Tankless On-Demand Water Heaters

Tankless water heaters eliminate standby thermal losses by heating water only when demand occurs. These systems pass water through a heat exchanger activated by flow detection, delivering continuous hot water without storage tank limitations.

Operating Principles

Flow-activated heating relies on a pressure differential switch or flow sensor that detects water movement and initiates the heating sequence. The fundamental energy balance governs instantaneous heating:

$$Q = \dot{m} c_p \Delta T$$

Where:

• $Q$ = heat transfer rate (Btu/hr or kW)
• $\dot{m}$ = mass flow rate (lb/hr or kg/s)
• $c_p$ = specific heat of water (1.0 Btu/lb·°F or 4.186 kJ/kg·K)
• $\Delta T$ = temperature rise (°F or K)

Converting to practical units for water heating:

$$Q_{Btu/hr} = \text{GPM} \times 500 \times \Delta T_F$$

$$Q_{kW} = \text{L/min} \times 0.07 \times \Delta T_C$$

The coefficient 500 represents the product of water density (8.34 lb/gal), specific heat (1.0 Btu/lb·°F), and time conversion (60 min/hr).

Gas Tankless Systems

Gas-fired units employ modulating burners that adjust firing rate based on flow and inlet temperature. Modern condensing models achieve thermal efficiencies exceeding 95% by extracting latent heat from combustion products.

Heat Exchanger Configuration

graph LR
    A[Cold Water In] --> B[Flow Sensor]
    B --> C[Primary Heat Exchanger]
    C --> D[Secondary Condenser]
    D --> E[Hot Water Out]
    F[Gas Burner] --> C
    C --> G[Flue Gases]
    G --> D
    D --> H[Condensate Drain]
    I[Control Module] --> F
    B --> I
    J[Temperature Sensor] --> I
    E --> J

The primary heat exchanger operates above the dew point of combustion gases (approximately 135°F for natural gas), while the secondary condenser recovers additional energy by cooling flue gases below this threshold.

Performance Characteristics

Gas tankless capacity depends on burner input and efficiency:

$$\text{GPM} = \frac{Q_{input} \times \eta}{500 \times \Delta T}$$

For a 199,000 Btu/hr unit at 96% efficiency with 45°F inlet temperature:

$$\text{GPM} = \frac{199,000 \times 0.96}{500 \times (120 - 45)} = \frac{191,040}{37,500} = 5.1 \text{ GPM}$$

Electric Tankless Systems

Electric units use resistive heating elements immersed in water flow passages. Multiple elements activate sequentially as flow rate increases, providing modulation without combustion controls.

Power Requirements

Electric tankless systems require substantial electrical service:

$$P_{kW} = \frac{\text{GPM} \times 500 \times \Delta T}{3412}$$

The conversion factor 3412 translates Btu/hr to kW. For 2.5 GPM at 70°F rise:

$$P_{kW} = \frac{2.5 \times 500 \times 70}{3412} = \frac{87,500}{3412} = 25.6 \text{ kW}$$

This power level demands 240V service with multiple circuits:

Electric efficiency reaches 99% due to direct resistance heating without combustion losses, but high electricity costs typically offset this advantage.

Temperature Rise Calculations

Inlet water temperature significantly affects capacity. The required temperature rise varies geographically based on groundwater temperature:

A unit rated at 4.0 GPM with 45°F rise delivers only 2.25 GPM with 80°F rise:

$$\text{GPM}_2 = \text{GPM}_1 \times \frac{\Delta T_1}{\Delta T_2} = 4.0 \times \frac{45}{80} = 2.25 \text{ GPM}$$

Sizing Methodology

Proper sizing requires simultaneous fixture demand analysis. ASHRAE establishes fixture flow rates:

• Shower: 2.0-2.5 GPM
• Lavatory faucet: 0.5-1.0 GPM
• Kitchen sink: 1.5-2.0 GPM
• Dishwasher: 1.0-1.5 GPM
• Clothes washer: 2.0-3.0 GPM

For simultaneous use of one shower and one sink (peak residential demand):

$$\text{Total GPM} = 2.5 + 1.5 = 4.0 \text{ GPM}$$

With 70°F rise required, the minimum capacity becomes:

$$Q = 4.0 \times 500 \times 70 = 140,000 \text{ Btu/hr}$$

At 95% efficiency, burner input must be:

$$Q_{input} = \frac{140,000}{0.95} = 147,400 \text{ Btu/hr}$$

Gas versus Electric Comparison

Efficiency Advantages

Tankless systems eliminate standby losses that consume 10-20% of storage tank energy. The Department of Energy estimates annual energy savings of $100-$150 for gas units and $44-$80 for electric units compared to standard storage tanks.

Energy Factor (EF) ratings quantify overall efficiency including cycling losses. AHRI-certified gas tankless models achieve EF ratings from 0.82 to 0.96, while electric units reach 0.99. However, Uniform Energy Factor (UEF), which better represents real-world performance, shows gas condensing tankless at 0.90-0.96 UEF.

The absence of tank standby loss becomes more valuable in applications with intermittent demand patterns, where storage tanks continuously lose heat during idle periods. Conversely, high-volume continuous demand may favor storage systems that pre-heat large quantities during off-peak hours.

Minimum Flow Requirements

Flow switches typically activate at 0.4-0.6 GPM to prevent short cycling and ensure adequate heat exchanger velocity. This threshold creates “cold water sandwich” effects when brief low-flow events fail to maintain heating, particularly problematic in point-of-use applications with single low-flow fixtures.

Advanced models incorporate buffer tanks or recirculation pumps to eliminate this limitation, though these additions reintroduce minor standby losses.