# Thermal Expansion Tank Cross-Reference: Amtrol ST-5 and ST-12, Watts PLT-5 and PLT-12, Zurn Wilkins XT-8 and XT-18, State and Reliance TW 5-1 and TW 12-1 by Capacity Class > Amtrol ST-5, Watts PLT-5, Zurn XT-8 and TW 5-1 fill the 2-gallon slot; ST-12, PLT-12, XT-18 and TW 12-1 the 4.5. All 3/4 in NPT, 150 psi, precharge 20 to 50 psi. | | | |---|---| | Source | https://ownerspec.com/plumbing/parts/thermal-expansion-tank-cross-reference/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-24 | | Updated | 2026-09-24 | | Facts verified | 2026-09-24 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer A residential thermal expansion tank comes in two slots, and every maker fills both. The 2-gallon class is the Amtrol Therm-X-Trol ST-5 (2.0 gal), Watts PLT-5 (2.1 gal), Zurn Wilkins XT-8 (2.1 gal) and the State or Reliance TW 5-1 (2.1 gal). The 4.5-gallon class is the Amtrol ST-12 (4.4 gal), Watts PLT-12 (4.5 gal), Zurn Wilkins XT-18 (4.8 gal) and the TW 12-1 (4.8 gal). All of them connect on 3/4 in NPT, all carry a 150 psi maximum working pressure, and the State and Reliance TW tables are identical number for number. The spec that differs is the factory precharge: Watts ships the PLT at 20 psi, Reliance the TW at 38 psi, Zurn the XT at 40 psi and Amtrol the ST at 50 psi, and every one of those sheets says to set the air side to the house's static supply pressure before the tank sees water. ## Fact sheet - **Answer:** The 2-gallon potable slot is the Amtrol ST-5 (2.0 gal), Watts PLT-5 (2.1 gal), Zurn Wilkins XT-8 (2.1 gal) and State or Reliance TW 5-1 (2.1 gal); the 4.5-gallon slot is the ST-12 (4.4 gal), PLT-12 (4.5 gal), XT-18 (4.8 gal) and TW 12-1 (4.8 gal). All connect on 3/4 in NPT with a 150 psi maximum working pressure. Factory precharge differs: Watts 20 psi, Reliance 38, Zurn 40, Amtrol 50. - **Applies to:** Amtrol Therm-X-Trol ST-5 and ST-12; Watts PLT-5 and PLT-12; Zurn Wilkins XT-8 and XT-18; State and Reliance TW 5-1 and TW 12-1 - **Compatibility basis:** Amtrol Therm-X-Trol quick sizing card MC8520 (06/24) and submittal sheet MC4400 (01/24), Watts engineering specification ES-PLT (2020) and Thermal Expansion product guide PG-ThermExpansion (2009), Zurn Wilkins spec sheets WXT-XT (Rev. E, 01/22) and WXT-WXTP (Rev. D, 01/22), State Water Heaters TW series spec sheet (November 2018), Reliance pump and expansion tank brochure RPTBRO0624 - **Assumptions:** Cross-fit means the same capacity class on the same 3/4 in NPT connection with a published acceptance volume, not an interchangeable part number. Diameter and height still have to fit the space above the heater.; State's TW spec sheet prints no factory precharge figure. The 38 psi value comes from Reliance's brochure for the tank with the identical specification table, and is attributed to Reliance only.; Watts prints the PLT acceptance volumes (1.26 and 2.8 gal) only in its 2009 product guide; the 2020 ES-PLT sheet prints volume, dimensions and precharge but no acceptance figure.; Zurn's XT sheet and Amtrol's card publish different sizing assumptions (a 100 F rise at 150 psi for Zurn; 40 F incoming, a 150 psi T&P valve and precharge equal to supply pressure for Amtrol), so a size chosen from one maker's chart is not automatically the same model from another. - **Method:** Volume, acceptance volume, connection, diameter, height, weight, maximum pressure and precharge copied row by row from each maker's own spec sheet or brochure, opened this session. Equivalence is by capacity class and connection only. OwnerSpec installed and measured nothing. - **Primary source:** [Amtrol, Therm-X-Trol Quick Sizing Charts and specifications, MC8520 (06/24)](https://www.amtrol.com/wp-content/uploads/2024/06/MC8520-06_24-TXT-Sizing-Card.pdf) (manufacturer or vendor document) - **Facts verified:** 2026-09-24 - **Last updated:** 2026-09-24 - **Canonical URL (cite this):** https://ownerspec.com/plumbing/parts/thermal-expansion-tank-cross-reference/ If you are about to buy the tank with the same gallon figure as the old one and screw it on, the number that will actually bite you is printed on none of the boxes: the precharge. ## Which tank fits the 2-gallon slot? | Model | Tank volume | Max acceptance | Connection | Diameter x height | Weight | Max working pressure | Factory precharge | |---|---|---|---|---|---|---|---| | Amtrol ST-5 | 2.0 gal | 0.9 gal | 3/4 in NPTM | 8 x 13 in | 5 lb | 150 psig | 50 psig | | Watts PLT-5 | 2.1 gal | 1.26 gal | 3/4 in male | 8 x 11 in | 5.5 lb | 150 psi | 20 psi | | Zurn Wilkins XT-8 | 2.1 gal | 1.0 gal | 3/4 NPT | 8 x 10 1/2 in | 5.5 lb | 80 psi max supply, 150 psi relief | 40 psi | | State and Reliance TW 5-1 | 2.1 gal | 1.03 gal at 60 psi | 3/4 in NPT | 7.9 x 11 in | 4.5 lb | 150 psig | Not printed by State; 38 psi per Reliance | Acceptance volume is the water the diaphragm can actually take in, and it is the number to cross-fit on: Watts prints 1.26 gal for the PLT-5, Amtrol 0.9 gal for the ST-5. State rates the TW 5-1 for heaters up to 52 gallons. Amtrol's card puts the ST-5 on 40 and 50 gallon heaters set to 140 F at 40 to 80 psig supply, but a 50 gallon heater set to 160 F on 60 psig moves to the ST-12. Height is where the swap fails: the ST-5 is 13 in tall against 11 in for the PLT-5 and TW 5-1 and 10 1/2 in for the XT-8. ## Which tank fits the 4.5-gallon slot? | Model | Tank volume | Max acceptance | Connection | Diameter x height | Weight | Max working pressure | Factory precharge | |---|---|---|---|---|---|---|---| | Amtrol ST-12 | 4.4 gal | 3.2 gal | 3/4 in NPTM | 11 x 15 in | 9 lb | 150 psig | 50 psig | | Watts PLT-12 | 4.5 gal | 2.8 gal | 3/4 in male | 10.5 x 13.5 in | 10 lb | 150 psi | 20 psi | | Zurn Wilkins XT-18 | 4.8 gal | 2.4 gal | 3/4 NPT | 11 x 13 1/2 in | 10 lb | 80 psi max supply, 150 psi relief | 40 psi | | State and Reliance TW 12-1 | 4.8 gal | 2.19 gal at 60 psi | 3/4 in NPT | 10.6 x 13.7 in | 8.2 lb | 150 psig | Not printed by State; 38 psi per Reliance | State rates the TW 12-1 for heaters up to 100 gallons. State's sheet and Reliance's brochure print the TW 12-1 row to the same decimal, and Amazon lists the same model number under the American Water Heater name: one tank, three badges. The acceptance spread is wider in this slot: Amtrol prints 3.2 gal for the ST-12, Watts 2.8 gal for the PLT-12, Zurn 2.4 gal for the XT-18 and State 2.19 gal at 60 psi for the TW 12-1. The smallest shell here holds the most expandable water. Above this slot the connection changes: Zurn's WXTP-50V is 14 gal total, 5.6 gal acceptance, 1 in FNPT, 16 in across and 21 11/16 in tall at 32 lb. The [expansion tank size calculator](/plumbing/calculators/expansion-tank-size-calculator/) says whether the house needs that step. ## What differs: precharge, listing and pressure limit | Maker | Factory precharge | Instruction on the sheet | Certification printed | |---|---|---|---| | Amtrol ST | 50 psig (ST-5, ST-12); 40 psig (ST-25V up) | With the tank empty of water, adjust air precharge to match cold water supply pressure | Diaphragm listed as "Heavy Duty Butyl NSF/ANSI 61" on the submittal; no certifier named | | Watts PLT | 20 psi | Set to system pressure, under zero system pressure; do not exceed 80 psi | IAPMO listed, certified to ANSI/NSF 61 (PLT-5, 12, 20); wetted surface under 0.25% lead | | Zurn Wilkins XT and WXTP | 40 psi | Adjust to match incoming pressure with no water pressure in the tank; maximum precharge 80 psi; relief valve at 150 psi maximum | IAPMO listed, WQA certified to NSF/ANSI/CAN 61 and NSF/ANSI 372 | | State and Reliance TW | 38 psi (Reliance brochure only) | Match the inlet pressure, not higher than 80 psi (Reliance) | None printed | Every sheet lands on the same rule: the air side goes to the house's static pressure, set with the tank dry. Watts, Zurn and Reliance all cap the precharge at 80 psi, and Watts and Zurn both say a supply above 80 psi gets a pressure reducing valve first: see the [high water pressure guide](/plumbing/guides/high-water-pressure-80-psi-when-you-need-a-prv/). Only Watts and Zurn print a certifier and a standard number together. Amtrol names the standard but no certifier, and the TW documents print nothing, which means only that these sheets do not say. Whether the house needs a tank at all is the [thermal expansion tank guide](/plumbing/guides/thermal-expansion-tank-when-required/). Amtrol's manual puts it between the water heater and the backflow preventer, check valve or pressure reducing valve. ## What is not a thermal expansion tank? | Product | Built for | Why it is the wrong part | |---|---|---| | Hydronic expansion tank (Watts ETX, Amtrol Extrol) | Closed boiler heating loops | Watts prints the ETX line under a Nonpotable heading in the same guide that certifies the PLT to ANSI/NSF 61; Amtrol sells the ST series for closed potable hot water systems | | Well pressure tank (Amtrol WX-202 and its equivalents) | Pump drawdown between cut-in and cut-out | A 20-gallon class tank on a 1 in connection sized for drawdown, not thermal acceptance; see the [well pressure tank cross-reference](/water/parts/well-pressure-tank-cross-reference/) | Zurn adds that the XT and WXTP are not for sprinkler systems under NFPA 13 or 13D. ## When does it need replacing? None of the sheets opened here prints a service interval, so this page does not invent one. Amtrol sizes against a 150 psi T&P relief valve and Zurn requires a relief valve set at 150 psi maximum, so a tank that has stopped taking water shows up as that valve dripping after each heating cycle. Amtrol gives the one check: check the precharge while the tank is isolated and empty of water, and replace the tank rather than adjust it if it is corroded. Its in-line ST-5 and ST-12 also carry an InSight indicator cap, and a discoloured tip means tank replacement is required. The [anode rod page](/plumbing/parts/water-heater-anode-rod-by-model/) covers the part usually changed on the same visit. **Set the precharge before you fit it** The Amtrol ST-5 is the tallest 2-gallon tank here at 13 in and arrives at 50 psig with the smallest acceptance in its slot. The Watts PLT-12 carries an NSF 61 line on its own sheet and arrives at 20 psi. Neither matches your house out of the box: read static pressure at a hose bib with a gauge, then set the tank to it with the tank empty. ## The picks - **Amtrol Therm-X-Trol ST-5**: The 2-gallon slot, 40 and 50 gal heaters at 140 F (2.0 gal, 0.9 gal acceptance, 8 in diameter, 13 in high, 3/4 in NPTM, 150 psig, ships at 50 psig precharge, per Amtrol's MC8520 card) Amazon: https://www.amazon.com/dp/B08BZTFDRD/?tag=ownerspec-20 - **Watts PLT-12**: The 4.5-gallon slot with a printed NSF 61 listing (4.5 gal, 2.8 gal acceptance, 3/4 in male connection, 10.5 in diameter, 13.5 in long, 150 psi, 200 F, ships at 20 psi precharge, IAPMO listed and certified to ANSI/NSF 61, per Watts ES-PLT and PG-ThermExpansion) Amazon: https://www.amazon.com/dp/B002JEQGYO/?tag=ownerspec-20 - **American Water Heater TW-12-1**: The State and Reliance TW 12-1 under a third badge (4.8 gal, 2.19 gal acceptance at 60 psi, 3/4 in NPT, 10.6 in diameter, 13.7 in high, 150 psig, rated for heaters up to 100 gal, per State's TW sheet; the listing title rounds it to 5 gal) Amazon: https://www.amazon.com/dp/B01HTZD85W/?tag=ownerspec-20 ## Frequently asked questions ### Is the Watts PLT-12 a direct replacement for an Amtrol ST-12? Same slot, not the same part number. Watts publishes the PLT-12 at 4.5 gallons on a 3/4 in male connection, 10.5 in diameter and 13.5 in long; Amtrol publishes the ST-12 at 4.4 gallons on 3/4 in NPTM, 11 in diameter and 15 in high. Both are rated 150 psi and 200 F. The PLT-12 ships at 20 psi precharge, the ST-12 at 50 psi. ### Are the State TW 12-1 and the Reliance TW 12-1 the same tank? The published numbers are identical. State's TW spec sheet and Reliance's brochure both print the TW 12-1 at 150 psi maximum working pressure, 4.8 gallons, 2.19 gallons acceptance at 60 psi, 3/4 in NPT, 10.6 in diameter, 13.7 in high, 8.2 lb, rated for heaters up to 100 gallons. Amazon also lists the same model number under the American Water Heater name. ### What precharge should a replacement expansion tank be set to? The house's static supply pressure, per every sheet opened here. Amtrol says to adjust the precharge with the tank empty to match cold supply pressure. Zurn says to match incoming water pressure with no water pressure in the tank and caps the precharge at 80 psi. Reliance and Watts print the same 80 psi ceiling. Factory settings run from 20 to 50 psi, so almost every tank needs adjusting. ### Can I use a boiler expansion tank or a well tank on a water heater? No. Watts prints its ETX hydronic tanks under a Nonpotable heading in the same product guide that certifies the PLT line to ANSI/NSF 61, and Amtrol sells the ST series specifically for closed potable hot water systems. A well pressure tank is sized for pump drawdown, a different job in a different capacity class. Only a tank sold for potable thermal expansion belongs on the cold inlet. ### Which of these makers prints an NSF certification? Watts prints IAPMO listing and certification to ANSI/NSF 61 for the PLT-5, PLT-12 and PLT-20. Zurn prints IAPMO listing and Water Quality Association certification to NSF/ANSI/CAN 61 and NSF/ANSI 372 for the XT and WXTP lines. Amtrol's submittal sheet prints NSF/ANSI 61 against the butyl diaphragm without naming a certifier. The State and Reliance TW documents print no certification line. ## Sources - [Amtrol, Therm-X-Trol Quick Sizing Charts and specifications, MC8520 (06/24): ST-5 and ST-12 volume, acceptance volume, diameter, height, 3/4 NPTM connection and weight; 150 psig maximum working pressure; 200 F maximum operating temperature; 50 PSIG factory precharge for ST-5 and ST-12, 40 PSIG for ST-25V and larger; sizing based on 40 F incoming water, a 150 psi T&P valve and precharge equal to static supply pressure](https://www.amtrol.com/wp-content/uploads/2024/06/MC8520-06_24-TXT-Sizing-Card.pdf) (manufacturer or vendor document) - [Amtrol, Therm-X-Trol ST Series Non-ASME submittal sheet, MC4400 (01/24): construction table naming a heavy duty butyl NSF/ANSI 61 diaphragm, in-line and stand model dimension tables, 7-year limited warranty on ST-5 through ST-12](https://www.amtrol.com/wp-content/uploads/2023/11/MC4400_01_24_Submittal_Data_Book-34.pdf) (manufacturer or vendor document) - [Amtrol, Thermal Expansion Tanks for Potable Water installation and operation instructions, ST-5 through ST-210V, 9015-942 (04/23): with the tank empty of water adjust air precharge to match cold water supply pressure; install between the water heater and the backflow preventer; InSight indicator cap discoloration means tank replacement is required](https://www.amtrol.com/wp-content/uploads/2023/06/9015-942-04_23-Thermal-Expansion-Tank-IO.pdf) (manufacturer or vendor document) - [Watts, Series PLT Potable Water Expansion Tanks engineering specification ES-PLT (2020): PLT-5 2.1 gal and PLT-12 4.5 gal on 3/4 in male connections; 150 psi maximum pressure, 200 F maximum temperature, 20 psi air precharge; diameter, length and weight table; IAPMO listing and ANSI/NSF 61 certification statements; wetted surface under 0.25% lead by weight](https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/20441-source/es-plt-pdf) (manufacturer or vendor document) - [Watts, Thermal Expansion product guide PG-ThermExpansion (2009): PLT-5/DET-5-M1 acceptance volume 1.26 gal and PLT-12/DET-12-M1 acceptance volume 2.8 gal; ETX hydronic tanks listed under a Nonpotable heading](https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/24277-source/pg-thermexpansion-pdf) (manufacturer or vendor document) - [Watts, Model PLT-35 installation instructions IS-PLT-35: precharge must be set to the system pressure, normal precharge 20 psi, do not exceed 80 psi, adjust only under zero system pressure](https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/12108-source/638887905390000000/2915055-pdf.pdf) (manufacturer or vendor document) - [Zurn Wilkins, Model XT Water Thermal Expansion Tank spec sheet WXT-XT (Rev. E, 01/22): XT-8 and XT-18 table with 80 psi maximum supply pressure, volume, acceptance volume, 40 psi precharge, diameter, height, 3/4 NPT connection and weight; IAPMO listing, WQA certification to NSF/ANSI/CAN 61 and NSF/ANSI 372; not for NFPA 13 or 13D sprinkler systems; adjust precharge to incoming pressure with no water pressure in tank, 80 psi maximum precharge, relief valve set at 150 psi maximum; sizing chart based on a 100 F rise](https://files.zurn.com/spec-sheets/wxt-xt.pdf) (manufacturer or vendor document) - [Zurn Wilkins, Model WXTP Water Thermal Expansion Tank spec sheet WXT-WXTP (Rev. D, 01/22): WXTP-50V 14 gal total, 5.6 gal acceptance, 40 psi precharge, 80 psi maximum supply pressure, 1 in FNPT, 16 in diameter, 21 11/16 in height, 32 lb; NSF/ANSI/CAN 61 and NSF/ANSI 372 certification](https://files.zurn.com/spec-sheets/wxt-wxtp.pdf) (manufacturer or vendor document) - [State Water Heaters, TW Series Thermal Expansion Tank spec sheet (November 2018, hosted on State's regional site): TW 5-1, TW 12-1, TW 5-5 and TW 12-5 table with 150 PSIG maximum working pressure, tank volume, maximum acceptance volume at 60 psi, 3/4 NPT connection, diameter, height, weight and heater size (up to 52 gal and up to 100 gal); 200 F maximum temperature; no factory precharge printed](https://www.statewaterheatersme.com/wp-content/uploads/2025/11/Expansion-tanks_Spec-sheet.pdf) (manufacturer or vendor document) - [Reliance Water Heaters, Pump and Expansion Tanks brochure RPTBRO0624 (2024): TW 5-1 and TW 12-1 table identical to State's; tanks are pre-charged with 38 PSI; air pressure adjusted to match inlet pressure but not higher than 80 PSI; 150 PSI rating](https://assets.reliancewaterheaters.com/damroot/Original/10009/Reliance-Pump-Tank-Broch.pdf) (manufacturer or vendor document) - [Amazon listing, Amtrol ST-5 Expansion Tank, 2 Gallons, 8 in diameter (ASIN B08BZTFDRD): title and stock status only](https://www.amazon.com/dp/B08BZTFDRD) (retailer listing or retailer-hosted document) - [Amazon listing, Watts PLT-12 4.5-Gallon Potable Water Expansion Tank 3/4 in Male Inlet (ASIN B002JEQGYO): title and stock status only](https://www.amazon.com/dp/B002JEQGYO) (retailer listing or retailer-hosted document) - [Amazon listing, American Water Heater TW-12-1 Expansion Tank For Potable Water (ASIN B01HTZD85W): title and stock status only](https://www.amazon.com/dp/B01HTZD85W) (retailer listing or retailer-hosted document) --- Citation: OwnerSpec, "Thermal Expansion Tank Cross-Reference: Amtrol ST-5 and ST-12, Watts PLT-5 and PLT-12, Zurn Wilkins XT-8 and XT-18, State and Reliance TW 5-1 and TW 12-1 by Capacity Class", https://ownerspec.com/plumbing/parts/thermal-expansion-tank-cross-reference/, facts verified 24 September 2026. 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