Sizing LP pipe for BTU load

ArTrvlr · Mar 9, 2013

I am preparing to have a standby generator installed. I am going to install the LP pipe from an existing 500G tank to the generator pad. The route will pass by the existing heat pump, so I am going to stub up beside it in case I ever decide to convert to gas heat. I am trying to make sure I use a big enough pipe to handle the total anticipated load. I have already talked to two electrical companies to get estimates on the generator install, and neither of them could give me a definite answer on the proper size pipe to supply the generator. So, I asked my LP supplier, and they were even less help. I had a gas pipe sizing table from a gas pipe manufacturer with me when I went to the LP store. Amazingly, the guy said "I always had trouble reading those charts".

OK, I do too, but I am not in the gas business! Normally, questions like this on forums get replies of "Ask your gas company.", or "Ask the guys doing the install.", but I have already BTDT, no luck.

My real question is, am I reading the pipe sizing table correctly? Here is a link to one of many sizing charts I have found. The chart I am using is on page 7, the last chart on the page.

I am using a two stage system, with a first stage regulator (10psi) at the tank, and a second stage regulator (11" wc) will be installed at each load. It is 113' from tank to where the heater will be, and another 98' to the generator. I am using the 200' column in the table. I will be using yellow polyethylene gas pipe. It is commonly available in 1/2", 3/4" and 1" sizes. The way I read the 200' column, 1/2" will only supply 297,000 BTU/hr., (not enough for my needs) but 3/4" will supply 1,707,000 BTU/hr. (more than enough for my needs). I am basing "my needs" on a max of 340,000 BTU/hr. for a 20Kw generator and 100,000 BTU/hr. for heating. The heating figure is just a WAG, it doesn't need to be exact, and is probably generous.

Again, my question is, am I reading the tables correctly? In the table in the PDF linked to above I multiplied the numbers X 1,000. I just need someone familiar with LP pipe sizing to confirm or dispute my results.

Thanks in advance, jp

General Lee · Mar 9, 2013

Ugh, I'm not sure if I read the chart correctly either. I think your correct. Just for reference, My 20Kw uses a 5/8 inch line but its only 10-12 feet from the the LP tank.

forgeblast · Mar 10, 2013

I was told to run 3/4 black pipe. And then use fitting to attach to the appliance.

Bikewanderer · Mar 10, 2013

You'll be fine with 3/4" as long as you're not actually putting LP in the pipe, and vapor only.

Tractor Seabee · Mar 10, 2013

You are reading it right. The table is pretty clear to understand. If I was doing it I would use 1". The small difference in cost is good insurance when you are guessing at the furnace. Then you may decide to install a large outdoor barbecue, put gas for range, dryer and WH inside the house. Cheaper now than digging it all up to incerease latter. What is the cost of power down there? Either the gas company or electric company will have a BTU/BTU comparison chart and you can calculate which is cheaper. Some gas company's up her guarantee a fixed price for the year if you pay a year ahead for auto fill and no tank lease cost. Those other appliances may be cheaper to operate on gas.
Ron

MotorSeven · Mar 10, 2013

I went round & round with my gas company on this when I did my install. My instant hot water Heater calls for a 3/4" feed line. So I bought 3/4 copper line and ran it from the tank to the house/HWH. Gas company shows up to connect everything and says they can't hook it up because......they carry ONLY 1/2 inch fittings

So I get on the phone with the gas guy who says they have never installed a line from a tank over 1/2" even to commercial businesses that run multiple high use appliances. He went on to say that the line leaving the second regulator is 1/2" so running anything bigger between it and the first regulator at the tank is a waste of money. I still think the guy is wrong, but I caved since I wanted to be able to COOK, so I exchanged the 3/4 for 1/2....everything runs fine even when I have the HWH on, cooktop & 39K btu fireplace all running at the same time.

ArTrvlr · Mar 10, 2013

Tractor Seabee said:
You are reading it right. The table is pretty clear to understand. If I was doing it I would use 1". The small difference in cost is good insurance when you are guessing at the furnace. Then you may decide to install a large outdoor barbecue, put gas for range, dryer and WH inside the house. Cheaper now than digging it all up to incerease latter. What is the cost of power down there? Either the gas company or electric company will have a BTU/BTU comparison chart and you can calculate which is cheaper. Some gas company's up her guarantee a fixed price for the year if you pay a year ahead for auto fill and no tank lease cost. Those other appliances may be cheaper to operate on gas.
Ron

Thanks for a good answer. I first thought the table was pretty clear as well, but the local gas dealer not being able to read or understand it caused my self-doubts. Thanks for the confirmation.

I considered just going with 1", but it is more than a small difference in cost. Online pricing (zorotools.com) is approx. $230 difference. I can get 3/4" (I haven't priced 1") PE pipe and fittings locally much cheaper ($243 locally vs $600 at zorotools), but the local fittings are fusion style, so I would have to add the cost of a plumber making the joints. The $357 difference in materials should be more than enough to pay the plumber just to make the joints, as I am going to do the trenching myself. I just got the names of some plumbers with fusion splicers late Fri., so haven't had a chance to get quotes on splicing.

As for adding more gas appliances later, the most likely candidate is a tank-less water heater. That could be as much as 200,000 BTU/Hr, but most likely less. Even at 200K for tank-less water heater, 100K for central heat, and 340K for the generator (all generous estimates to give some cushion), I am still well under the 3/4" max of 1.7 million BTU/Hr.

A gas range is out, my cook and LOML wouldn't go for it. And I don't mess with the cook! She has a new ceramic top range that she likes, so I would never even suggest a change. Gas dryer is out because I don't think they are safe. A grill could be added, but still wouldn't put me over the 1.7 million mark. But, I will still consider 1". One inch IPS PE delivers 3.1 million BTU/Hr at 200 feet.I will compare the prices locally, and also get advice from the plumbers when I talk to them.

Thanks for the suggestions, jp

ArTrvlr · Mar 11, 2013

MotorSeven said:
He went on to say that the line leaving the second regulator is 1/2" so running anything bigger between it and the first regulator at the tank is a waste of money.

I think your gas man is related to mine:laughing:, maybe even the same guy! He is not considering distance, which is a major factor, just look at any gas pipe sizing chart. Also, second stage regulators are available with 3/4" connections.

kenmac · Mar 11, 2013

Your right on with the size. I use these 2 PUSH ON GAS RISERS MPT X CTS WITH PE COUPLING 1 INCH on eBay! when comming out of the ground ( they come in different sizes)when using this type of gas pipe.You'll need 1 at the generator with a gas stop ( shut off valve ) and 1 at the propane tank with gas stop.Be sure you install a tracer wire with the plastic pipe so, you (or someone ) will be able to locate it in the future

Tractor Seabee · Mar 11, 2013

All PE pipe is either welded or uses several different types push-on/compression fittings. Regular installers have the welding unit. It works even for tees. I would not use any fittings underground except at tee locations. Put the next size pipe over the riser and come above grade. LP is heavier that air so you do not want a leak to accumulate in the soil where you do not know exactly where it is. With the vent arrangement you will smell it. Think of the big boom if you drop a match where it has been absorbed into the ground for a month. Buy all your pieces long enough so no splices underground. I would also put a fence post along side that vent to protect the plastic from mechanical damage. Paint it red.

Ron