Navien 240A -- risks of using existing 4" venting?

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MarkusMarkus

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Hello all,

I purchased a townhouse a couple of years ago which already had a tankless water heater, a Takagi unit which uses direct venting through 4" PVC for both its intake and exhaust. The original installer *did not* install the flush valves needed for regular maintenance (descaling), and the previous owners surely never had the unit properly serviced as it is now prematurely on its last legs with hard water mineral buildup in the heat exchanger. The first signs were leaks springing from inside the chassis, and a licensed installer came out and confirmed this, so unfortunately the whole thing needs to be replaced.

I'm going to have it replaced with a Navien 240A, but here's the rub -- the Navien wants 2"-3" diameter PVC for its intake and exhaust, but the Takagi used 4" for both so that's the existing infrastructure. Naturally I'd prefer to use that existing venting rather than tear into the ceiling between the unit and where that venting reaches the exterior (probably 25' and running through 3 bends if I were to guess).

Navien specifies 3" max diameter for the venting. If I had the installer run 2" venting to and from the unit but then converting to the existing 4" venting above the ceiling, let's say five feet up, what risks am I taking? It's of course easy to understand why reducing to a smaller diameter vent would be bad, and most venting issues I find online concern blockages, but I've not found any explanations as to why going to a larger vent diameter to the exterior might be problematic.

Is it a turbulence issue for the intake side (i.e. incoming air going from 4" down to 2" before entering the unit), or something not ideal on the exhaust side (going from 2" to 4")? My plan would be to have the installer use 2" PVC as far as possible before it meets the existing 4" so that we're not going from large diameter to small immediately before it enters the unit.

Thanks very much for your input and expertise.
 

Jeff H Young

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I cant answer your question but I recomend following the manufacture either writen or at least a verbal explanation . If someone on here tells you go ahead its fine is that good enough? I only assume there is a maximum vent size and possible consequences to ignoring the instructions
 

Reach4

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I'm going to have it replaced with a Navien 240A, but here's the rub -- the Navien wants 2"-3" diameter PVC for its intake and exhaust, but the Takagi used 4" for both so that's the existing infrastructure.
Is that two separate 4 inch pipes that exist, or is it a single coaxial pipe?
 

MarkusMarkus

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Is that two separate 4 inch pipes that exist, or is it a single coaxial pipe?
There are two separate 4-inch pipes, and I should also add that the venting is makes a 90-degree turn to the horizontal after entering/exiting the unit, then exit to the side of the townhouse (i.e. they do not run vertically to exit at the roof).
 
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Fitter30

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I think you would be ok the manufacturer of the new heater what 2" because when their marketing 2" is cheaper than 4" and still works.
 

MarkusMarkus

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I cant answer your question but I recomend following the manufacture either writen or at least a verbal explanation . If someone on here tells you go ahead its fine is that good enough? I only assume there is a maximum vent size and possible consequences to ignoring the instructions
Thanks, Jeff, and point taken. I realize that I'm rolling the dice here and accept that in the end I might end up having to have my plumber return in order to go into the ceiling and replace the vent system. But hey, here's hoping. : )

I think you would be ok the manufacturer of the new heater what 2" because when their marketing 2" is cheaper than 4" and still works.
And here's hoping that's true, too, Fitter.

I'll let you all know how things go once the new unit is in and operating for a while, and at least that will be one more data point for posterity whichever way things go.
 

MarkusMarkus

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Hello all,

I wanted to return to this thread, now a little over three weeks since the install of my new Navien 240A, and just report that all is well and that there have been no problems regarding the new unit's 2-inch venting being connected to the existing 4-inch venting which was required by the previous unit. No error codes showing, and no issues with unit performance.

The crew that did the installation ran the new 2-inch venting up vertically from the unit and through the ceiling where it makes the switch from 2-inch to 4-inch diameter at the first 90-degree bend to horizontal. Total length of 2-inch venting is probably five feet before that turn. Anyway, no problems thus far and we're happy, as well as looking forward to regularly de-scaling the unit the way the previous unit should have been.

So, just another data point in case anyone here ever is considering doing something similar with regard to the venting, and of course I'll reply here should anything change over time.
 

Jeff H Young

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Hello all,

I wanted to return to this thread, now a little over three weeks since the install of my new Navien 240A, and just report that all is well and that there have been no problems regarding the new unit's 2-inch venting being connected to the existing 4-inch venting which was required by the previous unit. No error codes showing, and no issues with unit performance.

The crew that did the installation ran the new 2-inch venting up vertically from the unit and through the ceiling where it makes the switch from 2-inch to 4-inch diameter at the first 90-degree bend to horizontal. Total length of 2-inch venting is probably five feet before that turn. Anyway, no problems thus far and we're happy, as well as looking forward to regularly de-scaling the unit the way the previous unit should have been.

So, just another data point in case anyone here ever is considering doing something similar with regard to the venting, and of course I'll reply here should anything change over time.
So you just went for it with no comment or did you consult at all with manufacture ?
 

MarkusMarkus

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So you just went for it with no comment or did you consult at all with manufacture ?
My plumber contacted a Navien rep who advised not exceeding the maximum specified vent diameter of 3", so I took the chance. Should we start seeing any issues or error codes, I would just plain have to have my plumber back out to tear into the ceiling and replace the whole run of the venting.
 

tvwav

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I am facing the same venting issue. Had a Japanese Takagi i installed 2009 and control board went out and no replacement available. I also can not get any explanation why 4" venting would be a problem other than CSR stating i should only use the recommended diameter without any reason other than that's what is recommended. I just can't see the use in re-venting at additional cost and labor when i have perfectly good vents already.

This is the only info i've been able to find. Seems that few, including the manufacturer, address replacing an existing tankless. .

So i guess my question, since i'm about to do exactly what you have done is have you had any issues since?
 

Fitter30

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With the iom manual did u see a minimum distance for the flue/ combustion air?
Plumber setup the burner with a flue analyser?
 

Jeff H Young

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My plumber contacted a Navien rep who advised not exceeding the maximum specified vent diameter of 3", so I took the chance. Should we start seeing any issues or error codes, I would just plain have to have my plumber back out to tear into the ceiling and replace the whole run of the venting.
come on back and share your results . you joined came and got advice why not share and help someone else?
 
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