Gas line pressure drop

[Troutfish]

I'm a general contractor having an issue with a furnace in a newly built house. I have two furnaces in this 3500 sf two story home with one in the attic and the other in the basement. The furnace in the basement does not work yet the 2nd floor furnace does. I called the HVAC contractor out and he's replaced the valve inside the furnace 3 times with no luck. He's telling me the pressure in the line after his valve drops from 6.5 to 0.2 inches of water and he needs 4.0 to ignite the flame. He's gotten the same reading with all 3 valves inside the furnace. I've had the gas service company out to check the pressure at the meter and everything checks out fine. All the other gas appliances in the house work fine.

Everyone is pointing to the gas piping as the culprit. This furnace is about 15 ft from the gas meter and is the first appliance receiving gas. My plumber ran 1 1/2" service into the house and branched off that line with 1/2" hard pipe to service this furnace. So, there's about 7 ft of 1 1/2" reduced down to 8ft of 1/2" pipe running to this furnace.

Our question is this, does this sizing lead to inadequate pressure to the furnace? It seems oversized based on past experience but the other appliances further down the line all work fine. The plumber says it's not his issue so I'm at a loss here with cold weather bearing down on us.

 

[Reach4]

I am not a pro.

Can't the pressure be measured just before the furnace, such as at the drip leg? That would seem to be a good demarcation for determining whether the problem is the piping or the furnace.

While lighting the pilot light, there is very little gas trying to be used. With the burners firing, the amount of gas being used will depend on the furnace.


http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html "120" in the table would be 120 BTU/hour for 1/2 inch of pressure drop.

 

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[Dj2]

Have you checked the branch gas pipes for leaks? pressure test?

 

[Plumber69]

I'm a general contractor having an issue with a furnace in a newly built house. I have two furnaces in this 3500 sf two story home with one in the attic and the other in the basement. The furnace in the basement does not work yet the 2nd floor furnace does. I called the HVAC contractor out and he's replaced the valve inside the furnace 3 times with no luck. He's telling me the pressure in the line after his valve drops from 6.5 to 0.2 inches of water and he needs 4.0 to ignite the flame. He's gotten the same reading with all 3 valves inside the furnace. I've had the gas service company out to check the pressure at the meter and everything checks out fine. All the other gas appliances in the house work fine.

Everyone is pointing to the gas piping as the culprit. This furnace is about 15 ft from the gas meter and is the first appliance receiving gas. My plumber ran 1 1/2" service into the house and branched off that line with 1/2" hard pipe to service this furnace. So, there's about 7 ft of 1 1/2" reduced down to 8ft of 1/2" pipe running to this furnace.

Our question is this, does this sizing lead to inadequate pressure to the furnace? It seems oversized based on past experience but the other appliances further down the line all work fine. The plumber says it's not his issue so I'm at a loss here with cold weather bearing down on us.

I came across this once. Sometimes blk pipe is shipped with a plastic plug. There was a chunk of plastic in the pipe installed restricting the gas flow

 

[MACPLUMB]

The 1/2" "MAY" be undersized for the amount of BTU'S the furnace needs to operate

 

[FullySprinklered]

Plumber69 may be on the right track. Longer nipples come with a red cap to protect the threads from damage. Spotted one on a job just a few weeks ago that had the end of the cap missing. Looked in the end of the pipe and there was the rest of the cap jammed inside the pipe.

 

[Bgard]

Is there any trapped areas in the branch? I have seen water get in the pipe during construction, the pipe was only partially full of water which would allow the pressure to be read as good, but when the main burner would open the pressure would drop to zero as the flow of gas would cause the water in the pipe to “pile up” and stop the gas flow, when the gas flow stops the water levels out and you show good pressure again. I’ve also seen a similar thing with very fine sand or debris, the debris gets blown up against the inlet screen in the appliance gas valve when the gas flow starts and blocks the screen and stops the flow, when the valve closes the debris falls away from the screen and you once again show pressure at the valve.

 

[MACPLUMB]

Also seen this happen with a spider in the gas line, they spin a web inside pipe

 

[Plumber69]

This guy posted everywhere on the net. He got a shifty hvac guy that's blaming the gas company. I highly dought it's the gas company reg.

 

[Plumber69]

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