Correction required for this article: Please read italics below.
Making progress on the house and about ready to install new insulation, so I decided to see if we could get the gas pipes moved out of the way of new insulation. We are trying to get most of the wiring and piping out of the insulated spaces. You would think this would be a relatively easy task. I may just pick the wrong plumbing help, but the guy told me the entire system was grossly undersized and should be replaced. I guess I was just born yesterday, because although I didn’t take his word for the necessary sizing of the pipes, I took his word for the size of the pipe we have! He said it was 1″ pipe. But black iron is measured by the outside dimension and we have 1 1/4 pipe! That means the existing pipe is fine for the BTU’s we need it to deliver. Harumph.
What a difference a 1/4″ makes. We have about 80 lineal ft. to the boiler, which would be able to carry up to 480,000 BTU/hr. but if the pipe was only 1″, we would be limited to 220,000. Similarly, we have a pipe that goes outside to the old spa area for a gas spa heater. That distance is about 165′ so we would use the 175′ figures for a total of 300,000 BTU/hr.
Gas Pipe Sizing
I’m not sure how decreasing and increasing the pipe size impacts the distance and BTU’s. The pipe decreases to 1″ near the gas dryer hookup and increases again going to the outdoor pipe.
Decrease in main trunk pipe size to 1″
Increase in pipe size back to 1 1/4″
It seems that to carry the total BTU’s the trunk pipe must maintain the minimum diameter for the total BTU’s in the run. The branch lengths are not included in the trunk run figures, but the total BTU’s of the appliances they serve are.
In this example there are five appliances for a total BTU rating of 310,000 BTU/hr. The run to the furnace is 59′ and carries up to 270,000 BTU/hr. A 1″ pipe only carries 260,000 so a 1 1/4″ pipe is needed, according to the chart. To add the grill, figure the 30′ to the grill as a branch line. For a 30 ft. length to carry the 40,000 BTU/hr, a 1/2″ line would work and the total run will hold 460,000 BTU/hr. more than enough for all appliances on this line.
The gas run for our house could include the following appliances, distances and BTU/hr.
Our gas run
So we are not grossly undersized for the common appliances within our distances, but would be with 1″ pipe as the service guy thought. However if we were to run the extra distance to the outdoor heater, we might run into problems. A spa heater is about 100,000 BTU/hr or so and we would be limited in the entire line to 300,000.
That 1/4″ represents a pretty large difference in capacity. I just have to find someone who is willing to still work with black iron pipe–the bid I received was to replace all the pipe with flex, the new stainless steel (CSST) stuff but we should be able to keep black iron for most of our runs.
Months later when we DID get the pipe replaced, the Gas Connection representative also identified the main pipe as 1″. (Black pipe is measured by its “nominal” dimension which is neither the inside nor outside diameter.) Although in his opinion it was not grossly undersized, using the existing pipe would be too likely to leak and not pass inspection requiring costly leak locating and repair. The cost to replace the old pipe with a larger outside line directly to the utility room would be about the same as adding more pipe to the old and less difficult for them. So what I found about measuring black pipe was incorrect, it is not measured by outside dimension. The btu delivery information is accurate though and the Gas Connection used some of my btu figures when they computed the requirements for the job.