Sizing a gas tankless water heater primarily involves calculating the required Gallons Per Minute (GPM) flow rate and considering the necessary temperature rise (ΔT) to ensure adequate hot water for your household needs.
Understanding Key Sizing Factors
To properly size a gas tankless water heater, you need to assess your home's hot water demands based on simultaneous usage and local climate conditions.
1. Determine Your Household's GPM Needs
The most critical factor is the maximum number of hot water fixtures that might be used at the same time. Each fixture (shower, faucet, washing machine, dishwasher) has a typical GPM requirement.
According to general sizing rules, average GPM requirements for simultaneous fixture use are:
| Simultaneous Fixtures | Approximate GPM Needed |
|---|---|
| 1-2 Fixtures | 3.5 GPM |
| 2-3 Fixtures | 5 GPM |
| 3-4 Fixtures | 7 GPM |
- Examples of GPM per Fixture:
- Shower: 1.5 - 2.5 GPM
- Dishwasher: 1.0 - 2.5 GPM
- Washing Machine: 1.5 - 3.0 GPM
- Kitchen Faucet: 1.0 - 2.0 GPM
- Bathroom Faucet: 0.5 - 1.5 GPM
To calculate your peak GPM:
- List all hot water-using appliances and fixtures in your home.
- Estimate which ones are most likely to run simultaneously during peak demand (e.g., a shower, a dishwasher, and a bathroom sink).
- Add up the GPM for these anticipated simultaneous uses.
For instance, if you anticipate one shower (2.0 GPM) and a washing machine (2.0 GPM) running at the same time, you'd need a heater capable of at least 4.0 GPM.
2. Calculate the Required Temperature Rise (ΔT)
Temperature rise is the difference between your incoming cold water temperature and your desired hot water temperature. This is crucial because a tankless heater's GPM output decreases as the required temperature rise increases.
- Incoming Water Temperature: This varies significantly by geographic location and season. In colder climates, incoming groundwater can be as low as 35°F-40°F, while in warmer climates, it might be 60°F-70°F.
- Desired Hot Water Temperature: Most people set their water heaters to 105°F-120°F for comfort and safety.
Formula for Temperature Rise:
Desired Hot Water Temperature - Incoming Cold Water Temperature = Temperature Rise (ΔT)
- Example: If your desired temperature is 110°F and your incoming cold water is 40°F (common in northern winters), your required ΔT is 70°F. If your incoming water is 60°F, your ΔT is 50°F.
3. Consider British Thermal Units (BTU) Output
Gas tankless water heaters are rated by their BTU output, which indicates how much energy they can produce. A higher BTU rating allows for a higher GPM output at a given temperature rise, or the ability to achieve a larger temperature rise for a given GPM.
Most manufacturers provide performance charts that show the GPM output at various temperature rises. You'll need to match your calculated GPM and ΔT to a unit's specifications.
4. Consult Professional Installation
While these guidelines provide a strong foundation, consulting a qualified HVAC or plumbing professional is highly recommended for accurate sizing and installation. They can account for factors like:
- Local climate and specific ground water temperatures.
- Gas line size and availability.
- Ventilation requirements.
- Your home's unique plumbing layout.
