Vertical generally. Read the installation instructions and they will indicate mounting limitations. I was recently doing some re-pipe to connect the manifolds for the bathroom radiant I had added and when draining the boiler used the prv to let a little air into the system so the boiler could drain. I ended up after recharging the system, having to drain it back down to replace the prv, which was just weeping by a bit due to debris in the seat area. It is easy to be tempted to do as I did, but avoid it if you can.