A gas heater only delivers its rated BTU if the gas line delivers the rated fuel. This is the number every tech forgets to verify until the homeowner complains the heater “doesn't heat like it used to.” It did heat like it used to — the gas line has been starving it all along.
The two fuels and their realities
- Natural gas (NG)— low pressure (7–14 inches of water column typical at the heater). Bigger pipe needed for the same BTU because the gas itself is lower energy density (~1,000 BTU/ft³).
- Propane (LP)— higher pressure at the regulator, higher energy density (~2,500 BTU/ft³). Smaller pipe can carry the same BTU, but the second-stage regulator sizing matters as much as the pipe.
How to size the line (the short version)
- Find the heater's input BTU/hron the nameplate. Not output — input.
- Measure the longest runfrom the meter (NG) or second-stage regulator (LP) to the heater. Include the fitting equivalents; two elbows add ~2–3 feet of equivalent length.
- Add up all applianceson the same branch — house furnace, water heater, range, dryer — if they share the run. Gas lines are sized for simultaneous demand.
- Look up the pipe size in the appropriate sizing table (NFPA 54 for NG, NFPA 58 for LP, or the local plumbing code amendment).
Residential ballpark (natural gas, 7" WC)
| Heater input | Pipe size (50 ft run) | Pipe size (100 ft run) |
|---|---|---|
| 150,000 BTU/hr | 3/4" | 1" |
| 250,000 BTU/hr | 1" | 1 1/4" |
| 400,000 BTU/hr | 1 1/4" | 1 1/2" |
These are illustrative. Always confirm with the current code table and with a manometer testat the heater inlet under full fire. Static pressure is meaningless; the number that matters is what the heater sees when it's burning.
Field test: the manometer reading
- Isolate the heater. Attach a manometer to the gas-valve inlet test port.
- Fire the heater at full call-for-heat.
- Fire every other gas appliance in the house simultaneously — worst-case demand.
- Read inlet pressure. If it's below the manufacturer's minimum (commonly 5" WC for NG heaters), the line or the meter is undersized. Call the gas utility before touching the heater further.
Regulators and the propane two-stage rule
Propane installations always use two-stage regulation: a first-stage regulator at the tank drops tank pressure to about 10 psi; a second-stage near the appliance drops that to 11" WC. A single-stage system might work for months and then fail on the first cold morning when tank pressure dips. If the heater is more than about 100 feet from the tank, a two-stage system isn't optional — it's necessary.
If a heater is running rich, sooting, or randomly locking out on a slow heat rise, manometer the inlet before you condemn any part on the heater. Eight out of ten of those calls end at the gas line, not the heater.