Water Heater Size Planner – First-Hour Demand Worksheet

Water Heater Size Planner: Estimate Your First-Hour Demand

This private worksheet helps you list simultaneous hot-water uses to estimate your peak demand. It does not recommend a specific model or size. The result is a starting point for a conversation with a licensed plumber, who must consider your exact fuel type, climate, incoming water temperature, and local code. Do not buy equipment based solely on this estimate.

Denver, ColoradoResearch checked July 31, 2026Source-linked educational guide — verify current details and obtain a professional assessment when needed

Quick, low-risk steps

List every hot-water fixture and appliance in your home.

Note which ones might run at the same time during your busiest hour.

Add up the approximate gallons used in that hour using the worksheet.

Record your current water heater’s model, fuel type, and age.

Share the completed worksheet with a licensed plumber for professional sizing.

What the Worksheet Does

The worksheet helps you think through your household’s peak hot-water use. You list fixtures and appliances, then estimate how many gallons they might draw during the busiest hour. This is not a replacement for a professional load calculation.

A plumber uses your answers along with fuel type, incoming water temperature, and local code to recommend equipment. The worksheet is private—no data is saved or shared.

List showers, tubs, dishwashers, clothes washers, and sinks.
Note which uses happen at the same time, like morning showers and laundry.
Estimate gallons per use based on fixture flow rates.
The total is your approximate first-hour demand.

Why First-Hour Rating Matters

A water heater’s first-hour rating tells you how many gallons of hot water it can deliver in an hour starting with a full tank of hot water. It is a key number for matching equipment to your household’s peak demand.

For heat pump water heaters, sizing also considers recovery rate and backup electric elements. ENERGY STAR notes that site conditions and current utility rates affect performance and savings.

First-hour rating is listed on the Energy Guide label.
It combines tank capacity and recovery rate.
A higher first-hour rating can handle more simultaneous use.
Undersized equipment may run out of hot water during peak times.

Worksheet Limitations

This worksheet does not account for incoming water temperature, which varies by season and location. It also cannot evaluate your home’s electrical capacity, venting, or space constraints.

A heat pump water heater may need a larger tank or different location than a standard electric model. Always have a licensed plumber review your specific situation before purchasing equipment.

Incoming cold water temperature affects recovery.
Fuel type (gas, electric, heat pump) changes sizing needs.
Local code may require permits, expansion tanks, or drain pans.
The worksheet is a conversation starter, not a final answer.

Next Steps After the Worksheet

Once you have your estimated first-hour demand, gather your current water heater’s model number, fuel type, and age. Take photos of the installation area, including clearances and connections.

Contact at least two licensed plumbers for assessments. Ask each to explain how they sized the recommendation and what scope items are included in their quote.

Share your completed worksheet with each plumber.
Ask about permits, venting, drain pans, and expansion tanks.
Request a written quote that lists all scope items.
Compare first-hour ratings of proposed models to your estimate.

What homeowners ask next

Can I use this worksheet to buy a water heater online?

No. The worksheet is a planning aid. A licensed plumber must evaluate your home’s specific conditions before you purchase equipment.

What if my estimated demand is between two sizes?

Discuss with a plumber. They may recommend the larger size or a different fuel type to ensure adequate hot water during peak use.

Does this worksheet work for tankless water heaters?

Tankless sizing uses flow rate and temperature rise, not first-hour rating. This worksheet is designed for tank-type water heaters.

Need an on-site professional?

Use the guide and worksheet first so you can describe the situation and compare the full scope. A referral is optional; you can also contact a licensed local provider directly.

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We have not enabled a provider route for this site. Use the checklist to contact and verify local providers yourself; urgent hazards should go to the appropriate emergency or public service.