Aquastat Wells

Aquastat wells are immersion-type fittings threaded into a boiler or storage tank that hold the sensing bulb of an aquastat temperature controller in contact with the controlled liquid while preventing leakage of that liquid into the boiler jacket or surrounding environment. Aquastat wells are available in 1/2" and 3/4" NPT pipe thread sizes, in copper and stainless steel materials, and in varying insertion lengths and insulation depths to suit different boiler tapping configurations. Brands available at SupplyHouse include Resideo (formerly Honeywell), OMI Parts, and White Rodgers, covering both direct replacement wells for common Honeywell and White Rodgers aquastats and new-installation wells for hydronic heating, tankless domestic hot water, and storage tank applications. The well clamp assembly, sold separately, is required to secure the aquastat to the well in most configurations using the Resideo 121371 series.

KEY SUBCATEGORIES / PRODUCT TYPES

Resideo / Honeywell 1/2" Copper Aquastat Wells

The Resideo (Honeywell) 1/2" Aquastat Well, part number 121371A (SKU 121371A), is a copper immersion well with a 1/2" x 14 NPT spud thread, a 3" insertion length, a 3/8" internal shell diameter, and a bulb size of 3/8" x 3". The well clamp assembly (SKU 121371AA) is sold separately and not included. The 121371A is confirmed compatible with the L8148J1009 Aquastat Relay and the L6006A1145 aquastat, among other Resideo immersion-type aquastat models. An OMI Parts aftermarket equivalent is available as SKU 121371A-OMI. The 1/2" well is also available in stainless steel as the Resideo 121371E (SKU 121371E), which carries a 1/2" x 14 NPT spud thread, a 3" insertion length, a 1-1/2" insulation depth, and a 3/8" x 3" bulb size. The 121371E includes a mounting clamp and is recommended for applications such as pool heaters where corrosion resistance is a priority.

Resideo / Honeywell 3/4" Copper Aquastat Wells

The Resideo (Honeywell) 3/4" Aquastat Well, part number 121371B (SKU 121371B), is a copper well with a 3/4" x 14 NPT spud thread, a 3" insertion length, a 1-1/2" insulation depth, a 3/8" internal shell diameter, and a bulb size of 3/8" x 3". The well clamp assembly (SKU 121371AA) is sold separately. An OMI Parts aftermarket equivalent is available as SKU 121371B-OMI. The Resideo 121371M (SKU 121371M) is a long-insulation variant of the 3/4" copper well, sharing the same 3/4" x 14 NPT spud thread and 3" insertion length but with a 3" insulation depth rather than 1-1/2", serving installations where the tapping location requires additional insulation coverage along the well shank. The Resideo 123871A (SKU 123871A) is a 3/4" copper well with 3" insertion length and 3" insulation depth, designed for connections to Uponor's CL and other boiler series with a 3/4" NPT tap, as noted in its common appearance alongside New Yorker CL series boiler parts.

Resideo Copper Well Assembly 123870A

The Resideo 123870A (SKU 123870A) is a copper well assembly with 3" insertion and 1-1/2" insulation, offered as a packaged well assembly at 3/4" NPT. The 123870A serves the same hydronic boiler immersion applications as the 121371B and its variants and is an option when a complete copper well assembly is preferred over the separate well and clamp configuration.

White Rodgers Aquastat Wells

White Rodgers aquastat wells at SupplyHouse include the F89-0213 (3/4" well with 1-13/16" shank, SKU F89-0213) and the F89-0036 (immersion well with 1/2" shank, 3-1/2" x 7/16", SKU F89-0036). White Rodgers wells serve White Rodgers immersion-type aquastats and are sized to the specific shank dimensions required by the White Rodgers aquastat line. Confirming shank length and thread size against the specific White Rodgers aquastat model before ordering is required, as shank dimensions differ from Resideo well dimensions.

Well Clamp Assembly

The Resideo Well Clamp Assembly, part number 121371AA (SKU 121371AA), is the hardware kit required to mount a Resideo immersion-type aquastat onto a 121371-series well. The 121371AA includes the mounting clamp, spud set screw for armored capillary, plug, and nut. The well clamp assembly is not included with the 121371A or 121371B wells and must be purchased separately. The 121371E stainless steel well is an exception, as it includes a mounting clamp.

BUYING GUIDE / HOW TO CHOOSE

Selecting the right aquastat well starts with the pipe thread size of the boiler tapping where the well installs. The two sizes in this category are 1/2" NPT and 3/4" NPT. The tapping size on the boiler determines which well fits. Mixing thread sizes requires an adapter and adds unnecessary complexity to a straightforward replacement.

Material is the second decision. Copper wells, including the 121371A and 121371B series, are the standard for residential hydronic boiler applications. The stainless steel 121371E is specified when the application involves corrosive liquids or environments, such as pool heaters, where copper can degrade faster.

Insulation depth is the third variable, and it affects how the well sits in relation to the boiler jacket. The 121371B has 1-1/2" insulation depth. The 121371M and 123871A both have 3" insulation depth. A longer insulation depth keeps more of the well shank outside the insulation layer, which can matter when the boiler tapping location and jacket thickness require additional standoff length to position the aquastat properly. Confirming the existing well dimensions or measuring the boiler tapping before ordering is the cleanest way to get this right on a replacement job.

For direct replacement of a Resideo 121371-series well, confirming the part number from the existing well is the fastest path to the correct replacement. The 121371A (1/2" NPT, 3" insertion, 1-1/2" insulation) and 121371B (3/4" NPT, 3" insertion, 1-1/2" insulation) cover the most common residential hydronic configurations.

The well clamp assembly (SKU 121371AA) must be ordered separately when specifying a 121371A or 121371B well for an installation that requires the aquastat to be mechanically secured to the well. It's not included in the well itself. The stainless steel 121371E is the exception, as it ships with a mounting clamp.

FAQs

What is an aquastat well and what does it do?

An aquastat well is an immersion fitting threaded into a boiler, storage tank, or other liquid-containing vessel that holds the sensing bulb of an aquastat temperature controller in contact with the liquid being monitored, while preventing that liquid from leaking out of the system. Aquastat wells are used with immersion-type aquastat temperature controllers in hydronic heating systems, tankless domestic hot water systems, and storage tank applications where temperature control is required. Without a properly installed well, the aquastat sensing bulb cannot accurately read the boiler water temperature or maintain a sealed connection to the boiler.

What is the difference between the Resideo 121371A and 121371B aquastat wells?

The difference between the Resideo 121371A and 121371B aquastat wells is the pipe thread size. The 121371A uses a 1/2" x 14 NPT spud thread and the 121371B uses a 3/4" x 14 NPT spud thread. Both wells are copper, share the same 3" insertion length, 3/8" internal shell diameter, and 3/8" x 3" bulb size, and both require the well clamp assembly 121371AA purchased separately. The pipe thread size at the boiler tapping determines which of the two wells fits the installation.

What is the difference between the 121371B and 121371M aquastat wells?

The difference between the Resideo 121371B and 121371M aquastat wells is the insulation depth. The 121371B has a 1-1/2" insulation depth. The 121371M has a 3" insulation depth. Both are 3/4" x 14 NPT copper wells with a 3" insertion length and a 3/8" internal shell diameter. A longer insulation depth on the 121371M provides additional standoff between the boiler tapping and the aquastat mounting position, which can be required when the boiler jacket thickness or tapping geometry demands more shank clearance than the standard 121371B provides.

Is the well clamp assembly included with Resideo aquastat wells?

The well clamp assembly is not included with the Resideo 121371A or 121371B aquastat wells and must be purchased separately. The Resideo Well Clamp Assembly, part number 121371AA (SKU 121371AA), includes the mounting clamp, spud set screw for armored capillary, plug, and nut required to secure the aquastat to the well. The Resideo 121371E stainless steel 1/2" aquastat well is an exception, as it includes a mounting clamp.

When should the stainless steel 121371E aquastat well be specified instead of a copper well?

The Resideo 121371E stainless steel aquastat well should be specified in applications where corrosion resistance is a higher priority than standard residential hydronic service. Pool heaters are one confirmed application for the 121371E, where the chlorinated water environment can accelerate copper degradation compared to a more protected boiler installation. The 121371E carries a 1/2" x 14 NPT spud thread, a 3" insertion length, a 1-1/2" insulation depth, and includes a mounting clamp. For standard residential hydronic boilers using potable or treated water, copper wells such as the 121371A and 121371B cover the vast majority of installations.