How to Fix Atwood Water Heater Mixing Valve 90029 Leak - Step-by-Step

Short Answer

Your Atwood mixing valve 90029 can likely be repaired rather than replaced. There are internal O-rings and seals that commonly fail after extended storage periods like your 8-month sit. The black plastic cover houses the adjustment mechanism, but the actual leak is probably coming from internal seals that can be serviced. However, given the age of these valves and the cost of rebuild parts versus a new valve ($45-65), replacement is often the more economical choice.

Understanding the Problem

The Atwood 90029 mixing valve is a thermostatic valve that blends hot and cold water to deliver safe water temperatures to your RV's fixtures. After sitting unused for 8 months, the internal rubber seals and O-rings have likely dried out and shrunk, creating the slow drip you're experiencing. This is extremely common with RV water systems after extended storage periods.

The valve contains several internal seals: a main body O-ring, cartridge seals, and inlet/outlet O-rings. The black plastic cover you mentioned houses the temperature adjustment dial and spring mechanism, but it's not typically where leaks originate. Most leaks occur at the threaded connections or from the internal cartridge assembly.

Extended storage without proper winterization or periodic operation allows seals to dry out and lose their flexibility. Temperature cycling during storage can also cause expansion and contraction that compromises seal integrity. The slow drip indicates the seals haven't completely failed but are no longer maintaining proper pressure.

What You'll Need

If attempting a repair, you'll need an Atwood mixing valve rebuild kit (part number varies by manufacture date), which includes new O-rings, seals, and possibly a new cartridge. You'll also need basic tools: adjustable wrenches, channel lock pliers, pipe thread sealant, and possibly a seat removal tool.

For replacement, you'll need a new Atwood 90029 mixing valve (or compatible replacement like the 90030 or Suburban equivalent), pipe thread sealant or Teflon tape, and the same basic tools. The valve typically costs $45-65 depending on your supplier.

Essential supplies include shutoff valve for your water system, towels for cleanup, and possibly PEX or copper fittings if your connections need updating. Having basic testing tools handy can help diagnose any associated components if your system includes monitoring.

How to Fix It

First, check that your water heater bypass valves are in the correct position for normal operation - this is the most common cause of mixing valve issues. Then shut off your water pump and open faucets to drain the system pressure. Remove the inlet and outlet water lines from the mixing valve - these are typically 3/8" NPT threaded connections. Take photos before disconnection to ensure proper reassembly.

For repair attempts, carefully remove the black plastic adjustment cover by unscrewing it counterclockwise. Inside you'll find the cartridge assembly and spring. Remove the cartridge by hand or with pliers (requires a quarter-turn counterclockwise, not complete rotation) and inspect all O-rings and seals. Replace any that appear cracked, hardened, or deformed using the rebuild kit components.

However, I strongly recommend replacement over repair for your situation. Remove the entire valve from its mounting location and install the new 90029 valve in the same orientation. Apply pipe thread sealant to all threaded connections and hand-tighten plus 1-1.5 turns with wrenches for 3/8" NPT fittings - don't over-tighten as this can crack the valve body.

Reconnect your water lines, turn on the water system, and check for leaks. Adjust the temperature setting by turning the black dial - typically clockwise increases temperature. Test the output temperature at your faucets and fine-tune as needed. The entire replacement process typically takes 30-45 minutes for a DIY repair.

Leave It to the Experts

Consider professional service if you're uncomfortable working with plumbing connections or if your RV has a complex hot water distribution system with multiple zones. Mobile RV technicians can typically replace the mixing valve in about an hour for $150-200 including parts and labor.

Professional service becomes essential if you discover additional issues during the repair, such as corroded fittings, damaged water lines, or if the valve is integrated with a more complex temperature control system. Many RV service centers stock these valves and can complete same-day repairs.

Also consider professional help if this leak has caused water damage to surrounding areas, as there may be hidden moisture issues that require proper assessment and remediation. A qualified technician can also inspect your entire hot water system to prevent future issues after extended storage periods.

Tools & Supplies You'll Need

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