Tankless Water Heater Inlet Size Question

GlargleApril 28, 2014

We have decided on purchasing the Rinnai RU98i tankless water heater (natural gas). Our builder has told us that we have to have a 1/2" gas inlet. However, this model has a 3/4" inlet.

I checked another brand it, too, has a 3/4" inlet. I've also briefly searched and learned that a reducer can be placed. However, the builder said that it will reduce the flow.

I have no plumbing background (clearly) and was wondering if anyone can help me understand what direction I need to go.

Thanks!

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aidan_m

You need a MINIMUM 3/4" gas pipe running from the meter all the way to the heater. The longer the run, and the more branches before the water heater, the more likely the main gas line needs to be oversized to 1" A reducer will not suffice. Adequate gas service is essential for this concept to work.

    Bookmark   April 28, 2014 at 4:42PM
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Glargle

Ok, I've learned more. We have a 1" line running from the meter through the basement. The 1/2" lines split off the 1" line. The builder, who admits that he has not researched it and is only relaying info, suggested Navien as the brand to go for so that we can use a 1/2" line. Does this seem ok? I still feel as if the person who ran the line should have run a 3/4" line to the tankless water heater.

    Bookmark   April 28, 2014 at 5:01PM
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snoonyb

Call RINNAI, it's their warranty.

    Bookmark   April 28, 2014 at 7:48PM
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jakethewonderdog

No tankless gas water heater I'm aware of uses 1/2" gas line.

Please get someone who knows what they are doing to do this for you. Your builder is not that person.

Look, I have a tankless myself. I think they are great. But they've gotten a bad reputation because people like your builder don't understand how to size or install them.

If your builder is telling you 1/2" gas line he has absolutely no idea what he's doing.

This post was edited by jakethewonderdog on Mon, Apr 28, 14 at 23:01

    Bookmark   April 28, 2014 at 10:27PM
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jakethewonderdog

In what city are you located (trying to understand how cold your water is). Also, how many people in your family, how many bathrooms?

If you have 2 or more bathrooms, get the 199,000 btu/hour unit - from any manufacturer you want. Esp if you live in cold weather climate.

You absolutely have to have 1" pipe going almost all the way up to the 199,000 btu/hr water heater.

Also, use a 92% or better condensing unit. The unit itself cost more but the venting is much cheaper than stainless steel required by 82% units.

Take a look at the Rheem condensing: They have very low flow requirements to turn it on. At ~$1200 they are reasonably priced for 199,000 btu unit. Also gets good reviews on Amazon.

Here is a link that might be useful: Rheem Condensing

    Bookmark   April 28, 2014 at 10:59PM
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Glargle

Thanks, everyone! I held my ground and told him that this is ridiculous and that we absolutely need a 3/4" line going to the heater. He admitted that he was unfamiliar and was leaning to the person who installed the gas line and that they were trying to do it on a budget. We explained that we want it done right.

    Bookmark   April 29, 2014 at 10:21AM
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snoonyb

The next thing to ask, is the meter sized correctly to handle
all the appliances operating simultaneously, with the Rinnai operating off the 1"trunk or will it need it'd own line from the meter?

    Bookmark   April 29, 2014 at 10:58AM
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Glargle

Thank you. I'm hoping that it will get a direct line from the meter at this point. We have a 1" line coming from the meter sprouting 1/2" lines to a gas lantern on the porch, gas hvac, and gas cooktop.

    Bookmark   April 29, 2014 at 3:18PM
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jakethewonderdog

You are limited to about 15' of 3/4 pipe on the 199,000 BTU unit.

You do need to be sure that your meter and service will deliver enough gas for everything.

    Bookmark   April 29, 2014 at 3:35PM
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snoonyb

Since, at this point, the vendor would be suspect, pose the "every lite in the house is on" in BTU's, to the service provider.

And to those that think that I'm mixing metaphor's, I am, so, embrace the concept of a starved appliance, not the metaphor.

    Bookmark   April 29, 2014 at 6:04PM
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padd123

YOU WILL HAVE TO RE CALCULATE YOUR BTU OUTPUT OF NAT GAS THIS NEEDED FOR YOUR HOME BASED ON A HALF POUND OF PRESS AND.6 SPECIFIC GRAVITY AT 60 DEGREES SATURATED VAPOR CALCULATE ALL OF YOUR GAS APPLIANCES IF I NOT MISTAKEN YOUR FLU HAS CHANGED TO 4 IN ON TANKLESS SYSTEM

    Bookmark   May 11, 2014 at 1:43PM
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jakethewonderdog

Padd123:

The caps lock button is just to the left of "A" on your keyboard. Nobody likes being shouted at.

BTW: I stand corrected: Navien has a line of tankless heaters even 200k btu/hr model that can use 1/2" gas line. It's not entirely clear how they do this, but it's not at all standard. I think it's asking for trouble...and wouldn't count on 1/2" gas line to supply 200k btu/hr ... but there it is.

Here is a link that might be useful: half inch gas line capable Tankless

This post was edited by jakethewonderdog on Mon, May 12, 14 at 10:23

    Bookmark   May 11, 2014 at 5:39PM
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aidan_m

Maybe LP, not gas?

    Bookmark   May 12, 2014 at 12:14AM
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jakethewonderdog

aidan_m

Nope. Natural Gas.

Edited my post to show link.

    Bookmark   May 12, 2014 at 9:41AM
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xedos

Jake - you are missing the micetype disclaimer:

1/2" rigid pipe can be used; refer to the sizing
tables on the proceeding pages for LIMITATIONS !!!!!!!!!!!!!

There will be few instances where 1/2 will work in most installs.

    Bookmark   May 22, 2014 at 11:04PM
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jakethewonderdog

If you look at my previous post, I thought I was pretty clear that it wasn't a good idea and that it was asking for trouble. But, since it's out there and I was previously adamant that it didn't exist, I thought I should mention it. Looks like the limits are ~24' of 1/2" with 6" WC or greater.

Seems crazy to me to keep 24' of 1/2" gas line when it's clear it would be a marginal installation.

    Bookmark   May 23, 2014 at 8:40AM
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zl700

Navien newer units allow this because of the low allowable inlet pressure.
Navien's input to maintain full fire is just 3.5" WC N/G. So if you have a system at 5-6" and lose 1.5 to 2.5" on the pressure drop of the piping including the 1/2" and still have 3.5" at the unit it will work.

Their unit does not have a internal gas pressure drop with their combustion system, so no more elevated inlet pressures required.

They do a webinar once a month to show how the product works with a video to prove.

https://attendee.gotowebinar.com/rt/7113686655298627329

Here is a link that might be useful: Navien NPE tankless webinar

    Bookmark   May 26, 2014 at 6:59PM
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cleverchimp

Dont go with Navien, their unit seems to have issues in the past. I plan to change my BOSCH unit to Rinnai RU98e condensing unit. The way it was installed now it share the 1 inch gas line with the Jacuzzi. 3/4 line going to the tankless, also I put a 5 micron prefilter before the tankless, maybe I'll be installing pump (GTK15) on the hot water line as well.

    Bookmark   June 4, 2014 at 2:25PM
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jakethewonderdog

cleverchimp,

When you said "they had issues in the past" what does that mean? They did have some problems when they started out and those seem to have been resolved.

So did most of the manufacturers.

I'm not a fanboy for Navien, but they have some innovative features and I'm not hearing that they still have design issues.

    Bookmark   June 5, 2014 at 8:23AM
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cleverchimp

I just like to google a brand and avoid brands that show many complaints then praise. Navien seems to be right there, Rinnai and Noritz seems to be the top player, I choose Rinnai because they have a pump with timer that can be installed with their unit (the unit control board support this) so that I have almost instant warm water during the time I shower.

    Bookmark   June 10, 2014 at 10:30PM
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