looking for mixing valve
 

I currently have an underfloor heating system with a 10 gal hot water heater that I’m hope to convert to a GSHP system next summer. My current system is missing a mixing valve. At the timer the HVAC guy said I didn’t need one, the system has been working fairly well. I have the water set at 175°.
What I would like to do, is to use an Arduino to control a mixing valve. What I’m looking for is a motorized 4-way mixing valve. Hot water in from boiler and return to boiler. Mixed water supply to and return from floor. thank you

What's wrong with a mechanical thermostatic mixing value?

what changes the set point when the outside temp drops?

I see some holes in the story here, and details matter. Before anyone can recommend a system upgrade, the existing one needs described.

the system is: 10gal hot water heater set at 175° to an expansion tank to a temp/psi gage to the 4 zone manifold supply to the underfloor hanging 3 inches down back to the 4 zone manifold to the pump to a air trap back to the heater. what holes in my story do you see?

Lots on eBay! So many different flavors. You'll have to dig through the spec sheets.

Generally you purchase the valve body and actuator separate from each other. If you're trying to do this on a budget, it can take a lot of digging to find a cost effective pair.

The valve body choices are pretty obvious. The actuators are available in different voltages and modulating techniques.

Most on eBay are on/off control, no modulation.

Personally I like the floating actuators. Easy to control. Two pins: pulse one to close, one to close, and it'll hold its part-way position when power is removed.

You can get remote sensing mechanical thermostatic mixing valves.
I work in industrial automation and instrumentation.

Maybe the HVAC guy is correct, you don't need a mixing valve.
 

Just control the water heater temp our of the heater and run the water directly to the distribution manifold.

Ok, so you have a staple up or free hanging setup. That's not going to work real well with the lower water temperature that a heat pump provides. With the high water temperature you're using now, natural convection and radiation do a lot. These 2 effects go away drastically when you lower the loop water temperature.

With a solid slab, running a low temperature heating loop makes sense because of all of the thermal mass. With a thin floor, the heater pipes have to maintain direct contact with the surface they're going to heat. With most underfloor setups, this means heat spreaders. There's not much thermal mass, either. Lots of details to consider before you commit to a big system change.

that's what I do now but when the temp drop's from 25° during the day to 5° at night the water temp I have set does not keep up. that's way I'm looking at a mixing valve