Venting Dual Hot Water Heaters

I'm looking at replacing my 17-year-old 75-gal Hot Water Heater with two 50 gallon units.
The current one vents into the chimney by way of galvanized ductwork.
What is the recommended way to vent the dual heater setup? I am concerned that if one heater goes off, exhaust gas from the other one might find its way back into the house by traveling backwards thru the ductwork of the heater that is off.
Thank you.
Newt snipped-for-privacy@vuden.com
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why not buy a 100 gallon tank? it will save energy by 1/2 the standby losses
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That's a thought. Actually, the 75 has been more than adequate for my needs, so another 75 would do me just fine
The reason I was looking at two 50s was mainly because they would be easier for me to get down the steps into the basement. The existing one was put in by professionals, so they had ample manpower to manhandle it down the steps. I have learned to plumb in the time since that installation.
I asked a plumber once about using two 40s, but he suggested that if two 50s fit, then I should go for 2 50s.
Also, on a scouting trip to Home Depot, I did not see any 75s. Can these be had other than by going through a plumber? I'm not sure if the local plumbing supply place sells HWHs to DIYers. It's pretty much a wholesale place.
Finally (and least important) I was thinking that with two heaters, aprropriately plumbed, if one failed, I could bypass it, thus taking the "emergency" out of any failure scenario.
Newt
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Get a high output 50 so you can move it. HD stuff is not the good stuff.
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Do you mean a furnace-boiler and water heater cannot share a chimney, Its code here. *******************************************************
My old house had both too, very common. I recall recently that it is no longer allowed, but I don't have any firm information. It may have been a local code. I know that woodstoves and oil/gas burners are not allowed. I've not had any reason to check so I'm not up to date on that.
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It may not be new code now, but they wont make you change it in Chicago on even commercial apartments, I just passed an inspection.
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wrote:

AOSmith is good, but so is Bradford White if you end up hiring a plumber to do the job. Ours is serving us well and vents up the chimney.
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Get a high output single tank. When you shop buy with comparing by EF Energy Factor rating, most of what you get is 55-65 EF on standard tanks so at best 35 cents of ever dollar is wasted, 2 tanks is not smart. Your old tank may never of had the btu to keep up. You would want a faster recovery. www.energystar.gov has ratings on all tanks, there are higher efficency tanks to also consider. AO Smith has a good commercial line.
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I have a Ng tankless and wont put in a tank again, but here its like convincing folk their V8 car gets crap milage, they give you 100 reasons they want another V8.
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nope there are good reasons to avoid tankless, no hot water at all during power failures, excess up front cost, new gas lines and possibly new meter because a large tankless has such high BTU, no hot water at all with low flow like valve just opened a little, delay from water on till hot water is available, tankless must detect water use then yurn on burner. tankless payback period will likely exceed life of unit, plus unit needs regular service from trained technician.
regular tanks tend to be install and forget till they leak
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On Sun, 4 Jan 2009 07:17:12 -0800 (PST), " snipped-for-privacy@aol.com"

Actually Bob, tankless can be serviced by a DIYer. Power failure is not a problem. My fake fire place can be lit with a match if the low-voltage has failed. Same with a gas tankless heater.

See: http://www.tanklesswaterheaterguide.com /
Okay you hate tankless
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a power vent tankless can NOT be lighted with a match to operate during a power failure!
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On Sun, 4 Jan 2009 19:09:51 -0800 (PST), " snipped-for-privacy@aol.com"

Not all tankless heaters have power vents, right? Do you know?
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Your really minimizing the number of availble models by saying no power vent.
many larger BTU units are direct power vent. normal chimneys may be too small for a high BTU tankless
The number of overall constraints make the tankless a real adjustment and hard to live with.
Like buying a tiny car to save gasoline when your 6 foot 4 and 250 pounds.
Sure you can buy it, but will you be happy with your purchase?
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Mine has battery ignition, there are models with Hydro power.
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Your biased and full of crap again. I have HW with no power on a Bosch 117000 btu, I have battery ignition. 600$, well I got a 4.5 year 100% return, show me any investment as good.
My new gas line was about 8 ft or so, big fkn deal.
So dont open it a little, get used to it. A V8 doesnt accelerate like a 4, do you still buy V8 cars.
Delay is maybe 30 seconds more but the burner lights in 3-5, I have like most all a delay any way. Is your life that rushed 30 seconds is such a big deal to save alot of money on utilities, CFLs have a delay till they are bright.
What regular service you cant do your self, that is total crap what you say.
A new meter. doughtfull, meters have excess capacity
Tankless, dont loose efficency from bottom scale every year as tanks do,Tankless are the highest effecency waters heaters made with only a few condesing tanks that can even come close to 82 EF, Tankless Takagi is 94EF. Tankless coils last by design 30 years. For a few inconviences you get a produst that can save you alot and last longer. The lowest efficency tankless Ng is 82, avg tank is 55-65 EF, Tankless go from 82-94EF Tanks go from 55-82 EF. EF is the only true rating that is to be used comparing water heaters.
Lets not forget you never had one. I have one and I have tanks
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