Choosing between gas and electric is often the biggest decision when buying a tankless water heater, and it is driven as much by what your house can support as by preference. Gas units need fuel supply and venting; electric units need serious electrical capacity. This guide compares the installation requirements of each so you know what questions to raise before you collect quotes.
General information only. Requirements vary by model, local code and utility. Gas, venting and electrical work should be done by licensed professionals, with permits and inspections where required.
Quick Comparison
| Factor | Gas (natural gas or propane) | Electric |
|---|---|---|
| Fuel/power source | Gas line plus 120V outlet for controls | High-amperage 240V circuits |
| Venting | Required (type depends on model) | Not required |
| Typical flow capacity | Generally higher | Generally lower for whole-home use |
| Main upgrade risks | Gas line size, vent routing | Panel capacity, wire gauge |
| Location flexibility | Needs exterior wall or vent route | Easier to place indoors |
| Condensate | Condensing models need a drain | Not applicable |
| Combustion safety | CO detectors and clearances | No combustion |
Gas Tankless Requirements
1. Gas supply and line size
Gas tankless heaters have a high input rating compared with a typical tank unit. Your existing gas line may be too small to feed the new heater alongside furnace, stove and dryer. The installer should calculate total gas load and pipe sizing based on code. Upgrading a gas line can add significant cost.
Propane models use similar logic but need a suitable orifice or conversion and sufficient supply from the tank and regulator.
2. Venting
Venting removes combustion gases safely. Options depend on the model:
- Non-condensing units often use stainless steel or specified metal venting, since exhaust is hot.
- Condensing units produce cooler exhaust and often use PVC, CPVC or polypropylene, subject to manufacturer approvals.
- Direct vent (sealed combustion) units draw combustion air from outside through a second pipe or a concentric kit.
- Power-vented configurations push exhaust out with a fan.
Vent length, number of elbows, termination height, and distance from windows and doors are all regulated. Using the wrong vent material or length is dangerous and can void the warranty. Only use components approved for your specific unit.
3. Combustion air
Some models take air from the room, so the space must be large enough or have adequate openings. Sealed combustion models pull air from outside.
4. Electrical connection
Most gas tankless units still need a standard 120V outlet to power the control board, fan and ignition. Consider surge protection and backup power if outages are common.
5. Condensate drain
Condensing models produce acidic condensate that must drain to a floor drain, laundry sink or condensate pump, often through a neutralizer cartridge, depending on local rules.
6. Water connections and valves
Isolation valves, a pressure relief valve and proper piping support. Many installers also add a recirculation setup if the unit supports it.
7. Clearances and location
Maintain required clearances from combustibles, walls and other appliances. Outdoor-rated models can be mounted on an exterior wall in some climates, with freeze protection considerations.
Electric Tankless Requirements
1. Electrical capacity
This is the make-or-break item. Whole-home electric units can draw very large currents, often requiring multiple double-pole breakers and heavy-gauge wiring. Your main service panel may need an upgrade, and in some homes a service upgrade from the utility. An electrician should perform a load calculation. Smaller point-of-use models draw less but still require dedicated circuits.
2. Wiring and breakers
Use the wire gauge and breaker sizes specified in the manual. Undersized wiring can overheat, which is a fire risk. Do not guess.
3. No venting
Electric heaters need no flue, no combustion air and no gas line, so placement is flexible. You can install them in closets, under sinks or in utility rooms.
4. Water connections
As with gas models, isolation valves, a pressure relief valve and proper support are needed.
5. Cold-climate performance
Cold incoming water requires a bigger temperature rise, which demands more power for the same flow. Whole-home electric models can struggle to meet high demand in cold regions. Read our sizing guide.
Cost Considerations
Costs depend heavily on what your home already has. Typical scenarios:
- Replacing a gas tank with a gas tankless: may need larger gas pipe and new venting.
- Replacing an electric tank with an electric tankless: may need panel capacity and new circuits.
- Switching fuels: can involve all of the above plus structural changes.
Ask each contractor to itemize unit, venting, gas or electrical upgrades, permits, labor, haul-away and any trim work. Check for rebates and incentives with your utility or government programs.
Permits and Inspections
Most jurisdictions require permits for new or replacement water heaters, especially when gas or electrical work is involved. Inspections confirm safe installation. Skipping permits can cause problems with insurance claims or home sales.
Safety Essentials
- Install carbon monoxide detectors near sleeping areas and on each level, per local requirements.
- Never modify or shorten vent pipes.
- Keep combustibles away from appliances.
- Do not attempt gas or high-voltage work without proper qualifications.
- Schedule periodic inspections and maintenance; see how to flush a tankless heater.
Which One Should You Choose?
Gas may suit you if:
– You already have natural gas or propane service
– You need higher flow for a large family or cold climate
– You can route venting easily
Electric may suit you if:
– You have no gas supply or venting options
– You have ample electrical capacity or only need modest flow
– You want simpler siting and no combustion
If you are unsure, ask for a site assessment. Our broader overview of water heating options also covers heat pumps and other choices.
Questions to Ask Your Installer
- What size gas line or electrical service does this unit need, and does my home meet it?
- What venting material and routing will you use?
- Will you handle permits and inspections?
- What is included in the quote, and what could change it?
- How will condensate be handled?
- What maintenance does the unit need?
Sizing a Tankless Water Heater: Flow Rate and Temperature Rise
Questions to Ask Before Buying a Tankless Unit
Frequently Asked Questions
Do I need a new gas line for a gas tankless heater?
Often a larger line is needed because tankless units have a high gas input. A licensed plumber or gas fitter can calculate it.
Can I install an electric tankless heater without upgrading my panel?
Small point-of-use units might fit. Whole-home models often require significant capacity. An electrician can tell you.
Does a gas tankless heater work in a power outage?
Most do not, because they need electricity for controls and fans. A backup power option may help.
Is venting always required for gas?
Yes. Combustion gases must be vented to the outdoors according to the manufacturer and code.
Conclusion
Gas tankless heaters ask for gas capacity and proper venting; electric ones ask for serious electrical capacity. Check what your home has, get itemized quotes and use licensed professionals. A clear view of installation needs prevents surprises and helps you pick the right fuel.
