What is the bottom connection on the 551 Series DISCAL?

It is common practice to connect a boiler trim kit, or just an expansion tank, to the bottom of a Caleffi 551 Series DISCAL air separator. The 551 has a 1/2" NPT female thread on the bottom and can be ordered with, or without, a "service check valve" (561402A). We highly recommend using the service check valve if an expansion tank is going to be piped directly to the 551.  That way you can easily unscrew and replace the tank without having to isolate the 551 or shut down the system. If the 551 will not be connected to an expansion tank, a 538402 FD drain valve screws into the 1/2" NPT female connection on the 551 and provides a male garden hose outlet, for a convenient drain option.