot water at all in a power failure if your tankless is a power vent model.
around here many neihborhoods are low pressure main lines with no regulator at meter.
local plumbers refuse to install tankless in these neighborhoods because of poor performance.......
so lets say your new tankless works 100% PERFECT:)
and your teenagers love their endless 1/2 showers. Gas bill be going up:(
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snipped-for-privacy@aol.com wrote:

rented from the gas company, with includes yearly maintenance, and it has not failed me yet. It is cheaper then electric, does not fail in a power out, it never runs out of hot water. Now the last one can be a disadvantage when you have children, they love it.....
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Your local plumbers are obviously like you, uninformed and under educated, my and my neighbors Ng tankless work , save money and beet our expectations.
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ransley did you note in certain low pressure gas line areas.
my home is high pressure with a regulator
much older neighborhoods are low pressure, with no regulators on meter, just neighborhood.
plumbers report low flow in these areas sometimes, so they refuse to install tankless.the gas company adjusts the pressure by remote control seasonally, this causes heavy load issues
the regulator is for 100s of homes and the infrastructure undersized for the load.....
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