Abstract: A combination check valve, shutoff and seal cartridge is provided for a thermostatic valve, wherein the thermostatic valve includes hot and cold water inlets communicating with an ON/OFF/volume control and/or diverting water valve. The water output of the water volume control is then plumbed to a tub spout, faucet, showerhead or other plumbing device(s). The thermostatic valve has a body with both the cold water inlet and the hot water inlet, and a mixed temperature water outlet for mixed water of a predetermined temperature. Each inlet has respective ports accepting a corresponding combination check valve shutoff and seal cartridge. The cartridge has a housing with a spring therein. The cartridge housing has a seat element, and a pair of elastomeric stop seals fitted in respective sides of the cartridge housing. The seals seal a fluid flow path to the inner sides of the ports of the thermostatic valve body at the hot and cold water inlets of the thermostatic valve body.