Adding a Garage Heater to an Existing 1/2-Inch Gas Line
An existing 1/2 in. Schedule 40 black iron gas line supplies a 100,000 BTU/h furnace and a 40,000 BTU/h water heater. The developed length from the meter to the farthest appliance is 45 ft with four elbows and one tee. The meter outlet pressure is 7 in. w.c., and the furnace manifold is set to 3.5 in. w.c. The owner wants to add a 50,000 BTU/h garage heater teed off the line after the meter. A 10 psig air pressure test on the existing line drops 0.4 psig in 30 minutes. The technician suggests leaving the 1/2 in. line, raising manifold pressure to 4.0 in. w.c., and running 1/2 in. CSST to the garage heater. The local code requires a sediment trap and a shutoff valve at each appliance and prohibits ignition sources within 18 in. of a garage floor unless the appliance is listed for that location.
1. Determine whether the existing 1/2 in. line can supply the added load. State the size you would install and your basis.
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New total load is 100,000 + 40,000 + 50,000 = 190,000 BTU/h. The longest developed length is 45 ft with four elbows and one tee, so equivalent length is higher than 45 ft. Using a standard natural-gas capacity table for a 0.5 in. w.c. drop and 0.60 specific gravity, 1/2 in. Schedule 40 at this length is far below 190,000 BTU/h, roughly in the 60,000-70,000 BTU/h range depending on the table. 3/4 in. is also likely short at 45 ft for 190,000 BTU/h. A 1 in. Schedule 40 main, or an equivalent CSST size from the manufacturer's table, is the minimum practical choice. The branch to the garage heater must be sized separately. Raising manifold pressure does not increase pipe capacity and can cause overfiring at the appliances.
2. Interpret the pressure test result and describe the proper leak-testing and repair process.
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A 0.4 psig drop during a 30-minute 10 psig air test is a failed test; the line is leaking. Isolate sections to narrow the leak, then use soap bubbles or an electronic combustible-gas detector on joints, fittings, and valve bodies. Never use a flame. Repair or replace the leaking components with approved gas piping materials, then repeat the pressure test at the required test pressure and duration. After the test passes, purge the line, reconnect appliances, and leak-test operating joints. Also confirm that the meter and regulator can supply 190,000 BTU/h at the required pressure; a pipe upgrade alone is not enough if the regulator is undersized or locked up.
3. Describe the required sediment trap, shutoff, and garage installation details, and explain why raising manifold pressure is not an acceptable capacity fix.
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Install a sediment trap or drip leg at each appliance, arranged so dirt and scale fall into the trap rather than into the gas valve. Provide a readily accessible manual shutoff valve in the supply line at each appliance. For the garage heater, use a unit listed for garage installation, protect it from vehicle impact, and keep ignition sources at least 18 in. above the floor unless the listing permits otherwise. A 1/2 in. branch may be adequate only if the manufacturer's table and equivalent length show it can carry 50,000 BTU/h at the available pressure drop. Raising manifold pressure does not fix an undersized pipe; it increases input, can cause incomplete combustion, and may overload the regulator. Correct the piping size and verify input by clocking the meter.