Technical Foundation
What Is a Gas Tankless Water Heater?
A gas tankless water heater fires its burner only when a hot water tap is opened, heating water as it flows through a heat exchanger rather than maintaining a constantly heated storage tank — eliminating standby heat loss and, when properly sized, providing continuous hot water without running out the way a tank heater can during high-demand periods. Sizing is based on required flow rate (gallons per minute) at a specific temperature rise (incoming groundwater temperature to the desired output temperature), since the unit's maximum GPM output drops as the required temperature rise increases — a unit rated for 9.5 GPM at a 35°F rise delivers meaningfully less flow at a 70°F rise common in colder climates.
Condensing units extract additional heat from flue gases before venting, achieving higher efficiency (often 90%+ versus 80-85% for non-condensing) and allowing lower-cost PVC venting instead of the stainless steel venting non-condensing units require, but at a higher unit cost. Fuel type (natural gas vs. propane) is not interchangeable without a conversion kit specific to the model, and gas supply line sizing must be checked against the unit's BTU input rating, since an undersized gas line will starve the burner and prevent the unit from reaching rated output.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Flow Rate (GPM) | Rated at a specific temperature rise, e.g. 9.5 GPM at 35°F rise | Actual usable flow drops as required temperature rise increases — size for your coldest incoming water |
| Fuel Type | Natural gas or propane, not interchangeable without a conversion kit | Using the wrong fuel type without proper conversion risks improper combustion and unsafe operation |
| Combustion Type | Condensing (high efficiency, PVC vent) vs. non-condensing (stainless vent) | Condensing units cost more upfront but reduce venting cost and operating fuel expense |
| BTU Input Rating | Commonly 150,000-199,000 BTU/hr for residential/light commercial | Gas supply line must be sized to deliver adequate flow at this input rating without pressure drop |
| Venting Configuration | Direct vent (sealed combustion) vs. natural draft | Determines installation location flexibility and combustion air source requirements |
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Applications
Where Gas Tankless Water Heaters Get Used
Gas tankless water heaters are specified anywhere continuous, on-demand hot water is preferred over a stored tank's standby loss and finite capacity.
Residential Whole-House Hot Water
Sized tankless units replacing tank heaters to provide continuous hot water without standby energy loss.
Point-of-Use Applications
Smaller tankless units serving a single remote fixture without long hot water wait times.
Commercial Kitchen & Restaurant Hot Water
Higher-capacity commercial tankless units meeting simultaneous multi-fixture demand in food service.
Multi-Family & Apartment Buildings
Individual unit tankless heaters providing per-unit billing and eliminating shared tank heater downtime.
Cabin & Off-Grid Propane Applications
Propane tankless heaters providing hot water where natural gas service isn't available.
Radiant Floor Heating Systems
Tankless heaters sized to supply continuous heated water for hydronic radiant heating loops.
FAQ
Gas Tankless Water Heaters Questions, Answered
How do I size a tankless water heater correctly for my home or building?
Determine your peak simultaneous hot water demand in GPM (add up the flow rates of fixtures likely to run at the same time) and the required temperature rise (desired output temperature minus your coldest incoming groundwater temperature, which varies significantly by climate and season). Then select a unit rated to deliver that GPM at that specific temperature rise, not just the unit's maximum advertised GPM, which is typically rated at a lower, more favorable temperature rise.
What's the difference between a condensing and non-condensing tankless heater?
A condensing unit extracts additional heat from the flue gases before they're vented, achieving higher overall efficiency and allowing lower-cost PVC venting material. A non-condensing unit vents hotter exhaust gases and requires more expensive stainless steel venting, but has a lower upfront unit cost. The efficiency gain from a condensing unit typically pays back over time through reduced fuel consumption, especially in units run frequently.
Can I switch a natural gas tankless heater to run on propane?
Not without a manufacturer-specific conversion kit designed for that exact model — natural gas and propane burn differently and require different orifice sizing and gas valve calibration for safe, complete combustion. Running a unit on the wrong fuel type without proper conversion is a safety hazard and will typically void the manufacturer's warranty.
Why does my tankless heater struggle to reach full output even though it's rated high enough?
The most common cause is an undersized gas supply line that can't deliver adequate gas flow to match the unit's BTU input rating, effectively starving the burner even though the unit itself is correctly sized. Have the gas line sized specifically for the tankless unit's BTU rating, since tankless heaters typically require significantly more instantaneous gas flow than a tank heater of similar output capacity.
Do I need direct vent or natural draft venting for my installation?
Direct vent (sealed combustion) units draw combustion air from outside through a dedicated intake and are more flexible for installation locations, including smaller mechanical rooms, since they don't depend on room air for combustion. Natural draft units draw combustion air from the surrounding space and typically need more installation clearance and room ventilation — check your unit's specific venting requirements against your installation location before finalizing the model.
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