Low propane pressure issue with new install, 96% 2 stage 60k btu furnace
I’m doing a startup on a new propane furnace install and I’m getting a major inlet pressure drop when the furnace fires.
The LP conversion kit for the furnace calls for around 11” WC manifold pressure on high fire and 5” WC on low fire.
What I’m seeing:
Propane tank was recently filled.
Brand-new two-stage regulator installed at the tank.
About 75–100 ft total run from the tank to the furnace.
Gas line is 3/4” galvanized pipe.
Roughly 50 ft of the line is underground from the tank toward the house.
Static pressure at the furnace looks normal.
As soon as the furnace fires, inlet pressure at the furnace drops to about 3.5” WC.
The low gas pressure switch then opens and shuts the gas valve off.
If I check pressure directly at the outlet of the propane regulator while the furnace is firing, it only drops to around 11” WC.
So it seems like I have roughly 11” WC leaving the regulator under load, but only about 3.5” WC reaching the furnace.
I’m not a propane expert, but 3/4” pipe seems like it should be large enough for a roughly 60k BTU furnace over this distance.
Does this sound like a restriction somewhere in the underground/house gas piping rather than a regulator sizing issue? Could a partially blocked pipe, valve, fitting, debris, or damaged underground section cause this much pressure drop?
What would you guys check next?