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I have developed a very small drip. The expansion tank appears to be
fine and the pressure on the gauge is at an appropriate level. It
seems these things are not the longest lasting devices. This is a
WGO-3 model with the PRV at the upper left corner. Can anyone give me
some pointers on how to replace this?

1) Shut off the unit and water input
2) Reduce the pressure inside the boiler (how can I do this manually?)
3) Unscrew the PRV and replace it (any particular sealant needed or
just regular pipe dope?)
4) Turn on water source and turn unit back on. (Do I have to purge
any air on this or is that done automatically?)

Thanks for your help.

1 yes
2 open the drain valve
3 pipe dope or Teflon tape
4 purge if you can but minimal water should be lost