Mixing valve for hydronic heating

[Minimac]

Just wondering if a shower mixing valve would be an acceptable alternative to the higher priced mixing valves needed when combining a Pex system to a conventional hydronic system. I've seen people replace perfectly good shower valves when remodeling a bathroom. They are easily adjustable and cheap on craigslist. I know the Pex needs to run at a lower temp than the existing copper heating system I have. I'm thinking that this may be an inexpensive (cheap) way to combine the different zones. I'm planning on possibly running the pex for an under the floor system for my basement. I already have a 1/2 backflow preventer at my disposal for the cold side of the valve and the hot side would connect to the supply side on my boiler. Worth it or not?