Proper Venting of Rheem Condensing tankless water heater

**saphman** · 04-09-2012, 12:25 PM

I thought I would attach this gas table for reference and comments. Given that he has a 1" line in the attic, Maybe he could upsize his gas meter and inlet pressure. Using CSST really eliminates elbows and fittings. Of course the system should be grounded.

**Killer95Stang** · 04-09-2012, 03:58 PM

I'm only running 3/4" for less than 20 ft from where it tees off of the 1" line. Add in a couple elbows and I should be good. Our meters are supplied with .5lb at the meter.

I worked as an ME for a heat treating equipment manufacture, so I've sized piping manifolds for systems in the 1-5 million BTU range.

I used this chart.
http://home.mchsi.com/~gweidner/pipe...g-chart-ng.pdf

**Hackney plumbing** · 04-09-2012, 05:12 PM

Originally Posted by saphman

I thought I would attach this gas table for reference and comments. Given that he has a 1" line in the attic, Maybe he could upsize his gas meter and inlet pressure. Using CSST really eliminates elbows and fittings. Of course the system should be grounded.

I was about to tell you that you oversized your line but I see you already know that. I use 2lb systems on a regular basis and would have installed 1/2" i.d. copper tube for a 200,000 btu burner.

I have done hybrid low and med pressure systems also. The existing gas appliances would be running on low pressure but the one line to the tankless or generator would be 2lb med pressure.

**saphman** · 04-09-2012, 07:00 PM

I do believe you are correct. I had an existing 1/2" gas line right where I installed my tankless but I got so much feedback that was negative on using that I upsized mine to 3/4" right from the inlet manifold in my attic. Based on the gas table i posted earlier the 75 feet CSST run I added still gives me over 500 CFH which translates into ~ 500,000 btu to run a 200,000 btu heater. More than enough with 2 PSI system. I tried in fact turning everything on that is gas in my house at the same time: 2 gas furnaces, 6 burner viking stove, nat gas generator, natural gas grill, new tankless heater, gas dryer, outdoor gas lamp and gas fireplace. All ran without a hitch!!

**Killer95Stang** · 04-13-2012, 12:04 PM

Originally Posted by Dana

Think again about using 3/4" goods- with a 199K burner a 3/4" line would only get you 10' from the meter, with no ells or tees.

^ (the numbers in the grid are max-kbtus) ^

Almost every real-world installation needs 1-1/4" gas lines up to the unit, with a reducer for making the connection. Undersizing the gas lines introduces a whole load of operational misery, especially when the furnace fires up. Do it right the first time and save yourself the pain of having to rip it out and start over. Very-short sections of 1" or 3/4" aren't much of a problem, but I do mean VERY short, with as few turns in the skinnier pipe as possible.

Thanks for your post that got me thinking.. then rethinking ... then calculating.. and then came the slap in the forehead. Solution.. I'm running 1" pipe about 35 feet from the meter to about 5 feet shy of the WH (drop from attic to unit), then going down to 3/4" for the connection. I also down sized the WH to the next smaller model of tankless units. I had nowhere near the need of the unit I originally bought, so going smaller will hopefully keep me from having error codes a year or so down the road.

Sometimes I just wish that unions would meet code!!!

So.. $40.00 in shipping costs.. lesson learned..