Installing gas heaters & tankless water heater, Jacuzzi J-sp-180w

Just want to run this by you to get good information. I've got a very small cabin for weekends. I want to install a couple of small propane radiant heaters and this tankless water heater. At least initially I'll be running them off of a 100# propane tank with plans to eventually converting to a 250 gallon propane tank. My question is about what is acceptable for the gas lines. The water heater will be about 10 ft from the small propane tank which is outdoors. The water heater will be mounted in the garage and vented outside about 6 ft above the enclosure (vented) where the tank is to be located.
The tankless WH comes with a 3/4 gas line connection. My intention was to run 1/2 soft copper from the regulator to all the appliances with appropriate shut off valves. This is the type of set up and plumbing I've seen "normally" on propane systems. What's making me question this is the 3/4" connection. I don't get involved in gas or propane plumbing very often so don't know the rules or code for what type or size piping is allowed by code. My #1 concern is always SAFETY, especially with gas.

The BTU-input size of the burners matter, as well as which end of the run the piping run the tankless lives. Typically the lines inside the house are on the low-pressure side of the regulator, and there are charts for calculating how far you can run for so many BTU.

At 10 feet you might be able to get away with half-inch with a 180,000 BTU tankless if you keep the radiant heaters off when running at max flow. Some tankless units are pretty flaky when fuel-starved, so whatever the BTU charts say, give it some margin. At a typical 2-2.5 gpm shower flow it'll be running under 100,000BTU/hr, but cranked to the max to fill a tub you can hit the high-range pretty easily. On page 14 of the manual it looks like you could get away with up to 20' of half-inch, if you keep all other burners off while running it, but I wouldn't recommend pushing it that far.

Ideally the tankless would have a dedicated gas line back to the regulator rather than having it on a trunk line with tees to the radiant heaters, in which case you may be able to run them all at once, with minimal interactions.

If you can find a YouTube instructional on installing tankless units (or better yet a manufacturers training seminar through a distributor) it pays to study up a bit before diving in- it's more than just another plumbing job- it's a bigger (and more sensitive, since it's modulating) burner than most people would have for their space heating furnaces & boilers.

Thanks so much for your reply. Oddly, because of the size of the fittings at the regulators I don't know that it would do any good to go to 3/4" pipe but sounds like a best scenario would be to make the 1st run of about 10 ft with 3/4 then tee off to 1/2" for the small space heaters. If it were natural gas I'd go 3/4" all the way to the shut off valve & supply lines.
Another question... is black iron pipe acceptable or galvanized or just go on with the soft copper? All the old houses that have gas lines that I'm familiar with used black iron (steel) pipe but I see many with copper nowadays. Times have changed a lot...and I'd really like for this to be acceptable to any code.

Black iron, even if you paint, wrap, or treat it, will eventually rust. Copper used to have problems reacting with some gas, but I do not think that's an issue anymore. You may want to ask the supplier how many BTU the regulator on the tank is designed to provide...it may not be big enough on its own. It may be marked. One large enough to provide all of your appliances at full tilt may require a bigger one and it likely would have a larger fitting on it.

The code that it has to be acceptable to is Alabama code, which on a state level follows the 2009 International Fuel Gas Code, which IIRC calls out galvanized or black pipe or stainless for distribution within the house and bars use of cast-iron pipe, but I'm not sure if it continues the prior ban on using copper gas lines for the interior distribution- it might, and I'm sure even if it's allowed there will be juridictions where the prior ban continued as an amendment. I've personally NEVER seen copper used as a natural gas or propane line, but there are lots of things I haven't seen. Apparently it's becoming common, see A.D.Miller's comments on this thread. It sounds like the source of the fuel (which determines it's contaminants) determines whether copper can be used. No matter what the source it black iron or galvanized works.

Stubbing off a tee to another substantial load 10' from the regulator and dropping back to 1/2" isn't really great. There can still be pressure interactions with the modulating controls on the tankless if you do. (With natural gas you wouldn't really have much option- it's a lower-BTU gas than propane, and may even have to up-size to 1" to make it behave.)

Thanks Dana for all the good, helpful info. I've dealt with lots of old river and lake cabins and mobile homes or campers and the copper tubing is quite common. Tho', I think that in a mobile application I would stay away from it. I've swapped out many gas stoves and water heaters where at least the supply lines were nothing more than copper tubing. I've also seen buildings blown up by leaking gas lines... so I am extremely cautious when messing with gas of any kind. I've been seriously thinking about studying up and taking the test to get licensed for gas. Might be a very good time to get the books and start studying. I've hardly ever even had a call to work on gas for other people so have not concerned myself with it much.

quote; and bars use of cast-iron pipe

Have you EVER seen "threaded, or any other type of cast iron pipe" used for ANYTHING other than the occassional "Durham" waste and vent system? Copper gas systems must either have flared connections or be brazed. There is not, and never has been, any "reaction" between LP gas and copper tubing, which is why it is almost always used between the tank and the building.

hj said:

quote; and bars use of cast-iron pipe

Have you EVER seen "threaded, or any other type of cast iron pipe" used for ANYTHING other than the occassional "Durham" waste and vent system? Copper gas systems must either have flared connections or be brazed. There is not, and never has been, any "reaction" between LP gas and copper tubing, which is why it is almost always used between the tank and the building.

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I agree on the cast iron pipe. Black Iron was a common name used by lots of old plumbers for plain old steel pipe that was not galvanized. However, on the LP gas issue concerning reaction with copper... I did read something about that on the internet. I too have always used flared connections on copper for LP & Natural gas supply lines. I never really understood why I've seen lots of steel pipe in the ground but now it seems that copper is required. In my home town the public gas dept "ballooned" or sleeved the inside of all the old steel gas lines...