The Cast Air AFD3 is a float-operated automatic drain valve designed specifically for Castair THB-series filter bowls, priced at $47.74. It connects via a 1/4" NPT fitting and provides passive, fully automatic condensate drainage from THB bowl assemblies without any electrical connection, timer, or manual intervention. If your THB filters are accumulating water that is making it downstream into your air lines and tools, the AFD3 is the direct-fit solution.
The THB-series filter bowls are a common configuration in Castair compressed air systems. Like all inline compressed air filters, THB bowls collect condensate as part of their job — separating moisture from the air stream before it reaches downstream equipment. The problem comes when that condensate is not removed regularly. An overfull filter bowl passes moisture downstream, effectively making the filter useless. The AFD3 keeps the bowl evacuated continuously, so the filter keeps working the way it is supposed to.
How the Float Drain Works
The AFD3 uses a mechanical float mechanism — the same principle used in carburetor float bowls and fluid level controls. As condensate collects at the bottom of the THB filter bowl, the float inside the AFD3 rises with the water level. When the water reaches the drain threshold, the float opens the drain port and evacuates the condensate. When the bowl is clear, the float drops and the drain port closes. The cycle is passive and continuous — no programming, no power, no maintenance schedule.
The 1/4" NPT connection is larger than the 1/8" NPT used on the AFD1 (Watts bowl series). This is not interchangeable — thread size and bowl geometry are matched to the specific THB bowl design. Using the wrong drain on a bowl will result in a poor seal or incorrect float actuation. If you have THB bowls, use the AFD3. If you have standard Castair filter bowls (not THB series), use the AFD1.
Key Features
- THB Bowl-Specific Design: Direct-fit for Castair THB-series filter bowls — correct thread size, correct float geometry, no adapters needed
- 1/4" NPT Connection: Matches the drain port on THB-series bowls; not interchangeable with AFD1 (1/8" NPT)
- Float-Operated Automatic Drain: Mechanical float evacuates condensate automatically as it accumulates — no electricity, no timer
- Passive Mechanical Operation: No consumable electronic components, no power supply, no scheduled maintenance cycles to program
- Continuous Bowl Protection: Drains in response to actual condensate accumulation, not a fixed time interval
- Prevents Filter Overflow: Keeps THB bowls within capacity so they continue to filter effectively
- Simple Thread-In Installation: Replaces the existing manual drain fitting on the THB bowl drain port
- Cast Air Build Quality: Manufactured in Minnesota to Castair's standard for shop-environment reliability
Why This Matters in a Working Shop
Inline filter bowls do their job by pulling moisture out of the compressed air stream. The condensate has to go somewhere — it collects in the bowl. In a high-usage shop or in humid conditions, a THB bowl can fill up in a matter of hours. Once the bowl is at capacity, moisture carries over downstream.
The downstream effects are well known to anyone who has dealt with them: fisheye and blushing in spray finishes, rust in air tool internals, erratic operation in pneumatic controls, and deterioration of air hose interiors. None of these failures are dramatic or sudden — they accumulate over time as moisture does slow, consistent damage. The AFD3 prevents that accumulation from happening by keeping the bowl clear automatically.
What's in the Box
- 1x Cast Air AFD3 THB bowl float drain valve
- 1/4" NPT thread fitting for THB-series Castair filter bowl drain ports
Available Configurations
Cast Air offers two bowl float drain options. Bowl series determines which part you need:
- AFD3 — $47.74 (this part): THB Bowl float drain. 1/4" NPT connection. Fits Castair THB-series filter bowls specifically. The lower-cost option of the two bowl drain valves.
- AFD1 — $77.77: Watts Bowl float drain. 1/8" NPT connection. Fits all standard Castair filter bowls (non-THB series). If you are unsure which bowl series you have, call us and we will confirm.
For automatic draining of the receiver tank itself (not a filter bowl), see the air-operated tank drain valves:
- ADVS — $119.43: Air-operated tank drain valve only, for 1–10 HP compressors
- ADVSM — $154.00: Same valve with complete installation kit (strainer, shut-off, hardware)
When Does a Shop Need the AFD3
Choose the AFD3 when:
- You have Castair THB-series filter bowls that currently require manual draining
- You are upgrading from the manual drain petcock on existing THB filter assemblies
- Your THB bowls are in a high-humidity environment or on a high-demand air line where condensate accumulates quickly
- You want automatic bowl draining at the lowest cost — the AFD3 is $30 less than the AFD1
- You are installing new THB filters and want automatic drainage from day one
Choose the AFD1 instead when:
- Your filter bowls are standard Castair bowls, not the THB series — the AFD1 fits Watts-style bowls with 1/8" NPT drain ports
- You are unsure of your bowl series: call us at 866-217-0063 and we will confirm the right part before you order
Note: Neither the AFD3 nor the AFD1 mounts on or drains the compressor receiver tank. For tank drain automation, use the ADVS or ADVSM.
Technical Specifications
- Part Number: AFD3
- Manufacturer: Cast Air (Castair), Minnesota
- Type: Bowl float drain valve — automatic, mechanical
- Connection Size: 1/4" NPT
- Compatible Bowl Series: Castair THB-series filter bowls
- Operation: Mechanical float — no electricity required
- Drain Trigger: Float-activated when condensate reaches threshold level in bowl
- Price: $47.74
- Country of Origin: USA (Minnesota)
Why Buy From Air Compressor Factory Direct?
Air Compressor Factory Direct has been a Castair specialist since 2016. We stock the full Cast Air filter and drain accessory line because shops need more than just the compressor — they need the parts to keep the whole air system working correctly. The AFD3 is one of those parts that is easy to overlook until a filter bowl overfills and ruins a paint job or accelerates tool wear.
If you are not certain whether your filter bowls are THB series or standard Castair, call us. We will identify the correct drain from your filter part number and get you the right part the first time. Every order ships free. Full manufacturer warranty. No guessing, no returns for the wrong part.
Call 866-217-0063 for quick answers and help!
