Best thermal expansion tank for a tank water heater: Amtrol ST-5 and ST-12, Watts PLT-12, Zurn XT and the State/Reliance TW compared on the makers' own spec sheets
Amtrol ST-5 for a 40 to 50 gal heater (2.0 gal, 50 psig precharge, 7-year warranty), ST-12 for 60 to 80 gal, Watts PLT-12 when IAPMO and NSF 61 must be printed.
- Verified
- Updated
- 12 min read
- 7 sources
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Short answer
The best thermal expansion tank for a 40 or 50 gallon tank water heater is the Amtrol Therm-X-Trol ST-5: Amtrol’s sizing card assigns it to those heaters at 140 F on 40, 60 and 80 psig supply, and prints 2.0 gallons volume, 0.9 gallon acceptance, 150 psig, a 50 psig factory precharge and a 7-year limited warranty. A 60 or 80 gallon heater, or a 50 gallon heater at 160 F on 60 psig or more, moves to the 4.4 gallon ST-12. When the certifier has to be printed on the maker’s own sheet, the pick is the Watts PLT-12: Watts’ ES-PLT prints Listed by IAPMO, certified to ANSI/NSF 61, 4.5 gallons and a 20 psi precharge. Every tank here is 3/4 in and 150 psi, and Amtrol, Zurn and Reliance set the air precharge to the house’s static supply pressure. Skip all of them until a gauge gives you that number, and skip them above 80 psi: Zurn calls for a pressure reducing valve first.
The picks
A 40 or 50 gal heater set to 140 F, on supply up to 80 psi
Amtrol Therm-X-Trol ST-5 Expansion Tank, 2.0 gal, 3/4 in NPTM
Amtrol sizing card MC8520: 2.0 gal volume, 0.9 gal maximum acceptance, 150 psig, 200 F, 50 psig factory precharge, 8 in x 13 in; submittal MC4400: butyl diaphragm NSF/ANSI 61, 7-year limited warranty
A 60 or 80 gal heater, or a 40 to 50 gal heater set to 180 F
Amtrol Therm-X-Trol ST-12 Expansion Tank, 4.4 gal, 3/4 in NPTM
Amtrol sizing card MC8520: 4.4 gal volume, 3.2 gal maximum acceptance, 150 psig, 200 F, 50 psig factory precharge, 11 in x 15 in; submittal MC4400: butyl diaphragm NSF/ANSI 61, 7-year limited warranty
Wants the certifier and the standard printed on the maker's own sheet
Watts PLT-12 Potable Water Expansion Tank, 4.5 gal, 3/4 in male
Watts ES-PLT: 4.5 gal, 150 psi, 200 F, 20 psi factory precharge, 10.5 in x 13.5 in, Listed by IAPMO and certified to ANSI/NSF 61, wetted surface under 0.25% lead; acceptance 2.88 gal at 40 psi precharge, 2.49 at 60, 1.85 at 80
A heater up to 100 gal, from the water heater brands, with a 1-year warranty
American Water Heater / State / Reliance TW 12-1 Expansion Tank, 4.8 gal, 3/4 in NPT
State TW spec sheet: 4.8 gal, 2.19 gal acceptance at 60 psi, 150 psig, 200 F, 3/4 in NPT, 10.6 in x 13.7 in, 8.2 lb, rated for heaters up to 100 gal, one-year limited warranty; Reliance brochure: 38 psi factory precharge. No certifier or standard printed
If the T&P valve drips at all hours and a gauge reads a steady 85 psi, this is the wrong page: that is supply pressure, not expansion, and the 80 psi guide comes before any tank.
How was this shortlist built?
From the makers’ spec sheets, not from the boxes. Amtrol’s June 2024 sizing card and January 2024 submittal, Watts’ ES-PLT, Zurn’s XT sheet, State’s TW sheet and Reliance’s brochure were opened on 10 October 2026 and compared on volume, acceptance volume, factory precharge, printed certification, warranty and each maker’s own sizing chart. Nothing was installed or pressurised.
Four tanks are linked; the Watts PLT-5 and both Zurn XT sizes were listed as scarce on the day, so they sit in the table unlinked. Flexcon, SharkBite and Cash Acme are absent: no potable expansion tank spec sheet from any of them could be found this session.
Whether the house needs a tank at all is the thermal expansion tank guide. The arithmetic behind the size is the expansion tank size calculator, and every cross-fit by capacity class is on the cross-reference page.
How do the tanks compare on paper?
A thermal expansion tank is a small vessel with an air cushion behind a diaphragm that accepts the water a heater expands as it warms. Acceptance volume is the water the diaphragm can take, and it is the figure that does the work, not the gallon number on the label.
| Tank | Volume | Acceptance | Connection | Factory precharge | Max pressure | Certification printed | Warranty |
|---|---|---|---|---|---|---|---|
| Amtrol ST-5 | 2.0 gal | 0.9 gal | 3/4 in NPTM | 50 psig | 150 psig, 200 F | Diaphragm NSF/ANSI 61; no certifier named | 7 years |
| Amtrol ST-12 | 4.4 gal | 3.2 gal | 3/4 in NPTM | 50 psig | 150 psig, 200 F | Diaphragm NSF/ANSI 61; no certifier named | 7 years |
| Watts PLT-5 | 2.1 gal | 1.48 gal at 20 psi; 1.0 at 60; 0.8 at 80 | 3/4 in male | 20 psi | 150 psi, 200 F | Listed by IAPMO, ANSI/NSF 61; under 0.25% lead | Not on the ES-PLT |
| Watts PLT-12 | 4.5 gal | 3.42 gal at 20 psi; 2.49 at 60; 1.85 at 80 | 3/4 in male | 20 psi | 150 psi, 200 F | Listed by IAPMO, ANSI/NSF 61; under 0.25% lead | Not on the ES-PLT |
| Zurn Wilkins XT-8 | 2.1 gal | 1.0 gal | 3/4 in NPT | 40 psi | 150 psi relief; 80 psi max supply | IAPMO Listed; WQA certified NSF/ANSI/CAN 61 and NSF/ANSI 372 | Not on the sheet |
| Zurn Wilkins XT-18 | 4.8 gal | 2.4 gal | 3/4 in NPT | 40 psi | 150 psi relief; 80 psi max supply | IAPMO Listed; WQA certified NSF/ANSI/CAN 61 and NSF/ANSI 372 | Not on the sheet |
| State/Reliance TW 5-1 | 2.1 gal | 1.03 gal at 60 psi | 3/4 in NPT | 38 psi (Reliance) | 150 psig, 200 F | None printed | 1 year (5 on the TW 5-5) |
| State/Reliance TW 12-1 | 4.8 gal | 2.19 gal at 60 psi | 3/4 in NPT | 38 psi (Reliance) | 150 psig, 200 F | None printed | 1 year (5 on the TW 12-5) |
Volumes, acceptance figures, precharges and pressure limits are the makers’ published figures. Watts prints acceptance as a table by air-side precharge (20, 40, 60 and 80 psi), which is why the PLT rows carry three numbers: acceptance falls as the precharge rises. State prints its TW acceptance at 60 psi only, and Amtrol and Zurn print one maximum figure each.
Read down the precharge column before anything else. Amtrol’s manual says to adjust the air precharge, with the tank empty of water, to match the cold supply pressure; Zurn and Reliance say the same and cap it at 80 psi. A house at 65 psi raises an Amtrol from 50 and a Watts from 20, so neither works out of the box.
Which tank does each maker’s chart assign to your heater?
Each chart runs on its own assumptions, so the same heater can land on a different model from a different maker. Cells are copied from the sheets, with each chart’s assumptions beside them.
| Chart and its basis | 40 gal heater | 50 gal | 60 gal | 80 gal | 100 to 120 gal |
|---|---|---|---|---|---|
| Amtrol MC8520 at 140 F (40 F incoming, 150 psi T&P, precharge equal to supply) | ST-5 at 40, 60, 80 psig | ST-5 at 40, 60, 80 psig | ST-12 at 40, 60, 80 psig | ST-12 at 40, 60, 80 psig | 120 gal: ST-12 at 40 and 60, ST-25V at 80 psig |
| Amtrol MC8520 at 160 F | ST-5 at 40 and 60; ST-12 at 80 psig | ST-5 at 40; ST-12 at 60 and 80 | ST-12 at 40, 60, 80 psig | ST-12 at 40 and 60; ST-25V at 80 | 120 gal: ST-12 at 40; ST-25V at 60 and 80 |
| Zurn WXT-XT (100 F rise, 40 to 140 F, 150 psi max tank pressure) | XT-8 to 70 psig; XT-18 at 80 | XT-8 to 60 psig; XT-18 from 70 | XT-8 to 55 psig; XT-18 from 60 | XT-18 at 80 psig; lower columns not captured | 100 gal: XT-35 at 80 psig |
| State TW sheet (rating only, no pressure columns) | TW 5-1 (to 52 gal) | TW 5-1 (to 52 gal) | TW 12-1 (to 100 gal) | TW 12-1 (to 100 gal) | TW 12-1 (to 100 gal) |
| Watts ES-PLT (150 psi relief, 50 F rise) | Chart is a colour graphic that did not extract; no assignment printed here |
Two things fall out of the table. Set temperature moves the answer as much as heater size does: Amtrol keeps a 50 gallon heater on the ST-5 at 140 F and moves it to the ST-12 at 160 F on 60 psig. The water heater temperature guide covers what that setting should be.
And supply pressure moves it the other way: Zurn’s chart keeps a 60 gallon heater on the 2.1 gallon XT-8 at 55 psig and needs the XT-18 at 60 psig. That is the number a gauge has to give you before the tank is ordered.
Amtrol ST-5: buy it or skip it
Buy the Amtrol ST-5 for a 40 or 50 gallon heater set to 140 F: Amtrol’s card assigns it at every pressure column it prints, from 40 to 80 psig, and Amtrol’s submittal gives it a 7-year limited warranty, the longest printed on this page.
The flaw is on the card: 0.9 gallon acceptance is below the 1.0 gallon printed for the Zurn XT-8 and the 1.03 gallons for the TW 5-1 at 60 psi. Amtrol’s own chart accounts for that, so inside the chart it is fine; outside it, at 160 F on 80 psig, Amtrol moves the same heater to the ST-12.
Skip it if the heater is set above 140 F, if it is 60 gallons or larger, or if the space above the heater is under 13 inches: the ST-5 is the tallest 2 gallon tank here. Amtrol’s in-line ST-5 and ST-12 carry an InSight indicator cap, and the manual says a discoloured tip means the tank is to be replaced.
Amtrol ST-12: buy it or skip it
Buy the Amtrol ST-12 for a 60 or 80 gallon heater at 140 F, which Amtrol’s card puts on the ST-12 at 40, 60 and 80 psig, or for a 40 or 50 gallon heater at 180 F, which the card puts on the ST-12 at every pressure.
Its 3.2 gallon acceptance is above the figures the others print at 60 psi: Watts prints 2.49 gallons for the PLT-12 at 60 psi, Zurn 2.4 for the XT-18 and State 2.19 for the TW 12-1 at 60 psi.
Skip it if the chart says ST-5: a bigger tank costs more and buys nothing inside Amtrol’s chart. Skip it at 11 inches across and 15 inches tall if the tank has to hang from the cold line in a tight closet; the Watts PLT-12 is 10.5 by 13.5 inches. The listing title on Amazon is a distributor code, so check that the page says ST-12 before paying.
Watts PLT-12: buy it or skip it
Buy the Watts PLT-12 when the inspector, the landlord or you want the certifier and the standard on the same page: Watts’ ES-PLT prints Listed by IAPMO, certified to ANSI/NSF 61 and a wetted surface under 0.25% lead for the PLT-5, PLT-12 and PLT-20.
The flaw is the 20 psi factory precharge, the lowest here. Every PLT needs air added before the water goes on, and Watts prints the cost of skipping that step: 3.42 gallons acceptance at 20 psi becomes 2.49 at 60 and 1.85 at 80. The ES-PLT prints no warranty line, so this page prints none.
Skip it if the house is above 80 psi: Watts’ acceptance table stops at an 80 psi air side, and the PRV and gauge review lists both.
State and Reliance TW 12-1: buy it or skip it
Buy the TW 12-1 , sold under the American Water Heater name with the State and Reliance spec table, for a heater up to 100 gallons from the people who make the heater: State prints 4.8 gallons, 2.19 gallons acceptance at 60 psi, 150 psig and 200 F.
Two flaws, both printed. The warranty is one year on the -1 model (State gives the TW 12-5 five years), against Amtrol’s 7. And neither State’s sheet nor Reliance’s brochure names a certifier or a standard number, which means only that the documents do not say. The Amazon listing title calls it 5 gallons; State’s sheet says 4.8.
Skip it if the house is above 80 psi: Reliance says to match the inlet pressure but not above 80 psi. Reliance also prints the 38 psi factory precharge that State’s sheet leaves out, so a 60 psi house adds 22 psi before the tank sees water.
Before you buy any of the four
Put a gauge on a hose bib with no water running and read the static pressure; the PRV and gauge review lists gauges with a recording hand. That one reading picks the column in every chart above, sets the precharge on whichever tank arrives, and tells you whether a tank is the wrong purchase altogether: above 80 psi, Zurn wants a pressure reducing valve first and Reliance caps the precharge at 80.
What goes wrong?
| Failure | What it costs | What the sheets say |
|---|---|---|
| Tank fitted at factory precharge | No chart holds for a charge it did not assume: Watts prints 3.42 gal acceptance at 20 psi air side and 1.85 at 80 | Amtrol: set precharge to cold supply pressure with the tank empty; Zurn and Reliance cap it at 80 psi |
| Precharge never rechecked | Air bleeds off over years and the tank stops accepting water | Amtrol: check yearly, more often as the system ages, with the tank isolated and empty of water |
| Corroded or waterlogged tank adjusted instead of replaced | A diaphragm that has failed cannot be re-pressurised | Amtrol: replace, do not adjust, if corroded; a discoloured InSight tip means replacement |
| Tank placed on the wrong side of the one-way device | Expanded water still has nowhere to go | Amtrol: install on the cold supply between the water heater and the backflow preventer or other one-way device |
A tank that has stopped taking water shows up as the relief valve dripping after each heating cycle, and the T&P valve dripping guide separates that from a valve that has simply failed. The anode rod page covers the part usually changed on the same visit.
Frequently asked questions
Which expansion tank do I need for a 50 gallon water heater?
On Amtrol's sizing card, a 50 gallon heater at 140 F takes the 2.0 gallon ST-5 at 40, 60 and 80 psig. At 160 F the same card moves it to the ST-12 at 60 and 80 psig. Zurn's chart, built on a 100 F rise, keeps a 50 gallon heater on the XT-8 up to 60 psig and moves it to the XT-18 at 70 psig.
Which expansion tank do I need for an 80 gallon water heater?
A 4.4 to 4.8 gallon tank. Amtrol's card puts an 80 gallon heater at 140 F on the ST-12 at 40, 60 and 80 psig, and at 160 F keeps it on the ST-12 to 60 psig, then the ST-25V at 80. State rates the TW 12-1 for heaters up to 100 gallons. Zurn's chart keeps an 80 gallon heater on the XT-18 at 80 psig supply.
What should the precharge be set to, and does the factory setting matter?
Amtrol, Zurn and Reliance say to set the air side to the house's static supply pressure. The factory setting only tells you how far to adjust: Watts ships the PLT at 20 psi, Reliance the TW at 38, Zurn the XT at 40 and Amtrol the ST-5 and ST-12 at 50 psig. Zurn and Reliance cap the precharge at 80 psi.
Which thermal expansion tanks are NSF 61 certified?
Only two sheets opened here print a certifier and a standard together. Watts' ES-PLT says the PLT-5, PLT-12 and PLT-20 are Listed by IAPMO and certified to ANSI/NSF 61. Zurn's XT sheet says IAPMO Listed and Water Quality Association certified to NSF/ANSI/CAN 61 and NSF/ANSI 372. Amtrol prints NSF/ANSI 61 on the diaphragm without a certifier; State and Reliance print neither.
How long does a thermal expansion tank last?
No maker sheet opened here prints a service life. The warranties are the only durability figure the makers commit to: Amtrol gives the ST-5 and ST-12 a 7-year limited warranty, State gives the TW 12-1 one year and the TW 12-5 five years. Amtrol's manual says to check the precharge yearly with the tank isolated and empty, and to replace a corroded tank rather than adjust it.
Fact sheet and how to cite this page
- Answer
- For a 40 or 50 gallon heater at 140 F the pick is the Amtrol ST-5: Amtrol’s sizing card assigns it at 40, 60 and 80 psig and prints 2.0 gal volume, 0.9 gal acceptance, 150 psig and a 50 psig precharge. A 60 or 80 gallon heater moves to the 4.4 gal ST-12. The Watts PLT-12 is the pick when a certifier must be printed: Listed by IAPMO, ANSI/NSF 61, 20 psi precharge.
- Comparison basis
- Amtrol Therm-X-Trol quick sizing card MC8520 (06/24), submittal sheet MC4400 (01/24) and installation manual 9015-942 (04/23); Watts engineering specification ES-PLT; Zurn Wilkins Model XT spec sheet WXT-XT Rev. E (01/22); State Water Heaters TW series spec sheet (November 2018); Reliance pump and expansion tank brochure RPTBRO0624. Compared on volume, acceptance volume, connection, factory precharge, maximum pressure, printed certification, warranty years and each maker’s own sizing chart.
- Assumptions
- A residential storage water heater on a closed supply with a 150 psi T&P relief valve, which is the basis every maker’s sizing chart prints
- The air precharge is set to the house’s static supply pressure with the tank empty of water before installation, as the Amtrol, Zurn and Reliance sheets instruct; a tank left at its factory charge does not perform to its chart
- Sizing charts use different assumptions (Amtrol 40 F incoming water, Zurn a 100 F rise from 40 to 140 F, Watts a 50 F rise), so the same heater can land on different models from different charts
- A certifier and a standard number are attributed to a tank only where the maker’s own sheet prints both; the NSF and IAPMO listing databases were not opened this session
- Method
- OwnerSpec compared manufacturer-published specifications read on 2026-10-10 and confirmed each listed tank in stock on Amazon the same day. It installed, pressurised and measured nothing, prints no price, and reports only the figures the cited document states. Watts’ PLT heater-size selection chart is a colour graphic that did not extract as text, so no PLT heater-size assignment is printed.
- Primary source
- Amtrol, Therm-X-Trol Quick Sizing Charts and specifications, MC8520 (06/24) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Best thermal expansion tank for a tank water heater: Amtrol ST-5 and ST-12, Watts PLT-12, Zurn XT and the State/Reliance TW compared on the makers' own spec sheets", https://ownerspec.com/plumbing/reviews/best-thermal-expansion-tank/, facts verified 10 October 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Amtrol, Therm-X-Trol Quick Sizing Charts and specifications, MC8520 (06/24) (manufacturer or vendor document)
- Amtrol, Thermal Expansion Tanks for Potable Water installation and operation instructions, 9015-942 (04/23) (manufacturer or vendor document)
- Amtrol, Therm-X-Trol ST Series submittal sheet, MC4400 (01/24) (manufacturer or vendor document)
- Watts, Series PLT potable water expansion tanks engineering specification, ES-PLT (manufacturer or vendor document)
- Zurn Wilkins, Model XT thermal expansion tank specification sheet, WXT-XT Rev. E (01/22) (manufacturer or vendor document)
- State Water Heaters, TW Series thermal expansion tank specification sheet (November 2018) (manufacturer or vendor document)
- Reliance Water Heaters, pump and expansion tanks brochure, RPTBRO0624 (manufacturer or vendor document)