Mixing valve for hydronic heating
 

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?

Nope, They are designed to have 60psi on one side and nothing on the other. The ports are too small to get any real flow. Besides, you can get thermostatic mix valves for less than $100 in some places

I don't see how the backflow will help. Can you make a drawing of what you are thinking?

Just a mental( a very weak one) exercise. Since the cold side would be connected to my domestic water, the back flow preventer would eliminate the possibility of a cross connection with the boiler connected on the hot side.

Yes, it prevents boiler water from contaminating the potable water.

I picked up a bathtub/shower mixing valve for $0. A neighbor is doing a bathroom remodel and gave it to me. Our water pressure is regulated down to 35psi in our area by a regulator before the water meter. Water pressure in my shower or faucets has never been an issue. To get an idea how much flow through the shower valve is available,I tried this very unscientific experiment. I tried my existing tub/shower valve. I pulled of the shower head, then ran the water. Sure seemed to be pleI ty of flow! To verify adequate flow, I turned the diverter to the tub spout and it sure seems to be mucho flow. I'm pretty sure I'm going to go ahead with this project using the shower valve. Worse case scenario I'll have to buy a mix valve, but $0 is a whole lot better than over $100.

If you really want to find out how much flow there is, get a 5 gal bucket and time the flow to fill it...presto, gallons (or L) per minute. Easier than a subjective observation.

shower valve will adjust on pressure, if hot water side has a drop in pressure (someone opens a faucet in house) the cold side will also drop the pressure automaticaly.
Thermostatic valves will adjust on temperature. your output set point will remain the same regardless of water temp inlet.
This means if you have a 15 or 20 degree delta T set on heating device, your shower valve will fluctuate by this much after you have adjusted your infloor water temp, remember that the shower has a constant cold water temp for mixing whereas the heating system has a changing return temp (cold water connection to valve). and possibly as your return water heats up it will increase the supply water temp to floor and could create serious temps in infloor.
I hope my explanation makes sense.

I understand what your saying, but I don't think it applies. Once the shower valve is set, mixing cold water to hot water, there shouldn't be any fluctuation in pressure.The flow from my boiler is at a constant pressure, only the temperature would change. I don't think it will really effect my basement floor too much once I determine what will be the target temperature for that circuit. It would be a simple and inexpensive matter to install a small pressure reducing valve inline to that loop should it ever become necessary, but I doubt it would ever be needed.