Indirect Water Heater Sizing Calculator

What Is an Indirect Water Heater?

An indirect water heater uses your home’s boiler or hydronic heating system as the primary heat source. Instead of heating water directly with electricity or gas, it uses the boiler’s hot water to heat domestic water inside a separate storage tank.

This design has several benefits:

• Higher efficiency – the boiler does double duty for both space heating and water heating.
• Longer lifespan – fewer moving parts compared to traditional tank water heaters.
• Flexible sizing – can meet high peak demands when sized correctly.

But efficiency and comfort depend on choosing the right tank capacity—which is exactly what the calculator is designed to solve.

Why Proper Sizing Matters

If your indirect water heater is too small:

• Morning showers may turn cold halfway through.
• Running a dishwasher while filling a tub can leave the tank empty.

If your water heater is oversized:

• You’ll pay extra for equipment you don’t need.
• The boiler will cycle more frequently, lowering efficiency.

Correct sizing ensures you always have enough hot water without wasting money or energy.

How the Indirect Water Heater Sizing Calculator Works

The calculator uses peak hourly demand (EPHD) and your system’s recovery rate to determine the tank size you need.

Here are the main inputs:

1. Number of People in Household – More people = higher hot water use.
2. Number of Master Bathrooms – High-usage spaces with large showers or soaking tubs.
3. Number of Standard Bathrooms – Low-to-medium demand fixtures.
4. Other High-Demand Fixtures – Dishwashers, jetted tubs, or multiple tubs add extra load.
5. Boiler Recovery Rate (BTU/hr) – How quickly your system can reheat water.
6. Temperature Rise (ΔT) – The difference between incoming cold water and desired hot water temperature.

Behind the Scenes: The Formula

• EPHD (Gallons per Hour) = People + Fixture Demand
• Recovery GPH = Boiler BTU ÷ (Temp Rise × 8.25)
• Minimum Tank Capacity ≈ 70–75% of EPHD

This approach balances stored water and boiler recovery, ensuring both steady performance and efficiency.

Example Calculation

Let’s say you have:

• 3 people in your home
• 1 master bathroom and 1 standard bathroom
• 1 jetted tub
• A boiler with 100,000 BTU/hr recovery
• 70°F temperature rise

The calculator would estimate:

• Peak Hourly Demand (EPHD): ~65 gallons/hour
• Minimum Recommended Tank Capacity: ~50 gallons

This means a 50-gallon indirect tank should comfortably meet your family’s hot water needs when paired with the boiler.

Benefits of Using the Calculator

• Fast and simple – No complicated math needed.
• Custom recommendations – Adjusts for household size, fixtures, and system specs.
• Energy-conscious – Prevents oversizing that wastes energy.
• Peace of mind – Confidence that your system can handle peak demand.

Tips for Sizing Your Indirect Water Heater

1. Plan for future demand – If you expect your family to grow, size slightly larger.
2. Consider luxury fixtures – Large tubs, multiple showers, and spas require extra capacity.
3. Check boiler performance – Your boiler’s BTU rating and recovery rate matter as much as tank size.
4. Consult local codes – Always confirm sizing with a licensed contractor and follow regulations.