You walk into the plumbing aisle at the home center. Two water heaters sit side by side — one says Natural Gas, the other says Propane. They look identical. Same tank shape, same burner layout, same basic controls. So you figure they must be interchangeable, right? Not quite. The short answer is that the appliances are mechanically similar, but the fuel they burn behaves differently. And that difference matters a lot for safety, performance, and your wallet.
This article will walk you through exactly what separates them. You’ll learn whether you can swap one fuel for the other, what conversion involves, how operating costs compare, and what to watch out for when installing a new unit. No fluff — just the specifics you’d want from a contractor who has done this work.
Rinnai
Rinnai RX199iN Condensing Smart Sense Tankless Water Heater | 11.1 GPM, 199k BTU, Smart Sense Supports Natural Gas or Propane, and Indoor/Outdoor, Endless Hot Water, Save Space
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If you’re shopping for a tankless model that can handle either fuel out of the box, the Rinnai RX199iN Condensing Smart Sense Tankless Water Heater is worth a look. Its Smart Sense gas control system lets it run on propane or natural gas without swapping the entire unit — you just adjust the internal settings. That kind of flexibility saves headaches if you move to a house with different gas service or if you’re not sure what fuel will be available.
What Makes Natural Gas and Propane Different as Fuels
Natural gas is mostly methane. Propane is a heavier hydrocarbon, C3H8. That chemical difference means propane packs more energy per cubic foot — roughly 2,500 BTU per cubic foot versus natural gas’s 1,000 BTU. So a given volume of propane delivers about 2.5 times the heat.
But that’s not the only difference. Natural gas is delivered at a lower pressure — typically 7 inches of water column for residential appliances. Propane comes in around 11 inches of water column. The gas orifice (the tiny hole that lets gas into the burner) must be smaller for propane because it’s denser. If you run propane through a natural-gas-sized orifice, you’ll get a dangerous yellow, sooty flame that produces carbon monoxide.
The venting also changes. Propane is heavier than air. If a propane water heater has an indoor vent leak, the gas sinks to the floor instead of rising like natural gas. That means propane requires different leak detection methods and, in some cases, a higher vent termination point to keep heavy gas from pooling around the appliance.
Can You Convert a Natural Gas Water Heater to Propane (or Vice Versa)?
Yes, most modern gas water heaters — tank and tankless — come conversion-ready. But here’s the catch: you need the correct conversion kit from the manufacturer. That kit includes new orifices, a different regulator spring, and sometimes a new burner or gas valve. You cannot just swap the orifice and call it done.
The conversion process is roughly:
- Shut off the gas and disconnect the supply line.
- Remove the burner assembly and replace the main burner orifice(s).
- Swap the pilot orifice if the unit has a standing pilot (most tankless models use electronic ignition, so no pilot).
- Adjust the gas valve pressure regulator spring to match the new fuel’s pressure.
- Reassemble, reconnect gas, and test for leaks and proper combustion.
I’ve done this on a dozen units. The job itself takes about 30 minutes for a trained technician. But the kit must be exactly right for your model. Using a universal orifice kit will void the warranty and may create unsafe operation. Some manufacturers, like Rinnai, now build dual-fuel models that detect the gas type automatically — no kit needed. That’s the direction the industry is heading.
One more thing: converting a water heater from natural gas to propane is more common than the reverse, because propane has higher energy density and the conversion involves reducing flow. Going the other direction often requires a larger orifice and different regulator, which some older units can’t handle.
Cost and Efficiency: Which Fuel Saves You More?
Here’s where the numbers get real. Natural gas is almost always cheaper per BTU than propane — typically 30% to 50% less in most parts of the U.S. But that’s a national average. Your local rates can swing wildly. Propane prices are more volatile because they depend on winter demand and regional supply (think rural areas versus urban).
Efficiency-wise, the water heater itself doesn’t care which fuel you feed it. A 0.98 UEF condensing unit — like the Rinnai RX199iN — achieves that efficiency on either propane or natural gas as long as it’s properly set up. The combustion process is the same; only the fuel flow rate changes. So your energy savings come down to fuel cost, not appliance design.
But there’s a hidden cost. Propane tanks require refills, which means delivery fees and rental costs if you lease the tank. Natural gas comes through a utility pipe with no storage cost. If you live off the gas grid and have to bring in propane, the convenience of natural gas is hard to beat.
Installation Differences You Need to Know
Gas line sizing is the biggest practical difference. Because propane carries more energy per cubic foot, your gas line can be smaller for the same BTU output. That often means cheaper piping for a propane install. But if you’re converting an existing home from natural gas to propane, you might need to reduce the line size or add a regulator — something many DIY folks miss.
Venting is another concern. Condensing water heaters (tankless and some tanks) use plastic PVC venting that resists corrosion from the acidic exhaust. Both fuels work with PVC. But for non-condensing units, propane exhaust is slightly hotter and may require a different vent material rating. Always check the manufacturer’s venting spec for your specific fuel.
Outdoor installation is simpler with propane because the heavy gas disperses more easily when outside. Natural gas, being lighter, doesn’t pool the same way. But modern tankless units can go either place. The Rinnai RX199iN, for example, installs indoors or outdoors with the same vent (they call it Versa Vent). An outdoor install requires an RX-OVC vent cap, but no extra chimney pipe.
Propane vs Natural Gas Water Heaters: A Side-by-Side Comparison
| Factor | Natural Gas | Propane |
|---|---|---|
| Energy per cubic foot | ~1,000 BTU | ~2,500 BTU |
| Typical supply pressure (residential) | 7 inches water column | 11 inches water column |
| Fuel cost per BTU | Lower (typically 30-50% less than propane) | Higher, more volatile pricing |
| Storage requirement | None — piped from utility | Requires tank (owned or rented) |
| Availability | Urban/suburban areas with gas mains | Rural areas, delivered by truck |
| Conversion to other fuel | Possible with manufacturer kit | Possible with manufacturer kit |
| Venting concern | Gas rises; leak dissipates upward | Gas sinks; pool at floor level |
| Carbon monoxide risk | Lower if properly vented | Higher potential if conversion done wrong |
Frequently Asked Questions
Can I use a natural gas water heater with propane without a conversion kit?
No. Running propane through a natural-gas orifice will produce a dangerous flame — too much fuel, incomplete combustion, and carbon monoxide. You absolutely need the manufacturer’s conversion kit. Even then, some warranties require a licensed professional to do the swap.
Which is cheaper to operate, propane or natural gas?
Natural gas is almost always cheaper per BTU in areas where it’s available. But if you’re off the gas grid, propane might be your only option. Compare your local utility rates and include tank rental or purchase costs for propane. A good rule: natural gas saves you 30–40% on the fuel bill, give or take.
Are tankless water heaters compatible with both propane and natural gas?
Many modern tankless models are, but check the specs. Some require a conversion kit. Others — like the Rinnai RX199iN — detect the fuel automatically and adjust internal settings. That dual-fuel flexibility is becoming standard in premium units.
Do propane water heaters need special venting compared to natural gas?
Yes, especially for non-condensing models. Propane exhaust runs hotter and requires a higher temperature-rated vent. For condensing units (most tankless with PVC venting), the same piping works for both fuels. But always follow the manufacturer’s vent length and termination requirements to avoid backdrafting or corrosion.
How long does it take to convert a water heater from natural gas to propane?
A qualified technician can do it in 20 to 45 minutes, assuming the kit is correct and the unit is accessible. The job involves swapping orifices, adjusting the gas valve pressure, and testing combustion. Don’t rush it — a leak or misadjusted regulator can cause serious problems. Expect to pay for one hour of labor plus the cost of the kit.
What You Should Actually Do Right Now
- Check your existing water heater’s label to confirm what fuel it’s designed for. If it says only one fuel, don’t assume you can switch without a kit.
- If you’re buying a new water heater and might move or change fuel later, invest in a dual-friendly model like the Rinnai RX199iN. You’ll avoid a costly swap down the road.
- Compare local fuel prices using your gas bill (per therm for natural gas, per gallon for propane). Divide the cost by the BTU content to get cost per BTU — that’s the real comparison.
- Never DIY a gas conversion unless you have the proper tools and a combustion analyzer. A bad flame adjustment can kill in your sleep.
- For propane installations, install a propane gas detector near the floor in the same room as the water heater. Natural gas detectors go near the ceiling.
- When shopping, look for the Energy Factor (UEF) rating. A 0.95-plus condensing unit saves money on either fuel, but only if installed correctly.
- If your home has an older non-condensing tank model (atmospheric vent), replacing it with a condensing tankless can cut fuel use by 20–30% — the fuel savings alone might pay for the upgrade in this area in 3–5 years.
