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Thermal expansion tank size calculator
A 50 gallon water heater heated to 120°F holds 0.50 gallons more water by volume than it did cold, per AMTROL's expansion table. Behind a check valve or a pressure reducing valve that water has nowhere to go, and the pressure climbs to the relief valve. On a 60 psi supply with a 125 psi limit, an expansion tank of at least 1.07 gallons, precharged to 60 psi, takes it; held to 80 psi, as the 2018 and later IPC asks, it takes 2.37 gallons. This page works that out for any storage heater with AMTROL's method and shows the Watts and IPC limits beside it. A tankless heater usually needs no tank; the tankless water heater calculator sizes those.
- Enter the storage gallons of every heater behind the check valve or pressure reducing valve, and pick the thermostat temperature.
- Enter the static supply pressure from a gauge and the maximum pressure the system may reach, well under the relief valve setting.
- The result shows the expanded water, the least total tank volume, the air precharge, and the tanks for the Watts limit and the 80 psi of the IPC. Pick a model whose total and acceptance volumes both meet them.
Example result for the default inputs
50 gal of storage heated to 120°F, 60 psi static supply, 80 psi maximum. AMTROL method, IPC 607.3.
Expanded water
0.50 gal
Least tank, 80 psi max
2.37 gal
Air precharge
60 psi
| Expansion factor | 0.0100 at 120°F, AMTROL Table 1 (water starting at 40°F) |
|---|---|
| Expanded water | 50 gal x 0.0100 = 0.50 gal. The tank's acceptance volume on the maker's chart has to be at least this. |
| Design pressure factor | (80 + 14.7) / (80 - 60) = 4.735. The air side takes 21.1 percent of the tank as water before the pressure reaches 80 psi. |
| Least tank volume | 0.50 gal x 4.735 = 2.37 gal total volume |
| Air precharge | 60 psi, equal to the static supply pressure, set before the tank is installed. Tanks ship at 40 psi. |
| Watts limit (supply + 50%) | The supply plus 50 percent is 90.0 psi, not below your 80 psi limit, so your limit is the lower one. |
| IPC limit (80 psi) | Your 80 psi limit is within the 80 psi of IPC 604.8, which 607.3 (2018 and later) holds the system to. |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the tank volume is worked
Water grows as it heats. On an open system the extra water pushes back into the street main, but a check valve, a backflow preventer or a pressure reducing valve closes that path, and the pressure climbs until the relief valve opens. A thermal expansion tank holds an air cushion behind a diaphragm. The tank has to take all the expanded water before the pressure reaches the limit you set, and Boyle's law gives the tank volume that does it.
- Expanded water
Ve = heater gallons x expansion factorThe factor is from AMTROL's Table 1, for water starting at 40°F: 0.0062 at 100°F, 0.01 at 120°F, 0.0124 at 130°F, 0.015 at 140°F, 0.0179 at 150°F, 0.0209 at 160°F, 0.0242 at 170°F, 0.0276 at 180°F. A 50 gal heater at 120°F expands 0.50 gal. The tank's acceptance volume on the maker's chart has to be at least Ve.- Design pressure factor
DPF = (Pmax + 14.7) / (Pmax - Pline)AMTROL's equation for the cells of its Table 2, with pressures in psi on a gauge. With 60 psi on the line and 125 psi allowed, DPF is 2.149, which the table prints as 2.1. 1 / DPF is the share of the tank the water can take, here 46.5 percent.- Tank volume
Vt = Ve x DPFIn AMTROL's worked example, 240 gal at 150°F gives 4.296 gal, printed as 4.3, and 4.3 x 2.1 = 9.0 gal. This page does not round between the steps, so it gives 9.23 gal. Either way a tank with at least that total volume and at least 4.3 gal of acceptance meets it.- Air precharge
precharge = static supply pressureAMTROL says "The Therm-X-Trol air pressure should be equal to static line pressure." Tanks ship at 40 psi. Watts says never to precharge above 80 psi.- Watts limit
Pmax = 1.5 x PlineWatts' potable sizing calculator sets the final pressure at the supply plus 50 percent and keeps it under 85 percent of the T&P relief rating. At 60 psi that is 90 psi, and the 50 gal heater at 120°F needs 1.75 gal instead of 1.07 gal. The page shows both.
Seven heaters through the same formula
The first row is AMTROL's worked example, which rounds to 9.0 gallons; the page keeps the unrounded steps. Rows two to seven are common residential heaters with a 125 psi limit. The last two columns hold the final pressure to the supply plus 50 percent, as Watts does, and to 80 psi, as IPC 607.3 does from the 2018 edition; on an 80 psi supply no tank holds the system within 80 psi, so the supply has to come down first.
| Heater and supply | Expanded water, gal | Tank, 125 psi limit, gal | Tank, supply + 50%, gal | Tank, 80 psi (IPC), gal |
|---|---|---|---|---|
| 240 gal at 150°F, 60 psi (AMTROL example) | 4.30 | 9.23 | 15.0 | 20.3 |
| 40 gal at 120°F, 60 psi | 0.40 | 0.86 | 1.40 | 1.89 |
| 50 gal at 120°F, 60 psi | 0.50 | 1.07 | 1.75 | 2.37 |
| 50 gal at 140°F, 60 psi | 0.75 | 1.61 | 2.62 | 3.55 |
| 50 gal at 120°F, 80 psi | 0.50 | 1.55 | 1.68 | lower supply |
| 80 gal at 140°F, 60 psi | 1.20 | 2.58 | 4.19 | 5.68 |
| 120 gal at 160°F, 70 psi | 2.51 | 6.37 | 8.58 | 23.8 |
The same 50 gallon heater needs 1.07 gallons of tank on a 60 psi supply and 1.55 gallons on 80 psi, so read the pressure with a gauge before choosing. Two 40 gallon heaters behind one valve are 80 gallons of storage and need 2.58 gallons at 140°F.
Standards and data sources
- AMTROL, Sizing the Therm-X-Trol (form 9017-112) - the equation Tv = DPF x expanded water, Table 1 expansion factors from 40°F, Table 2 and its DPF equation, the 240 gallon worked example, the precharge rule, and the tank location after the backflow preventer.
- Honeywell (Resideo) Thermal Expansion Absorber, sheet 62-3082 - the same tables and example, the 150 psi relief valve setting with the maximum held at least 10 percent below it, the 40 psi factory precharge, and what happens without a tank on a closed system.
- Watts, Expansion Tank Sizing Calculator (Potable) - the final pressure at the supply plus 50 percent, no more than 85 percent of the T&P relief rating, the 90°F start temperature, the 80 psi limits on supply and precharge.
- IPC 607.3 (2018), as quoted by the North Carolina OSFM (2019 interpretation) - when a tank is required (a storage heater supplied through a check valve, pressure reducing valve or backflow preventer), the rule that the tank is sized so the system pressure does not exceed 604.8, and no device for a tankless heater outside a closed recirculation loop. The office's 2011 interpretation quotes the 2012 wording, 607.3.2.
Limitations of this calculator
- This page gives preliminary numbers for estimating and for checking a quote. The plumber installs the tank under the local plumbing code, and the code official decides what is required. Nothing here approves an installation.
- The tank maker's chart comes first. Pick a model whose total volume and acceptance volume are both at least the numbers here, rated for potable water and for the maximum pressure.
- Table 1 starts the water at 40°F. Watts starts it at 90°F, which gives a smaller volume, so a tank from this page is on the large side of a Watts result at the same pressures.
- Table 1 stops at 100 and 180°F, and the page does not interpolate between its rows. Heaters set above 180°F and booster heaters are outside the page.
- The volume is the storage of the heaters only. AMTROL's method does not add the hot water piping or a recirculation loop, and neither does this page.
- The pressures are static gauge readings. A supply that swings during the day, or a pressure reducing valve that creeps, changes the line pressure the tank is charged to. Check it with a gauge before setting the precharge.
- This is a potable water tank on a domestic water heater. Hydronic heating systems, glycol and boiler expansion tanks are sized from a different set of numbers.
- IPC 607.3 is quoted from the 2018 North Carolina code as the state fire marshal's office prints it. It sizes the tank so the pressure stays within 604.8, which holds building pressure to 80 psi static as adopting jurisdictions print it; the ICC text itself was not opened. The 2012 text (607.3.2) had no pressure figure. The UPC and local amendments differ. Check the code your jurisdiction adopted.
Common mistakes with expansion tanks
- Installing the tank at the factory precharge. Tanks ship at 40 psi. On a 60 psi line the water pushes into the tank before it has heated at all, and with a 125 psi limit the tank takes 34.1 percent of its volume instead of 46.5 percent, about 27 percent less. Set the air to the static supply pressure with a tire gauge before the tank goes in.
- Sizing from the heater gallons alone. A 50 gal heater at 120°F needs 1.07 gal of tank on a 60 psi line and 1.55 gal on an 80 psi line, with the same 125 psi limit. The supply pressure changes the answer as much as the heater does.
- Using the relief valve setting as the limit. Sizing to 150 psi lets the pressure reach the T&P valve on every heating cycle, and it weeps. Honeywell keeps the maximum at least 10 percent below a 150 psi relief setting, and Watts at 85 percent of it, 127.5 psi.
- Putting the tank on the wrong side of the check valve. The tank has to sit on the cold supply between the check valve, backflow preventer or pressure reducing valve and the heater. Upstream of the valve it sees the street main and does nothing for the heater.
- Counting one heater when two share the supply. Two 40 gal heaters behind one pressure reducing valve are 80 gal of storage. At 140°F that is 1.20 gal of expanded water and a 2.58 gal tank, so a 2 gal tank is too small.
- Using a heating system tank on drinking water. Use a tank made for potable water. AMTROL's Therm-X-Trol, for example, has a polypropylene liner and non-ferrous materials for domestic water. A tank for a heating system is a different product; the label says which.
Common questions about expansion tanks
What size expansion tank do I need for a 50 gallon water heater?
At 120°F on a 60 psi supply, the water grows 0.50 gal. With a 125 psi limit the tank needs 1.07 gal of total volume, and with the Watts limit of 90 psi it needs 1.75 gal. At 140°F it is 1.61 gal and 2.62 gal. The smallest residential tanks, such as Honeywell's TX-5, hold 2.0 gal; check the maker's chart for both total and acceptance volume. Held to 80 psi, as the 2018 and later IPC asks, it needs 2.37 gal.
What size expansion tank for a 40 gallon water heater?
At 120°F and 60 psi, 0.40 gal of expanded water and a tank of at least 0.86 gal with a 125 psi limit, or 1.40 gal with the 90 psi Watts limit, and 1.89 gal held to the 80 psi of the 2018 and later IPC. Above 80 psi on the supply, fix the pressure first.
Is a thermal expansion tank required?
Under IPC 607.3, in the 2018 text the North Carolina fire marshal's office quotes: where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion tank shall be connected to its cold water supply, downstream of those valves, and sized so the system pressure does not exceed that required by 604.8 (80 psi). The 2012 text, 607.3.2, asked for a device for controlling pressure in the same case. If you do not know whether the supply has a check valve or backflow preventer, ask the water utility. Local codes can require more.
What should the air pressure in an expansion tank be?
The static supply pressure, set before the tank is installed. AMTROL says the air pressure should equal static line pressure. Tanks ship at 40 psi, and Watts says never to precharge above 80 psi. If the supply is above 80 psi, a pressure reducing valve comes first.
Does a tankless water heater need an expansion tank?
Usually not. IPC 607.3 is written for storage water heaters, and the North Carolina interpretation of the 2018 text says an expansion device is not required for a tankless water heater that is not in a closed loop recirculation system.
What happens if the expansion tank is too small or failed?
The pressure rises past the limit on every heating cycle. Honeywell's sheet warns that on a closed system without a working tank dangerously high pressures could result, or the relief valve will frequently expel water.
Can an expansion tank be too big?
Not for the pressure. The equation gives a least volume, and a larger tank with the same precharge holds the peak pressure lower. With a 100 psi limit instead of 125 psi, the 50 gal heater at 120°F needs 1.43 gal instead of 1.07 gal. The cost, the weight on the piping and the space are the limits.
Why does a tank chart give a different size?
Charts differ in the start temperature and the pressure limit. AMTROL's Table 1 starts the water at 40°F, Watts at 90°F; one chart sizes to 125 psi, another to the supply plus 50 percent, and the 2018 and later IPC to 80 psi. This page shows the AMTROL equation with your limit, the Watts limit and the 80 psi limit side by side.
Does this replace a plumber?
No. This page is for estimating and for checking a quote. A plumber installs the tank, the local code official decides what is required, and the tank maker's chart sets the model.