Rheem Tankless Water Heater Secondary Heat Exchanger Replacement — What This Part Does
The secondary (condensing) heat exchanger on Rheem tankless units extracts additional heat from exhaust gases before they vent outside. It captures that extra energy to preheat incoming cold water, improving efficiency. This exchanger runs cooler and wetter than the primary exchanger because it condenses water vapor from combustion gases. Over time, acidic condensate and mineral scale eat through the fins and tubes, leading to cracks, leaks, and low outlet temperatures that trigger error codes.
When the secondary exchanger corrodes or clogs, the unit cannot pull enough heat from the exhaust stream. The outlet temperature sensor detects readings below the expected threshold and the controller throws fault code 29. Restricted airflow from dirt buildup on the exchanger fins will also starve the combustion process and drop outlet temps. Once the exchanger itself is cracked or leaking, cleaning will not fix it and you need a full assembly replacement.
Signs It Needs Replacing
- Error code 29 on the display The controller shows fault 29, indicating heat exchanger outlet temperature is too low and the unit shuts down or will not ignite.
- Water dripping from the heat exchanger area You see water pooling under the unit or dripping from seams around the secondary exchanger, meaning the exchanger has corroded through.
- Weak or intermittent hot water flow Hot water runs cooler than setpoint or cuts out during use because the secondary exchanger cannot transfer enough heat.
- Visible rust or white mineral deposits on exchanger fins The fins and tubes show heavy corrosion, flaking metal, or thick white scale buildup that restricts airflow and heat transfer.
- Condensate backup or moisture on sensor wiring Condensate collects around the sensor area or drips onto electrical connectors, causing false temperature readings and triggering error 29.
- Persistent low outlet temperature after cleaning You have cleaned the air inlet, combustion chamber, and vent path but the outlet temperature remains low and the fault returns.
How to Replace It
- Turn off electrical power at the breaker and close the inlet gas shutoff valve, then close the cold water inlet isolation valve and open a hot tap downstream to relieve pressure.
- Disconnect the vent piping from the top of the unit and set it aside, then unplug the condensate drain hose from the secondary heat exchanger outlet.
- Locate the secondary heat exchanger assembly on the exhaust side of the unit and disconnect the temperature sensor connector and any mounting screws or clamps securing the exchanger to the chassis.
- Remove the old secondary heat exchanger by lifting it straight up or sliding it out of the bracket, noting the orientation of inlet and outlet ports and the position of any gaskets or seals.
- Install the new secondary heat exchanger assembly in the same orientation, seating any new gaskets or O-rings onto the mating surfaces and securing the mounting screws or clamps to manufacturer torque if specified.
- Reconnect the temperature sensor harness to the new exchanger, making sure the connector clicks fully and no condensate can drip onto the terminals.
- Reattach the condensate drain hose and vent piping, checking that all joints are sealed and the vent slope meets code for condensing appliances.
- Open the cold water inlet valve slowly to fill the system, then open the gas shutoff and restore electrical power at the breaker.
- Run a hot water tap to initiate a call for heat, watch the unit fire and monitor the display for stable outlet temperature and no error codes, then check all connections for water or exhaust leaks.
The Part You Need
| Part | Notes |
|---|---|
| Rheem secondary heat exchanger assembly | Amazon | Find your exact model and serial number on the data plate inside the front cover or on the right side of the unit, then cross-reference that model number on the Rheem parts website or call a parts supplier to get the correct secondary exchanger part number for your specific tankless model. |
| Heat exchanger gasket kit | Amazon | Some Rheem secondary exchangers ship with new gaskets or O-rings included, but if yours does not, order the gasket set that matches your model to prevent leaks at the inlet and outlet connections. |
Related Error Codes
If this part is failing you may also see one of these codes:
When to Call a Pro
This repair involves gas connections, combustion venting, and critical temperature sensors on a sealed-combustion appliance. If you are not comfortable working with gas shutoffs, vent disassembly, or verifying proper combustion after reassembly, call a licensed plumber or gas technician. Most jurisdictions require a licensed professional for any work on gas-fired water heaters. A pro will also pressure-test the new exchanger, verify vent integrity, and confirm that the unit meets local code before putting it back into service. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.