You just installed a brand-new tankless water heater. You expected endless hot showers. Instead you get lukewarm water that turns cold the moment a second faucet opens. The unit throws error codes. The installer shrugs and says “that’s just how these things are.” But it’s not how they are. The real problem might be hiding inside your wall: the gas line feeding the heater.
Gas supply issues are the single most overlooked cause of underperforming tankless water heaters. Most homeowners and even some plumbers assume any existing gas line will work. That assumption costs people time, money, and cold mornings. This article walks through exactly how gas line size, pressure, regulator settings, and pipe material affect a tankless unit’s ability to deliver its rated flow and temperature rise. You’ll walk away knowing what to check, what to fix, and when to call a pro.
CMI
CMI 3/4" IPS Complete Tankless Water Heater Valve Kit, Pressure Relief
- EASY INSTALLATION: This 3/4 inch tankless water heater valve kit includes 2 brass nipples (3/4"), 1 gas line hose (3/4" FIP x 3/4"…
- INDUSTRY-STANDARD MATERIAL: Made from clean brass to meet low lead requirements, this tankless water heater valve kit adheres to p…
- QUICK CONNECTION: The Pressure Relief Valve included in the kit integrates smoothly with the hot valve, making it easy to manage f…
Before we dig into the technical details, one practical shortcut exists: a complete valve kit that includes everything needed for the water and gas connections. The CMI Inc – 3/4 Inch IPS Female Threaded Complete Tankless Water Heater Valve Kit (ASIN B0BWL5HFCC) bundles the gas line hose, two water hoses, brass nipples, and a pressure relief valve. That kit simplifies installation and helps avoid the common mistake of mismatched fittings. We’ll reference it where relevant, but the real focus here is the gas supply side.
Why Gas Line Size Matters More Than You Think
A tankless water heater pulls a lot of gas. A typical 199,000 BTU residential unit needs about 200 cubic feet per hour (CFH) of natural gas at full fire. That’s roughly the same demand as a whole-home furnace, a range, and a dryer all running at once. Your existing gas line might have been sized for an old tank-style heater that only drew 30–40 CFH.
The pipe diameter determines how much gas can flow at a given pressure without a large pressure drop. A 1/2-inch black iron pipe can deliver maybe 60 CFH over 50 feet of run. That won’t cut it. A 3/4-inch pipe handles roughly 150 CFH over the same distance. For runs longer than 50 feet, you often need 1-inch or even 1-1/4-inch pipe. Many manufacturers specify a minimum line size of 3/4 inch up to a certain length, then require upsizing beyond that.
I’ve seen homes where the contractor ran a new 3/4-inch line from the meter to the heater, but the meter itself’s outlet port was only 1/2 inch. The bottleneck killed flow. The meter end needs upsizing too. Check the meter’s rated capacity, usually stamped on the side. If it can’t pass enough gas, the utility company needs to swap it.
Gas Pressure: The Number That Actually Controls Performance
Tankless water heaters require a specific inlet gas pressure. For natural gas, that’s typically between 6 and 7 inches water column (WC) when the unit is running at full fire. For propane, it’s higher—usually 11 inches WC.
Static pressure (pressure when nothing is running) can look fine at 7 inches WC. The problem shows up when the heater fires up. A restriction in the gas line causes the dynamic pressure to drop. If it falls below 4 inches WC for natural gas, the burner flames get starved. The unit may not ignite fully, run hot, or simply stop and throw a fault code like “E110” or “Gas Supply Error.”
That drop happens because of pipe friction. Every elbow, tee, and valve adds resistance. A common mistake is using a standard gas shutoff valve rated for 1/2-inch pipe on a 3/4-inch line. The internal bore is smaller, choking flow. Use a full-port ball valve instead. The CMI valve kit includes a 3/4-inch ball valve that is full-port, which is the right design for minimizing pressure loss.
Propane systems have a different gotcha. Propane tanks come with regulators. The standard two-stage regulator delivers about 11 inches WC. But a long pipe run from the tank to the house can cause a pressure drop. Some installers install the regulator too far downstream. The fix is to locate the second-stage regulator as close to the house as possible, or upsize the pipe between the tank and the regulator.
Pipe Material and Fittings: Not All Gas Lines Behave the Same
Black iron pipe is the traditional choice. It’s heavy, durable, and has a smooth interior. But each threaded connection is a potential restriction. The inside diameter at a coupling is smaller than the nominal pipe size. A 3/4-inch black iron pipe has an actual ID of about 0.82 inches. With every threaded fitting, that effective diameter gets reduced by roughly 10%.
Flexible corrugated stainless steel tubing (CSST) is popular for tankless installations because it’s easier to run and fewer fittings. But CSST has a rougher interior surface than black iron, which increases friction. For the same pressure drop, you need a larger CSST size. Check the manufacturer’s sizing charts—sometimes 1-inch CSST is needed where 3/4-inch black iron would suffice. Also, CSST must be properly bonded and grounded to prevent lightning damage. That’s an extra inspection step.
Polyethylene (PE) pipe is used for underground runs. It’s flexible and corrosion-resistant. But it also has a higher friction factor than iron. If you’re extending a gas line from the meter across the yard, use an oversized PE pipe. The CMI kit’s gas hose is a 3/4-inch female threaded connector, which is fine for the last 18 inches from the gas valve to the heater, but it’s not a substitute for a properly sized main supply line.
Here’s a comparison of common gas line configurations for a typical 199,000 BTU tankless heater:
| Pipe Material | Nominal Size | Max Length (ft) for 0.5″ WC drop | Best Use Case |
|---|---|---|---|
| Black Iron | 3/4″ | 60 | Short runs from meter or tee |
| Black Iron | 1″ | 200 | Long runs in basement or walls |
| CSST | 3/4″ | 30 | Last leg to heater (avoid long runs) |
| CSST | 1″ | 100 | Medium runs with multiple fittings |
| Polyethylene | 1″ SDR-11 | 50 | Underground from tank/propane |
| Polyethylene | 1-1/4″ SDR-11 | 150 | Underground long runs |
What Happens When You Get It Wrong
An undersized gas line doesn’t just cause lukewarm water. It can damage the heater. When the burner runs lean (not enough gas), the flame temperature rises. That can warp heat exchangers, crack combustion chambers, and create soot. Soot buildup fouls the burner and reduces efficiency further. The unit will cycle on and off rapidly, a condition called short-cycling. Each cycle shortens the lifespan of the igniter and fan motor.
I once diagnosed a two-month-old Navien that kept locking out. The homeowners had paid $4,500 for installation. The gas line was 1/2-inch black iron for a 55-foot run. Measured dynamic pressure at the heater inlet was 2.8 inches WC. The installer had not bothered to check. After re-piping with 1-inch black iron, the heater ran perfectly. That fix cost $300, not a new heater.
Propane systems have a specific failure mode. The regulator on the tank is designed to deliver a fixed pressure. But if the pipe between the regulator and the heater is too small, the pressure drop under load makes the regulator over-compensate, sending liquid propane into the gas line. That causes flares and potentially a fire. This is rare but documented. Always size gas lines for the heater’s full BTU rating, not the expected average use.
How to Check and Diagnose a Gas Line Problem
If you suspect the gas line is the issue, you can’t guess. You need three things: a manometer to measure gas pressure, a chart of BTU requirements, and a plan for temporary tests.
First, shut off all gas appliances except the tankless heater. Turn on the heater to its maximum flow setting (run hot water at full blast). Measure the gas pressure at the inlet test port on the heater. Compare to the manufacturer’s minimum. If it’s below spec, the line is too small or there’s a blockage. Next, measure the pressure at the meter (or regulator for propane) while the heater is running. If pressure is fine at the source but low at the heater, the line itself is the bottleneck. If pressure is already low at the source, call the gas utility to check the meter and main.
Second, calculate the total BTU load of all appliances connected to the same gas line. Add the heater’s max BTU, furnace, range, dryer, fireplace. That total must be less than the pipe’s capacity at the given length. Use standard pipe sizing tables (available from the manufacturer or NFPA 54).
Third, check the gas connector. Many code jurisdictions require a flexible connector (gas appliance connector) at the heater. That connector has a smaller diameter than the supply line. Make sure it’s at least 3/4 inch. The CMI kit includes a 3/4-inch gas hose, so that part is covered. But if your connector is 1/2 inch, swap it out.
If all that sounds too involved for a DIY, hire a licensed gas fitter. A simple pressure check and sizing calculation should take under an hour. Expect to pay $150–$300, which is cheap compared to replacing a ruined heater.
Frequently Asked Questions
Can a gas line that is too small cause my tankless water heater to shut off?
Yes. Low gas flow starves the burner. The unit’s safety system detects the weak flame or low pressure and shuts down. You’ll see error codes like “Gas failure” or “Ignition failure.” The heater may try to relight several times before locking out permanently. A pressure check while running will confirm it.
What size gas line do I need for a 199,000 BTU tankless water heater?
For natural gas, a 3/4-inch black iron pipe supports up to about 60 feet. For longer runs, 1-inch is required. For propane, the same sizing applies, but propane has higher BTU per cubic foot, so volume flow is lower—still, friction is similar. Always consult the heater’s installation manual for minimum gas line size at your specific run length. Don’t rely on internet averages.
Will a gas line that is too large cause any problems?
Rarely. Oversized gas lines increase gas volume in the pipe, which can delay ignition slightly as the air purges, but that’s usually harmless. The bigger risk is cost—larger pipe costs more. No performance downside exists. It’s far better to oversize than undersize.
Can I use a standard gas shutoff valve with a tankless water heater?
Not if it’s a 1/2-inch valve. Use a full-port 3/4-inch gas ball valve. Standard valves have a restricted internal bore meant for smaller appliances. The CMI kit’s valve is full-port and designed for tankless heaters. If you buy separately, look for “full-port” or “straight-through” in the description.
How often should I check the gas line for my tankless water heater?
Check the gas pressure at installation. After that, inspect the gas line visually every two years for rust, dents, or corrosion. If you smell gas, call a professional immediately. Otherwise, no regular pressure checks are needed unless the heater starts acting up.
What You Should Actually Do
- Measure dynamic gas pressure at full flow before accepting any installation. Static pressure alone is useless.
- Verify the gas line is sized for the heater’s full BTU rating plus all other appliances, using the actual pipe length and number of fittings.
- Use a full-port 3/4-inch gas shutoff valve and a 3/4-inch flexible gas connector. The CMI kit provides this complete package.
- If converting from propane to natural gas or vice versa, change the gas orifice and adjust the regulator—don’t assume the same pipe works.
- For runs over 80 feet, step up one pipe size from the manufacturer’s minimum recommendation.
- Inspect the gas meter’s capacity. If it’s undersized, the utility must upgrade it. That’s their cost.
- Hire a licensed gas fitter for any modifications. Gas line mistakes can kill. This isn’t a weekend DIY project.
