💧 System Water Volume
gal

Total water in boiler, pipes, radiators, and coils

🌡️ Temperature Settings
50°F

Temperature when system is filled cold

180°F

Max boiler/system operating temperature

📊 Pressure Settings
PSI

Typical residential: 12–15 PSI

PSI

Rating stamped on your relief valve

PSI

Usually same as fill pressure (set at factory)

1.25×

1.25× is standard. Use 1.5× for older systems.

⚙️ System Options
🏠 Include Boiler Water Volume
🔄 Open vs Closed System
— Estimated tank size

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Running ASME calculations…

📊 Your Expansion Tank Results
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Minimum Expansion Tank Size Required
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gallons · We recommend buying the next standard size up
💧
—
gal
Water Expansion
📊
—
%
Acceptance Ratio
📉
—
PSI
Max System Pressure
🔢
—
%
Expansion Factor
BOILER 180°F EXP TANK — gal RELIEF — PSI Heating System 30 PSI

How it works

When water heats up, it expands. The expansion tank absorbs that extra volume. Without it, pressure spikes open the relief valve — or worse, damage your boiler.

🛢️ Standard Tank Sizes — Best Fit

Tank Model Size
Volume
Acceptance
Fit
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🔧 Installation Recommendations

What Is an Expansion Tank? A Simple Explanation

Water expands when it heats up. Your heating system needs somewhere to put that extra volume. That’s exactly what an expansion tank does.

💧

Why Water Expands

Cold water at 50°F and hot water at 180°F take up different amounts of space. The hot water takes up about 3.6% more volume. In a sealed system, that extra volume has nowhere to go — pressure builds up fast.

🛢️

What the Tank Does

The expansion tank has a rubber diaphragm inside. Air sits on one side. Water enters the other side. When water expands, it compresses the air. Pressure stays safe. The relief valve stays closed.

⚠️

What Happens Without One

Without an expansion tank in a closed system, pressure spikes every time your boiler fires. The relief valve drips water. Over time, it damages the valve. In worst cases, it damages your boiler or pipes.

📐

Sizing Matters a Lot

Too small and the tank doesn’t absorb enough expansion — pressure still spikes. Too large wastes money and takes up space. The ASME formula gives you the exact minimum size. We run that calculation for you.

🔩

Pre-Charge Pressure

The tank comes with air pre-charged from the factory — usually 12 PSI. This must match your fill pressure. If it doesn’t, the tank doesn’t work correctly. Check it with a tire pressure gauge. Adjust if needed.

🔄

When to Replace It

A waterlogged tank is heavy and feels full of water when you knock on it. It means the diaphragm has failed. The tank cannot be repaired — replace it. This usually happens every 5–15 years depending on water quality.

📐 The ASME Formula We Use

Vt = (Vs × Ef) / (1 - Pa/Pf) Acceptance Ratio = 1 - (Pa / Pf)

Vt = Tank volume required (gallons)
Vs = System water volume (gallons)
Ef = Expansion factor (from water density tables)
Pa = Absolute fill pressure = fill PSI + 14.7
Pf = Absolute maximum pressure = (relief PSI × 0.9) + 14.7

This is the standard ASME method used by licensed mechanical engineers. The 0.9 factor gives a 10% safety margin below the relief valve set point.

Water Density & Expansion Factor Reference

These values come from ASHRAE and engineering water tables. Use them to verify your calculation.

Cold Fill TempMax TempExpansion Factor (Ef)Volume IncreaseRisk Without Tank
50°F (10°C)120°F (49°C)0.006950.70%Low
50°F (10°C)140°F (60°C)0.01521.52%Moderate
50°F (10°C)160°F (71°C)0.02682.68%Moderate
50°F (10°C)180°F (82°C)0.03603.60%High
50°F (10°C)200°F (93°C)0.04624.62%Very High
50°F (10°C)220°F (104°C)0.05705.70%Very High

Frequently Asked Questions

No. Open systems have an open overflow pipe that lets water expand out safely. But most modern heating systems and closed-loop plumbing systems are sealed (closed). If you have a pressure-reducing valve, check valve, or backflow preventer on your water supply, you almost certainly have a closed system — and you need an expansion tank.
The easiest method: drain your system into measured containers and count the gallons. If that’s not practical, estimate it. A typical residential hydronic system holds about 1–2 gallons per 1,000 BTU/hr of boiler output. Add boiler capacity (often printed on the label), pipe volume (about 0.04 gal/ft for ½” pipe), and radiator or baseboard volume. When in doubt, use a slightly higher number — oversizing the tank is always safer than undersizing.
Very likely, yes. A dripping relief valve is the most common sign of an undersized or failed expansion tank. First check if the tank is waterlogged (knock on it — if it sounds like a full water bottle, the diaphragm has failed). Then verify the pre-charge pressure matches your fill pressure. If both are fine, your tank may simply be undersized. Use this calculator to confirm the correct size.
Set the pre-charge to match your cold fill pressure — the pressure at which you fill the system. For most homes this is 12–15 PSI. Check the pre-charge with the tank disconnected from the system (or with the system pressure bled down to zero). Use a standard tire pressure gauge on the Schrader valve on top of the tank. Adjust with a bicycle pump or air compressor.
Install on the return line, on the suction side (inlet) of the circulator pump. This keeps the pump from pushing against the expansion tank. The tank can be mounted horizontally or vertically — just make sure the connection port faces the correct direction for your model. Install an isolation ball valve so you can replace the tank without draining the whole system.
No. Heating systems use a non-potable rated expansion tank. Domestic hot water needs a potable-water rated expansion tank (NSF 61 approved). They look similar but are made of different materials inside. Mixing them up is a code violation and a potential health risk. Use this calculator’s Potable Water tab for domestic hot water systems.
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