Tankless water heater vent sizes and configurations

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I plan to purchase a tankless water heater to replace a failing water tank. The current vent is a 6" diameter vent pipe. I need a 7.4 GPM unit (4 bathroom house with 3 showers) and considering a Rheem GT199PVN. I noticed some tankless heaters have two separate vent pipes (one intake and the other exhaust). Some have one vent pipe with one pipe inside the other. If I choose the later I will need an additional hole in the side of the house (no chimney). What are the pros and cons of two separate pipes than just one pipe with an interior pipe? Are two separate pipes functionally better than one pipe inside the other? I'm a bit concerned when I need to replace the new heater (hopefully in 10+ years) that I may have to deal with patching or resizing holes. I've seen so many different kinds and configurations in water heaters and vents. Unemployed right now, I don't have $1600 to spend on installation, I got the time to do it myself. Any tips on installation are appreciated.
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I am definitely a fan of tankless water heaters, but one alternative to consider is a condensing tank water heater. It will be even more efficient than a non-condensing tankless water heater. One issue is that a condensing tank will cost a lot more than a non-condensing tankless, and while that is often offset by less installation labor for the condensing tank, since you said you'd be installing it yourself, that wouldn't apply.

A couple comments. That GT199PVN won't really provide you with 7.4 GPM of hot water--that flow rate is for only a 45 degree Fahrenheit temperature rise. It's mainly a marketing exaggeration, it would only apply if your incoming cold water is 60 degrees already and you are happy with 105 degree hot water. Where are you located? If your incoming water is actually 50 degrees, and you want 110 degree water in your shower, then that's a temperature rise of 60 degree, so you'd get about (45/60) * 7.4 = 5.55 gpm.
Another comment is that 7.4 GPM (if you get a bigger unit that can handle it) is enough for three simultaneous uses, e.g. 3 showers, or 2 showers and a dishwasher, 2 showers and 2-3 vanities, etc. Depending on your lifestyle and how many people are in your house, then 2 simultaneous uses might be enough, in which case the GT199PVN is likely to be adequate (depending on where you live and the incoming cold water temperature).

Good question, I don't know if separate pipes is a functionally better arrangement, but it certainly seems like a combined pipe is more convenient.
Cheers, Wayne
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And will likely require a larger utility service (electrical or gas), making the economics worse.
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I agree with you that the economics of electric tankless are suspect. But when I installed a gas tankless, there was no need to increase the gas service size. Even if there were, I was under the impression the utility would have done it for me at no cost (unlike with the electric service).
Cheers, Wayne
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The utility will usually replace from the street to the meter. From the meter to the appliance is on you.
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A non condensing Bosch tankless I have has a EF of 82 or 83, A Condensing tank I have, a AO Smith Cyclone has an EF of 83 also, Condensing tank by the true rating of EF is lower than you think, try to find EF ratings on AO , they are not openly published for just that reason, most regular gas HD tanks rate 55-60 EF or true overall efficency.
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Dual pipe would be a cold air intake that saves you a bit. Have you checked your gas suply with a manometer, and all gas apliances on, heating system also? Also calculate a % reduction in supply that will happen on the coldest days, call your gas co. What is your winter water temp low? I use a 117000 btu bosch and have no complaints with 35-40f incomming , but you need to do your homework before you find it isnt up to your demands and you are not happy.
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While you are at it , does the gas supply have enough capacity to run the water heater ? I still think it is cheaper to just run the old style tank in the long run.
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I agree...Especially with 4 bathrooms , 3 showers , dishwasher , sinks , ect....Would have to be a big tankless unit with a huge gas pipe or large electrical service , if electric , which would require an upgrade for the gas or electric service...And your unemployed to boot....Get the old style tank and hook it up yourself without the major upgrades....Cheap and easy and not really that much more to run.......
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On Thu, 6 Aug 2009 13:06:41 -0500, "Ralph Mowery"

I'm in e.TN. Two adults in the house. The cold water is warm enough to shower without being heated, although somewhat uncomfortable. An old-style tank is about $990 (Rheem direct vent), and a tankless about $1400 (or $1200 if I order online).
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wrote:

Here is nice guide with some venting information.*
Exterior tankless heaters are not required to have outside venting, iirc.
If I was building "new" there would tankless installed. A local fire captain built a 5,000 sf home. He installed 3 tankless heaters, configured in "zones". One for the laundry, kitchen and garage (inside garage) Two exterior units at opposite ends of the house. His three daughter-units have one (3 BR, 3B)
The MBR, Bath and Powder room have one heater on their side of the house (exterior)
* http://www.tanklesswaterheaterguide.com /
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just wait for a storm and power failure, with tankless no hot water at all......
the vertex condensig qualifies for the 30% tax break.
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wrote:

Not with gas units. They have a spark igniter - just like your gas grill. Gas works when the power is out!

So does my new garage door.
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The flow of the water generates the spark? (I'm envisioning someone having to go press the red button on thhe unit late at night!)
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wrote:

Don't hold my feet to the fire, but Bosch TWH indicates the spark takes place after the hot water is requested from inside the house.
..hydro-generator..?
BTW TWHs can last many years and is easily serviced by the home owner.
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tankless have high tech electronics and controls, and may need perodic sediment removal. its best to get a tankless where local experienced service is available.
from the time water is first drawn, tankless burners turn on and heated water finally reaches the fixture theres a delay, and waste of water
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wrote:

Bob (Goggle Post),
Have you been up close to a tankless system? I understand if not.
You sell yourself as a "hater" of tankless. Not once have you typed the positive.OR even a negative. Not to point this out but tankless can be a perfect move/solution.
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dont hate them just dont believe they are practical except in very special circumstances.........
I post the downsides which unfortunately is a long list...
the vertex is very close efficency wise, without the downsides
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I talked with a licensed plumber and he does not recommend tankless due to the expense and complexity. I called GE and they are giving me a replacement under warranty. The replacement tank will need an expansion tank, the reason (according to GE) the first one failed. I know (liquid) water does not expand much when heated, but apparantly enough to cause a water tank to fail.
Tankless are generally not recommended by plumbers, but they are improving.
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Dude, you might want to do some shopping. You should be able to get a decent storage unit for 1/2 that installed as a replacement.
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