Few decisions in a building's plumbing design generate more debate than water heating. Tank or tankless? Gas or electric? One unit or multiple?
For homeowners, the choice often comes down to cost or marketing. For engineers, it starts with the code — and then gets more interesting from there.
1. The code foundation: UPC and water heater requirements
The Uniform Plumbing Code (UPC), which Nevada has adopted, sets the baseline requirements for water heater installation. Both tank and tankless systems are permitted under the UPC, but they're governed by different sections and have different installation requirements.
For storage tank water heaters
- Temperature and pressure (T&P) relief valves sized and installed per manufacturer specifications
- Proper seismic strapping in applicable zones
- Combustion air provisions for gas-fired units
- Expansion tanks on closed systems (required in most Nevada jurisdictions)
- Minimum clearances for access and service
For tankless (demand) water heaters
- Minimum gas supply pressure and pipe sizing to handle peak demand (tankless units draw significantly more gas at full fire)
- Electrical requirements for ignition and controls
- Venting — condensing tankless units use PVC; non-condensing units require Category III or IV stainless steel flue pipe
- Minimum flow rates to activate the unit (important for fixture selection)
Both types must be installed by licensed contractors and are subject to permit and inspection in Nevada.
2. Tank water heaters: pros, limitations, and when they make sense
Storage tank water heaters heat water and keep it ready in an insulated tank — typically 40 to 100 gallons for residential use, and much larger for commercial applications.
Advantages
- Lower upfront cost
- Simpler installation
- Compatible with existing gas and electrical infrastructure
- Reliable performance across a wide range of fixtures and flow rates
Limitations
- Standby heat loss — energy is consumed around the clock to keep the stored water hot
- Limited hot water supply (once the tank is depleted, you wait for recovery)
- Larger footprint
For standard residential applications with moderate hot water demand, a properly sized tank unit remains a cost-effective and code-compliant choice. The key word is properly sized — a tank that's too small means cold showers; a tank that's too large wastes energy on standby losses.
3. Tankless water heaters: pros, limitations, and when they make sense
Tankless units heat water on demand, firing a high-capacity burner or heating element only when a hot water fixture is opened. There is no stored water, no standby loss, and theoretically no limit to how long you can run hot water.
Advantages
- No standby heat loss — energy is only used when hot water is needed
- Compact footprint
- Longer service life than tank units (typically 20+ years with proper maintenance)
- Endless hot water supply (within flow rate limits)
Limitations
- Higher upfront cost
- High gas demand at peak fire — requires larger gas lines in many cases
- Flow rate limitations — a single unit may not supply multiple simultaneous high-demand fixtures
- Requires descaling maintenance in hard water areas
Important note on gas supply. This is where many tankless installations fail. A standard residential gas meter and 3/4" gas line that served a 40-gallon tank heater may be completely inadequate for a condensing tankless unit drawing 150,000–200,000 BTU/hr at full fire. Gas line resizing is often required, adding to installation cost. An engineer or qualified plumbing designer should verify gas supply capacity before specifying tankless equipment.
4. Large residential projects: when the UPC tables no longer apply
Standard UPC water heater sizing tables work well for typical single-family homes. But once a project exceeds the scope of those tables — an 8-bedroom home, a large custom estate, a multi-unit building — engineering calculations become necessary.
- Number and type of fixtures
- Probability of simultaneous use
- Recovery time requirements
- Storage volume vs. instantaneous capacity tradeoffs
For large residential projects, demand calculations follow methods outlined in the UPC and ASHRAE guides, accounting for the factors above. At this scale, a single tankless unit is often insufficient. A manifolded system of multiple tankless units, or a commercial tank system with a separate recirculation pump, may be the right answer. Getting this wrong means occupants running out of hot water — or a system that costs far more to operate than it should.
5. The bottom line
For most standard residential projects, a properly sized tank water heater remains the reliable, cost-effective default. Tankless units offer real efficiency advantages for the right applications, but they require careful engineering of the gas supply, venting, and flow conditions to perform as advertised.
For commercial projects and larger homes, neither a tank nor a tankless system should be specified without proper demand calculations. The code requires it — and so does good engineering practice.
Start with the code. Both tank and tankless systems are permitted under the UPC, but each has distinct installation requirements that must be met before permit approval.
Planning a large home or commercial project? When UPC sizing tables no longer apply, demand calculations and gas supply verification are not optional — they're required by code and good engineering practice.