How much gas line do I need? House natural gas content

When an electrician installs a 20A breaker, he is saying, "Everything I have installed downstream of this can safely handle a load of 20A".  The wiring, the devices, the connections.  It is safe to plug in an appliance that pulls a maximum of 20A (actually 1920 watts because electrical circuits have an 80% safety margin).  It DOES NOT mean the circuit has a 20A load on it. 

You are essentially saying "Don't worry.  I will never use that appliance at full speed.  So I'm gonna undersize the piping".

That doesn't work.  

Ok.  Math time.

If I add up all the possible BTUs of all my proposed gas appliances (5-burner gas range, tankless 4.5 gpm water heater, furnace) I get just shy of 400,000 BTUs.  That is everything running wide open.

Assuming a nice round number of 1000 btu per cubic foot of gas, that suggests that I need 400 cfm.  That suggests 1" territory.  But the furnace is only around 10' from the meter, and the other appliances (stove, water heater) would be another 20' past that. with a couple 90 degree bends.

I can figure out the flow, just don't know how to figure out how many BTUs is an actual, logical demand number.  It will never use 400kBTUs at one time.

Good job!  You did the math.

Sounds like you need 1". 

SVreX said:

In reply to Curtis :

Curtis said:

  If I add up the possible amperage of all the circuits in my house, I get 440A, but it is sized so that the 200 is more than enough to supply the things I would use at one time. 

I'm willing to bet you are wrong.

You don't add the BREAKERS.  You add the LOAD.  Just because you have a 20A circuit does not mean your vacuum cleaner uses 20A.  Go through your house, read the motor load plates on every device.  I guarantee you do not have 440A of load in that house.  Total of combined branch circuits can exceed the capacity of the main breaker, as long as the load does not.

But you are trying to de-rate the load.  If your furnace says it can burn 50,000 BTUs, that's how much you need to pipe for.  It's not a vacuum cleaner.

This is my exact point.  Of course, I don't have 440A of load.  My analogy was meant to describe how the supply of electrical current is not based on the peak of every possible load in the panel.  You give it 200A knowing that you'll never need more than 200A at any given time.  The panel is supplied with a potential for 200A based on the conductor size and transformer capacity.  From there, it could split to thirty 20A-capable circuits.  The 200A is supplied as "enough for what you'll need at any given time."

Same goes for the potential flow of water through the 1/2" lines.  1/2" isn't enough to run every faucet wide open, but it is enough for the shower and the kitchen sink simutaneously.

Why aren't  gas "loads" the same way?

Thanks for making this thread. It's putting some house shopping things into perspective for me. One of our favorites has gas, gas boiler and water heater in the basement, big furnace in the shop, but no berkeleying gas for an oven. I thought I could tap off the boiler or hot water tank, but it sounds like I'd need a professional. I'm glad I learned this sooner rather than later.

In reply to Curtis :

They ARE the same.

Check your electrical motor loads.  The breakers and wiring are sized to handle the maximum load they draw.

Same thing with your gas piping.  Check your equipment load plates.  The meter and piping is sized to handle the maximum load they will draw.

... and water piping is engineered the same way.  Maximum potential flow rate at every fixture with them all turned on at the same time.  (although this is rarely calculated in residential applications)

BTW...

The 20A receptacle branch circuit was a bad example.  Receptacle circuits are not known variables.

Motors, heating coils,  on-demand water heaters, furnaces, and stoves are all all KNOWN quantities.  Size piping (and wiring) for the known equipment.

If you undersize a gas line, it can cause incomplete combustion.  It can lead to damage to the burners or chambers of the appliances, potential premature equipment failure, and even the possibility of carbon monoxide buildup. 

RevRico said:

Thanks for making this thread. It's putting some house shopping things into perspective for me. One of our favorites has gas, gas boiler and water heater in the basement, big furnace in the shop, but no berkeleying gas for an oven. I thought I could tap off the boiler or hot water tank, but it sounds like I'd need a professional. I'm glad I learned this sooner rather than later.

It may not be that bad. We had a gas line run to the kitchen for a stove in our last place. The geography of the place helped - but if the kitchen is above or near the boiler, it's not that difficult a job. We didn't have to make any other changes. It wasn't very expensive to have a pro do the job and I never had to worry about it exploding.

I know your knowledge set is fantastic in this realm so I'll just have to take your word for it.  I just don't understand it and it frustrates me.

In my house

- a single 3/4" water line feeds a total of nine 1/2" lines and that's normal because no one will use all 9 at once
- a single 200A breaker can feed a potential of thirty 20A breakers and that's normal because no one will use every outlet at 20A at once.
- but a gas line has to be sized to accommodate the absolute max BTUs even though they will never be used all at once.

I understand the load plates and the actual amperage load, I just don't understand why you keep saying they're the same when they clearly aren't.  If they were the same logic, I would need a 2" water line and 500A service. 

Your largest gas loads are not manually turned on and off, they are run by thermostat. Ie, furnace, water heater, and stove. You set your oven to 425 and leave it to warm up. Your furnace kicks on putting out heat. The water heater is running. Then someone turns on a burner to boil some water. This is not an uncommon situation, and the only load you're aware of is the stove and oven.

It's not realistic to think that someone will turn on every electric appliance in the house. And if they did, the breakers would just blow. Not only is it realistic to hit max BTU demand on gas, there are no breakers on the line and the consequences of an over-demand are death. So of course the code is conservative. For it to be otherwise would be lunacy. Nobody would be OK with being told, "If the furnace and water heater are running, make sure not to turn on both the stovetop and oven or you might die" when buying a new house.

Death.  Got it.  I would like to avoid that if possible.

So maybe my analogy is more appropriately allocated to discussing the actual flow demands needed to properly size a tankless water heater.  Right now, its just me and one bathroom.  I can imagine running the dishwasher and the shower together.  Again, future occupants might be a family of four, but this is really a 2-3 person house at best.  I don't feel bad sizing a heater for shower, a sink, and a dishwasher... 4-4.5 gpm.  For my ground water temps in PA, I'm thinking a Delta of 55 degrees.  Does that sound like a 150kBTU unit to you?  Or am I way off with my gpm needs?

Curtis I still think you're missing the point.

Water is a bad analogy. It's under much higher pressure than gas. So flow charts would be much different. One 3/4 line may be plenty to supply the whole house running at once. Water is a bad analogy (or maybe it's not if you find a similar chart for water and do the math).

Breakers are a bad analogy because each one is a capacity, not a load. In most houses, I don't know of any devices that shut off gas flow if the cfm/btu is too large. Maybe the meters do that. Maybe.

Also, if you had 40+ gas receptacles in your house, like you do electric, then things might be different. But you don't.

HVAC duct sizes might be a better analogy. You can't just heat some of your house because you're not in all the rooms at once.

Sorry for sounding like a dick. 

I don't have a good answer for tankless sizing. I was just trying to point out the differences between electrical breakers and gas pipe size.

You didn't sound like a dick.  I appreciate the clarity

I like understanding things and sometimes it takes blunt force.

Robbie said:

Breakers are a bad analogy because each one is a capacity, not a load. In most houses, I don't know of any devices that shut off gas flow if the cfm/btu is too large. Maybe the meters do that. Maybe.

 

Again which is why I was confused.  With electrical, you supply enough amperage to cover the load and each circuit has a capacity.  I assumed with a gas appliance that it was the same way.... you supply enough gas to the house for how much you actually use, not the total capacity of all the branches' capacity combined.  Evidently that isn't the case.

If you supply 200A and all circuits combined have a capacity for 400A, it's not as if you are GOING to or even COULD use all of the capacity.  I assumed that gas was the same way when SVRex made that initial analogy.  I assumed it was a calculation of total BTU potential times 0.6 for one person, or 0.8 for two people, times efficiency, divided by cfm, then look at a chart for pipe size.  Evidently it's just total BTUs period, which now makes sense to me.

You are picturing this backwards.

You are imagining a source, like a large bucket full of electricity or gas which is pouring enough stuff into a pipe so whatever you've got at the other end gets what it needs.

Reverse it.

Picture a big device at the far end demanding a certain amount of juice to be poured into it, and PULLING what it wants through the lines.  You are not dumping energy off the energy stockpile.  You are installing an appliance that demands a certain amount, and your job is to build the pipeline big enough to meet the demand.  

In theatrical terms, imagine Audrey II (from "Little Shop of Horrors"). 

You are also comparing two different things.

You are comparing your 200A main breaker to the pipes, and assuming you can limit stuff because you won't need it.

Compare the pipes to the wires.  You ALWAYS over-rate the size of the wires.  Doing the opposite could risk catastrophe.  The 200A main can only be compared to the gas meter.  The gas meter adjusts for usage (like the 200A main).  NOT the pipe size.

You wouldn't wire a 1000W theatrical follow spot with lamp cord, just because you intended to use it very dim. 

You are also confusing yourself with 20A circuits.  Forget them.  Those are service circuits, and their load varies from zero to their full capacity.  There is no such thing as a "service outlet circuit" for gas.  You don't have random gas appliances with plugs you stick in the wall.  

Consider instead the larger electric appliances.  Stoves, heaters, AC units, etc.  ALL of them are wired according to their loads.  NONE of them have wiring that is too small for their maximum rating.  Same thing with your gas piping.  It needs to be sized according to the load of the appliances.

SVreX said:

You are picturing this backwards.

You are imagining a source, like a large bucket full of electricity or gas which is pouring enough stuff into a pipe so whatever you've got at the other end gets what it needs.

Reverse it.

Picture a big device at the far end demanding a certain amount of juice to be poured into it, and PULLING what it wants through the lines.  You are not dumping energy off the energy stockpile.  You are installing an appliance that demands a certain amount, and your job is to build the pipeline big enough to meet the demand.  

In theatrical terms, imagine Audrey II (from "Little Shop of Horrors"). 

So here was my brain after reading that clarification.....

I berkeleying get it now.

and now I'm like:

Happy to be of service. 

You rock.  Thanks all.

I called for an estimate on getting one installed, mostly because they can tell me right away.  I may still install it myself, but at least I'll have a pro telling me if it can or can't be done on current equipment.