A natural gas tankless water heater gives you unlimited hot water from a wall-mounted box the size of a carry-on suitcase, and it does it while burning less fuel than the 50-gallon tank it replaces. That combination — endless supply plus lower operating cost — is why gas tankless has become the default upgrade for homeowners with existing gas service. But the payoff only lands if you size it right, vent it right, and maintain it. Here’s everything that goes into a good install.
- How It Works
- Condensing vs. Non-Condensing
- Sizing: GPM and Temperature Rise
- Venting and Gas Line Requirements
- Installation Cost
- Top Brands Worth Knowing
- Maintenance and the Hard-Water Problem
- Is It Right for Your Home?
- Recirculation and the “Cold Water Sandwich”
- Cold-Climate Considerations
- Frequently Asked Questions
How It Works
There’s no tank storing pre-heated water. When you open a hot tap, a flow sensor detects the demand and fires a gas burner that heats water instantly as it races through a heat exchanger. Close the tap and the burner shuts off. Nothing is kept hot between uses, which is exactly why you dodge the standby losses a tank suffers 24 hours a day.
The burner is serious hardware — modern units fire anywhere from 120,000 to 199,000 BTU, roughly four times a tank’s output, because they have to heat water on demand rather than slowly over an hour.
Condensing vs. Non-Condensing
This is the first fork in the road, and it drives both efficiency and install cost.
- Condensing units use a second heat exchanger to wring extra heat out of exhaust gases, reaching 0.93 to 0.98 Uniform Energy Factor. Cooler exhaust means you can vent through cheap PVC or polypropylene. Higher unit cost, lower venting cost.
- Non-condensing units vent hotter exhaust and require Category III stainless steel venting, which is expensive. They cost less upfront but more to vent, and they’re slightly less efficient (around 0.82 UEF).
For most new installs, condensing is the smarter buy — the venting savings and lower fuel bills usually outrun the higher sticker price within a few years.
Sizing: GPM and Temperature Rise
Undersizing is the number-one regret with gas tankless. Size by two numbers: the flow rate you need in gallons per minute, and the temperature rise your climate demands.
Add up the fixtures you’d run at once. A shower uses 1.5 to 2.5 GPM, a kitchen faucet about 1.5, a washing machine 2. Two showers going together is roughly 4-5 GPM. Then figure temperature rise: if your groundwater enters at 45°F and you want 120°F output, that’s a 75°F rise. Cold-climate homes need bigger units because flow drops as required rise climbs.
- 1 bathroom, warm climate: 6-7 GPM unit is plenty.
- 2 bathrooms, mixed climate: 8-9 GPM.
- 3-plus bathrooms, cold climate: 9-11 GPM, and consider two units in parallel.
Venting and Gas Line Requirements
Two infrastructure items make or break the install. First, the gas supply: that big burner needs adequate gas volume. Many homes have a 1/2-inch line to the old tank, but a tankless often needs 3/4-inch to deliver enough fuel without starving. Your installer should run a gas-load calculation for the whole house.
Second, venting. A condensing natural gas tankless water heater vents through 2- or 3-inch PVC and needs a condensate drain because it produces acidic condensate. Concentric or two-pipe (intake and exhaust) configurations both exist. Sidewall venting through an exterior wall is common and avoids running pipe up through the roof. Never vent a tankless into an old masonry chimney designed for a natural-draft tank.
Installation Cost
Expect $1,500 to $3,500 installed for a gas-for-gas replacement, with the unit itself running $900 to $1,700. The variables are gas-line upsizing ($300 to $800), venting ($200 to $500 for PVC), a condensate drain, isolation valves, and permits. A clean garage-wall install with existing adequate gas supply lands at the low end; a conversion requiring line upsizing and a long vent run pushes toward the top.
Top Brands Worth Knowing
Four names dominate the residential market:
- Rinnai: Reliable, widely serviced. The RU and RSC series are workhorses. Strong dealer network makes parts easy.
- Navien: The NPE series pioneered condensing with a built-in recirculation pump on some models for faster hot water at the tap.
- Rheem/Ruud: Good value, available at big-box stores, solid mid-range performers.
- Noritz: Commercial pedigree, robust heat exchangers, popular with plumbers.
Look for a Uniform Energy Factor at or above 0.95 to qualify for the federal tax credit, and check the heat-exchanger warranty — 12 to 15 years is standard on quality units.
Maintenance and the Hard-Water Problem
The heat exchanger is the heart of the unit, and hard water is its enemy. Mineral scale builds inside the exchanger, chokes flow, drops efficiency, and eventually triggers error codes. If your water is above 7 grains per gallon of hardness, plan to descale annually; softer water can stretch to every two years.
Descaling means circulating white vinegar or a descaling solution through the unit for 45 to 60 minutes using a pump and bucket. It’s a job you can DIY in an hour if your installer put in isolation (service) valves — insist on those at installation. Skipping maintenance is the fastest way to void a warranty and shorten the unit’s life.
Is It Right for Your Home?
A natural gas tankless water heater makes the most sense if you already have gas service, want to reclaim the floor space a tank occupies, and value endless hot water for big households or deep tubs. The 20-plus-year lifespan means it often outlasts two tank heaters, softening the higher upfront cost.
It’s a weaker fit if your gas line and venting would require expensive reworking, or if your household uses very little hot water — in that case the efficiency savings take too long to recover the premium. Get an itemized quote that spells out gas-line and venting scope, confirm isolation valves are included, and you’ll have a system delivering hot showers on demand for two decades.
Recirculation and the “Cold Water Sandwich”
One quirk of tankless heaters catches new owners off guard: the cold water sandwich. If someone runs a hot tap briefly, shuts it, then reopens it, you can get a slug of cooled water between two bursts of hot while the burner refires. It’s minor, but real. A built-in or add-on recirculation pump — standard on some Navien and Rinnai models — keeps a small loop of hot water primed, delivering near-instant hot water at the tap and smoothing out that sandwich. Recirculation adds cost and uses a little energy, but in a large house with long pipe runs it’s a genuine comfort upgrade worth pricing out.
Cold-Climate Considerations
Where you live changes the sizing math more than any other factor. Incoming groundwater in northern states can drop to 40°F in winter, demanding a 75-80°F temperature rise to reach a comfortable 120°F. That heavy rise cuts a unit’s effective flow rate — a heater rated 9 GPM in mild conditions might only deliver 5-6 GPM against a deep winter rise. Undersize it and you’ll get lukewarm showers in January. Always read the manufacturer’s flow chart at your coldest expected inlet temperature, not the headline GPM number, and size up if you’re in a cold region running multiple fixtures at once.
Frequently Asked Questions
Do I still get hot water during a power outage?
No. Even a natural gas tankless water heater needs electricity to run its igniter, fan, and controls, so it won’t fire during an outage. A small battery backup or generator restores function if that’s a concern for you.
How long does a gas tankless unit last?
With annual descaling and a clean inlet filter, 20 years or more is realistic — often double a storage tank’s life. Skipping maintenance in hard water can cut that in half and void the warranty.
Can one unit really run two showers at once?
Yes, if it’s sized for it. An 8-11 GPM unit handles two simultaneous showers in most climates; in very cold regions or for three-plus fixtures, plumbers sometimes install two units in parallel.
