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Discussion Starter · #1 ·
Hello,

I would like to add a natural gas firepit to my backyard. I have designed the physical pit itself, which will be made out of concrete building blocks. I started researching the natural gas component and could not believe the complexity and cost.

My backyard currently has two natural gas lines in it. A 3/4" line that runs for 40 feet or so from the meter to a gas BBQ that consumes up to 48,000 BTU/hr. The 3/4" line is reduced down to 1/2" for the BBQ, which requires a regulator outlet pressure of 4 iwc. The second line is a much larger line, estimate 1.5 or 1.75" diameter that was installed when we bought the house new. It was meant to be used for a future swimming pool that we never installed, so it is unused.

I want to build the fire pit 22 feet away from the BBQ. I had one licensed plumber come out and received an estimate that was way over expectations. (I know this forum discourages how much does it cost, questions. I am listing the price here to give perspective. I plan to get bids from one or two other plumbers). The total cost to dig a 22-foot long trench and install a 3/4" line off from the BBQ with a permit was $4,100. If I dig the trench myself, the cost drops to about $2K. I can hire someone to dig the trench for about $500 so the total cost would be $2,500. This plumber did not recommend going off the pool line, he felt the 3/4 inch BBQ line was more than sufficient.

When I approached a second plumber they were concerned that the 3/4 inch line would not be big enough. The firepit burner I want to use has a rating of 297,000 BTU's per hour! I cannot believe how much gas it uses. I have no idea if that rating is for having flames 1-2 feet tall or just a warm glow. The firepit opening is a 36" square and I was going to use a round 24" stainless steel burner with two rings.

My main question is can I connect the firepit to the BBQ line? If we run a 3/4" line straight from the BBQ to the pit, would I be able to use both appliances at the same time? Or is 350K BTU/hr too much for a 3/4" line?

My second question is price related - but is NOT asking for bids. I plan to get prices from one or two other plumbers. If it is going to cost me $2,500+ just to get natural gas to the firepit, we either need to switch to propane or scrap the firepit entirely. I cringe at the idea of using propane. I was told I cannot have a burner over 18" with a standard 20-pound propane tank and that one tank would last for maybe 3 hours. Is this true? If I went with propane I would design the pit with two 12" rings each with their own 20-pound tank side-by-side like a figure 8. If a 20-pound tank would give me 8-10+ hours of fire time, this would be feasible. If I would only get a few hours of use, it is impractical due to the hassle factor. How many hours can I expect from a 20-pound propane tank connected to a 12-inch round burner set on high?

I attached a rough (not to scale) drawing of the layout of my yard in case it helps.

Thank you!!!

Backyard Firepit Schematic.jpg
 

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Discussion Starter · #3 ·
Wow Joey that reply was unbelievably fast!! Am I reading the chart correctly that the maximum total rating for a 3/4" natural gas pipe is 230,000 BTU/hr? And that is only if the total length including any splits is only 10 feet? If that is true, connecting to the 3/4" line would not work. I can measure the distance from the meter to be more accurate, but I would give a ballpark estimate of 60-80 feet of total run if I use the BBQ line. Does that mean with the BBQ off my firepit will only get about 80,000 BTU/hr?

Is the pipe size inner diameter? I estimate the outer diameter of the pool pipe is close to 2". Should I assume the ID is 1.5"? If it is 1.5", I would have enough gas as long as the total length of the pool line with firepit branch is 200 feet or less, which it is.
 

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I'm not a plumber, but I believe gas pipe sizes are ID.

Does the burner have a BTU range for high and low? 300k BTU is a lot, but if it's a large burner area than maybe not. It's difficult to say whether or not you can just not turn it all the way up. I would imagine (and again, I'm not a plumber...) that there's a code against connecting devices rated at a certain BTU to a gas line that's too small.

It's worth noting that your fire pit burner is pulling more BTUs then a lot of swimming pool heaters (unless you were planning on a huge pool). It might be worthwhile to see about downgrading the output of your burner, and utilizing that pool line.

Also, whether or not you can use a 20lb tank depends on the BTUs of the burner. I don't remember the cutoff, but basically at some point the burner will try and pull gas from the LP tank quicker than the tank can output, requiring a larger tank.
 

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Discussion Starter · #5 ·
Joey, I have a few more energy-related questions because now I am wondering how much this firepit is going to cost to run! Would you please tell me if the following calculations are correct? Hope I am not asking too much....

Last month I paid $25 to the local gas company for 14 therms. Per the bill, 14 therms are 1,400,000 BTUs. Let's assume the firepit burner uses 300,000 BTUs per hour. Does that mean that each hour I use the firepit would require 3 therms of gas at a cost of roughly $5.50?

When a burner has a rating of 300,000 BTUs/hour, isn't that how much it uses when you have the valve open 100%? I wonder if that would result in foot high flames? If we operated the pit half-open, would the natural gas cost drop roughly in half?

I read that a full 20-pound propane tank holds 4.73 gallons of propane that has 433,000 total BTUs at 60 degrees (which is a typical temp for San Diego). Ignoring the fact that a propane tank will not work with any burner over 18", a full propane tank would run my 300,000 BTU/hr burner for almost 1.5 hours. It costs me about $10 to fill up a tank at Costco, so propane would cost about $7 an hour.

So now I tried to calculate how many hours I would get from a 20-pound tank attached to a 12" round fire pit ring. I found one ring online with a BTU rating of 90,000. Am I correct that I would get 433/90 or almost 5 hours of use at max flames from one tank? Propane would no doubt be a hassle, but if I had a total of 6 of these tanks on hand (keep extra ones stored in a shed outside) would I get almost 15 hours of use before needing to get the tanks refilled at Costco? If using propane saves me $2,500+, then this may be a viable alternative.

THANKS SO MUCH FOR CHECKING MY MATH!!!
 

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That math seems correct.

Keep in mind you can use a larger tank, just can't swap them out at every gas station. If Costco is refilling and not just exchanging they shouldn't have a problem with a larger tank. Get a 30, 40 or even 100 pound tank if you're able to safely transport it.
 

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Your 25 dollar bill for 14 therms. Also included a lot of extra charges. Probably even a fixed fee for the meter, and or gas line ran to the meter.

While in theory a 20 pound LP bottle could run your fire pit for 1.5 hours. In relity, it couldn't due it, due to not enough surface area to keep the gas pressure up.
 
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Discussion Starter · #8 ·
Beenthere you are correct! It costs $1.06 per therm up to the first 49 therms, then $1.25 per therm after that. So a 300K burner will cost about $3.18 per hour or half the price of propane.
 

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Discussion Starter · #9 ·
That math seems correct.

Keep in mind you can use a larger tank, just can't swap them out at every gas station. If Costco is refilling and not just exchanging they shouldn't have a problem with a larger tank. Get a 30, 40 or even 100 pound tank if you're able to safely transport it.
Yes, Costco refills tanks - it is not an exchange program. I have a pretty good dolly (also from Costco, so I could manage a 40 pound tank. Just need to see what the dimensions are. Costco sells new 20 pound tanks for $40 and that includes your first fill up.
 

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Excellent discussion.

My suggestion will be to use smaller fire pit ring with high BTU rating. That will create a 2 ft high flame that is very good for throwing heat sideways. Otherwise most of the heat rises and gets lost.



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