Navien’s comb-boiler (NCB) has had its share of criticism and praise alike but in my opinion there is one thing that gets me a little more than excited, the fact that nearly half the boiler must be removed to replace a part that is known to fail more than most other parts on the unit. As you’ll see in the video the PCB, main control panel, some internal [and external] piping and circ pump all must be fully removed in order to gain access to a failed 3-way diverter valve.
If you’re not sure what the valve does, you may not be alone but its really quite simple; when idle the valve is in position to supply flow from the heat exchanger’s supply outlet to either the heating system load (hot supply to system) outlet or to the flat plate heat exchanger to satisfy a call for domestic hot water. The problem is that its a piston type valve that can and will fail in a partially open position when there’s a call for DHW. The results are low flow to a potable water faucet at higher than desired temps [hence the reason I always spec and install a thermostatic mixing valve on the domestic side outlet]. The fix, while very time consuming, is relatively simple.
Hopefully this step-by-step video will familiarize the process for you if you’re faced with replacing the valve for the first time.
If you’re interested in a tool that has helped tremendously in making the job just a little easier, check this out:
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