Gas Line Sizing - On Demand Water Heater

mmiller99

Hi folks;

Hoping to get some advice on gas pipe sizing for an on demand water heater upgrade in my RV. It runs on vapor at 11-14 WC

I'm currently running a 42,000 BTU on demand but I would like to upgrade to a 64,000 BTU unit for various reasons, primarily, the 42,000 unit is close, but ideally should be a bit hotter. Also, the water flow on the 42,000 unit is quite low.

The current 42,000 BTU unit has a 3/8 flare for the gas connection. Measuring the outside diameter of the existing copper tubing I come up with .377" so I'm guessing that is .375" which means it's not even 3/8" copper tubing but 1/4" copper tubing that feeds the 3/8 flare connector on my existing 42,000 BTU heater.....

This brings to mind numerous questions. If I am correct, is my current 42,000 BTU unit being starved for gas seeing it's using 1/4" copper pipe to feed it? Distance of the run from the manifold to the heater ~ 10'

The replacement 64,000 BTU unit uses a 5/8 flare connection.

Looking at gas pipe charts it seems that you would need a 1/2" line to feed the 5/8" flare connection on the new 64,000 BTU heater over a distance of ~10 feet.

Does any of this make sense or am I way off base? To me, a 1/2" line is pretty darn big and I bet it's hard to bend. I was under the impression my existing feed line was 3/8" until I measured the OD. Now it seems like the feed situation is much worse than I thought and I actually have 1/4" line :(

To me, a 1/2" feed line seems like overkill...I'm not a gas fitter but I did stay at a Holiday Inn Express!

Big Ed_4

Up grading 50% You will have a problem venting besides developing gas leaks, Being of RV construction ……I would recommending "let sleeping dogs lie "..

mmiller99

After more research it looks like the issue I'm up against is that with 64,000 BTU, a 3/8 line falls short and will starve the unit, and, while a 1/2 line is correct, it's also overkill with much more capacity then needed.

So it looks like the 64k number has left me right on the cusp between the two.

More thinking led me to the idea of not running copper tubing and instead running hose off the manifold inside to the heater. This makes it much easier. Hose is also used by the factory on the slide out so it's not a safety issue. I'm sure they used copper pipe everywhere else because it's more reliable and cheaper.

What I find bizarre though is that of the 600 or so reviews on the heater, only one person mentioned that they had to buy a 3/8 to 5/8 adapter to connect to it. This makes no sense to me because I can assure you, there's no way an RV has a 1/2" gas feed to their existing water tank. I should have seen many posts from people about the gas line size…

And yet…not one person out of 600 or so has mentioned they needed to replace their gas line to make it work…

psb75

You need to get an understanding of pipe materials and sizing. What is the size of the connection on the gas water heating unit? That is your first clue. Most normal residential units have 3/4" NPT. These units need a lot of gas for their BTUH needs, Copper gas line sizes refer to the O.D. Black iron NPT pipe sizes refer to the I.D. Soft copper comes in 5/8", 1/2", 3/8", 1/4". These are all O.D. dimensions.

mmiller99

The connection on the new heater is 5/8 flare male.

64,000 BTU appears to need a 1/2" line over 15 feet as 3/8 looks like it's too small.

The existing tubing is 1/4" (OD) so it seems to be to be questionable even with the current 42,000 BTU unit but certainly not enough for 64k.

So I guess I'm looking for confirmation that for 64,000 BTU's, over a distance of 15 feet, I really do need a half inch hose.

hot_rod

For such a short distance, in a RV, why wouldn't you error for the large tube, if it is on the border? You don't want to do it twice😚

psb75

You really do.

mmiller99

Thanks guys!

The only reason I doubted myself is because 600 people have bought, installed, and reviewed that unit and not one of them had to run a new feed line….

And I can tell you for a fact that no RV has a 1/2" line running to their water heater. Mine was 1/4" and I could see newer units MAYBE increasing to 3/8, but what that ultimately means is that none of those people are getting the full 64,000 BTU's out of their units.

..and I found it hard to believe that not one of them talked about that, leading me to doubt myself.

mattmia2

.375 is 3/8

mmiller99

https://forum.heatinghelp.com/discussion/comment/1826374#Comment_1826374

Odd….I looked at charts all over and I could swear they said it was 1/4"

COPPER PIPE I.D. & WALL THICKNESS CHART

1/4	0.375
3/8	0.500

leaking

follow instructions. Oversized is good if won’t starve of gas . Figuring in lowered pressure when all gas Apliances are on is needed . I’ve had several , do not skimp. Or you will regret it .

mmiller99

I'm more confused about copper piping.

So we know I need 1/2" copper pipe but the fitting that goes to the water heater is 5/8" male flare.

Are there any issues with putting a 5/8 nut on the 1/2" line flare?

These pipe sizes are very confusing…

Larry Weingarten

Hi, It seems the OD of 1/4" copper tube (in the US) is 3/8", so unless the pressure is cranked up, with a second regulator at the water heater, I don't understand that sizing. Have you looked into a two regulator system? First stage would take it down to two psi, and second stage would take it to appliance pressure. 🤔

Yours, Larry

mmiller99

That's an interesting idea but it may be more complex than required.

The basic system looks like this;

Propane Tank ⇒ Regulator ⇒ Manifold ⇒ 3 branch lines feeding appliances at 11-14 WC

It would be pretty easy for me to run a new line from the manifold to the heater as it's mostly a straight shot.

But the thing I'm wondering about is the fittings. If the line is 1/2" can you flare it and use a nut for a 5/8 connection at the heater? Or would you have to use a 5/8" line in order to connect it to the 5/8" connector?

Some folks seem to indicate that 1/2" line is the same as 5/8" - it's all beyond me.

Really all I need is a 15 foot copper line with female flares on both ends. One to connect to the manifold and the other to the heater.

Here's a picture of the 'manifold' which just looks to be an iron pipe T with another T

psb75

You need to know the difference between I.D. (inside diameter) and O.D. (outside diameter). Soft copper gas line is usually referenced by O.D. Example: 5/8" O.D. copper has a 1/2" I.D. (same for hard copper pipe).

yellowdog

You need 1/2 od copper tubing for the new heater. No way around it. Use a 5/8 x 1/2 reducing flare nut at the heater. Not knowing which copper line is your water heater at the manifold makes it hard to tell you how to adapt it on that end. If it is off of the flare tee, you will have to get a 1/2 x 38 swivelnut with a 1/2 union to get to your 1/2 flare nut. If it is off of the black iron tee, you can just change the adapter to get your 1/2 flare. The existing fittings all look questionable already. Something looks off about them with the pipe slendered down on the ends.

jimna01

I treat most online reviews critically . It is pretty easy to stuff the ballot box so to speak with positive (or negative ) reviews. You need to read between the lines and try to determine if the reviewer actually ever used the product or in case of food/recipe/restaurant every actually make or visit the establishment .

mmiller99

https://forum.heatinghelp.com/discussion/comment/1826390#Comment_1826390

Bam. That's exactly what I needed to know. Thank you!

On the manifold, 1 of the 3 lines is capped at the other end and no longer used. As such, I can rearrange them and just change the adapter, as you mentioned. I thought this would be the easy way.

mattmia2

https://forum.heatinghelp.com/discussion/comment/1826375#Comment_1826375

that is for pipe not tubing. tubing is sized by nominal od, pipe is sized by nominal id. typically soft copper tubing with flare fittings is used when copper is used with small gas lines.

mattmia2

Here is an excerpt from the model code for copper tube sizing for low pressure propane. It appears that they give size K & L pipe in the K & L line and tube size in the ACR line. They are the same size tubing, one is sized by OD and the other by nominal ID:

So it appears for 10' at 64,000 btu/hr you would need 1/2" od tubing assuming the manifold is large enough that the pressure drop under all loads in the manifold is negligible.

hot_rod

are there regulators on the heaters and appliances? Perhaps they are running a higher pressure to all the devices?