Indirect Water Heater Sizing Calculator

Estimate the right indirect storage tank by combining peak mixed-water demand, usable hot storage, and how much of your boiler can recover the tank during the busiest draw window.

📋Preset scenarios

Each preset loads a realistic indirect water heater situation, then runs the sizing math so you can compare smaller tanks, higher-output coils, and domestic-priority boiler setups quickly.

🔧Indirect sizing inputs

The calculator sizes to mixed gallons delivered at the fixture temperature. It converts boiler recovery into hot gallons at storage temperature, then compares that to the stored hot water your indirect tank can actually deliver.

Unit system Switch between gallons and liters, BTU/h and kW, plus °F and °C with live conversion.

Tank and coil package Profile sets drawdown efficiency, coil capture, and a practical boiler-output band.

Expected occupants Use normal peak occupancy, not occasional parties.

Bathrooms served More baths usually means more overlapping fixture draws.

Simultaneous showers Count the showers that can realistically run at once.

Shower flow rate (gpm) Use a real fixture flow rate if you know it.

Peak draw window (minutes) Morning peaks are often tighter than a full first hour.

Tub fill during peak (gal) If no tub fills in the peak window, leave this at zero.

Dishwasher hot-water load (gal) Enter only the hot-water portion used during the peak window.

Laundry hot-water load (gal) Warm or hot cycles can stack with showers on busy mornings.

Incoming water temperature (°F) Use winter inlet temperature for conservative sizing.

Tank storage temperature (°F) Indirect tanks often store hotter water and mix down at fixtures.

Mixed delivery temperature (°F) Fixture delivery temperature changes the usable mixed-gallon multiplier.

Boiler output available to DHW (BTU/h) Enter boiler output in the selected unit system before domestic share is applied.

Domestic priority share (%) Use 100% if the boiler gives full priority to the indirect tank.

Candidate indirect tank volume (gal) Check the exact tank size you are considering against the peak draw.

Design buffer (%) Add margin for overlap, short bursts, or less predictable schedules.

Indirect sizing summary

Run the calculator to compare required storage, boiler recovery, and how your selected indirect tank covers the busiest draw window.

Awaiting inputs

Recommended indirect size

53 gal Rounded from the calculated nominal storage need.

Boiler output needed

110k BTU/h Required to make the candidate tank cover the peak.

Estimated first-hour delivery

78 gal Mixed gallons the candidate tank can deliver.

Peak-hour margin

+8 gal Positive means the candidate tank clears the design peak.

Calculation breakdown

Peak mixed draw

Diversity and buffer factor

Mixing multiplier

Stored hot gallons required

Boiler recovery during peak

Usable hot storage from candidate tank

Recovery at mixed delivery temp

Sizing decision

📦Indirect device and coil profile grid

These profile bands are built into the calculator so the storage recommendation reflects realistic drawdown efficiency and heat-exchanger capture rather than nominal tank gallons alone.

0.78

Compact single-coil

Lean drawdown factor for smaller steel tanks. Best where peak draws are short and the boiler is dedicated to domestic recovery.

0.88

High-output smooth coil

Improved coil capture lifts recovery, so a 53 gallon tank can often perform like a larger standard indirect in the same draw window.

0.90

Dual-coil indirect

Higher usable storage and broad heat-transfer surface help when tubs, showers, and laundry overlap during the first half hour.

0.96

Plate-assist package

Fast boiler-side transfer supports smaller tanks with larger boilers, especially when domestic priority is enabled and heating zones can pause.

📊Reference tables

Use the tables below to sanity-check the recommendation against fixture mix, boiler recovery, storage temperature, and common residential indirect sizing targets.

Peak mixed draw guide

Household	Peak mix	Indirect	Notes
1 bath condo	35-45 gal	40 gal	Single shower with light appliance use
2 bath family	60-75 gal	53-60 gal	Two showers plus dishwasher overlap
Tub primary suite	85-100 gal	70-80 gal	Tub fill added to normal shower demand
4 bath whole home	110-130 gal	100-119 gal	Large loads need tank and boiler matched

Boiler recovery at 145 °F storage

Net DHW input	Hot gal/h	Mixed gal/h	Typical fit
70k BTU/h	74	108	Compact indirect
100k BTU/h	105	153	53 gal standard
140k BTU/h	147	214	80 gal with tub loads
180k BTU/h	189	275	100 gal large family

Storage temperature mixing benefit

Storage	Mix factor	60 gal output	Use case
130 °F	1.29	77 gal	Lower storage, less lift
140 °F	1.57	94 gal	Common mixed-valve target
145 °F	1.71	103 gal	Strong indirect balance point
150 °F	1.86	112 gal	Higher usable gallons, watch scalding

Common sizing checkpoints

Project	Peak load	Tank	Boiler
Starter home	42 gal	40 gal	80-90k
Family colonial	72 gal	53-60 gal	100-120k
Suite plus tub	96 gal	80 gal	130-150k
Large four bath	120 gal	100-119 gal	160-180k

💡Sizing tips

Size to winter inlet temperature

Indirect tanks look stronger on paper when the inlet water is warm. Use the coldest expected inlet temperature so the mixing multiplier and boiler recovery stay realistic all year.

Check tank volume and boiler share together

A larger indirect tank will not fix a boiler that cannot recover during the draw window. If the margin is negative, compare a larger tank with more domestic priority before choosing either one alone.

Indirect water heater is device that employs heating system of the home for warm water. You know them also as sidearm tanks. They do not depend on own burner but take heat from already existent installation for instance furnace or boiler.

The furnace heats liquid that circulates through coil of heat exchanger in separate water tank. Usually deal about water or mix with antifreeze. The furnace water does not mingle with the water of the tank but only pass through the coils for warm it for domestic use.

How Indirect Water Heaters Work and Why They Save Energy

Base the furnace execute everything during the indirect heater only help heat switch.

Using such tanks you can reach big efficiency. The water in the tank preserves heat so the furnace rarely must light what spares energy. Because the tank keeps the heat like this well it answers for families with high need of warm water and give whirlwind delivery.

If you do not have problems with warm water here change to apt indirect unit probably will provide more warm water no less. Furnaces have usually more than 40000 BTU comparatively to typical direct gas water heater so the coil deliver much more of heat to the domestic water supply.

For such systems exist various variants. For example the PowerStor Series of single wall indirect water heaters have single wall heat exchanger what gives faster heat delivery and water flow. Other models use superb stainless steel for optima heat transfer and long service life of the tank.

Well installed indirect unit can serve 20 until 30 years during electric water heater commonly only 10. They coincide with furnaces of any fuel. That is good solution for those that requires much warm water and use oil for heat.

Well isolated indirect heater can reach almost the efficiency of tankless system while it costs less for installation and attention.