Caleffi offers a wide selection of models with and without check valves for your convenience to compliment the 521 Series Thermostatic Mixing Valve family. Models with checks have a "...C" suffix, for example 521519AC.
If you have an existing model that does not have checks, for example 521519A and you need them, you will need to replace the "standard" tailpieces with new tailpieces that come with checks (see below).
These are a little longer than the standard tailpieces and have an internal machined groove that the check clips into. For a complete list of tailpiece choices, see the SMALL MIXING VALVE AND ZONE VALVE FITTINGS page in Section 9 of our List Price Catalog.
Don't order the plastic check insert (for example R39204) and try to install it in the "standard" tailpiece; it won't fit. The check inserts are made available for replacement purposes only.