Hi guys I have been slowly putting together my radiant system I have a navien 210 with grundfos pump going through a plate heat exchanger on the closed side I have 3 zones was thinking of the alpha and a taco zone controller for actuators and primary loop pump Question what is the simplest most unexpensive way to stop primary pump when there is a call for domestic hot through the navien? just worried when I am done that hot water will be lukewarm when heat is on any Ideas would be greatly appreciated. I am not an expert on this type of heating but am sure having fun putting it together thanks
                                                         Heath

It's unlikely that you'll run out of burner capacity with both heating & hot-water loads running unless you 're simultaneously starting up a cold slab and you have cold/very-cold incoming water and a high rate of flow on the hot water (like 2 high-flow showers.)

What is your design-condition heating load, space heating water temp, and type of radiation?

Dana I was going to try running the domestic with the primary pump on and see what happens but want to be prepared there should not be a problem since the domestic goes from navien to a 65 gal electric but the electiric will be bypassed if there is a power outage I still should be able to live with it just was wondering if they make anything like that. I am heating 5 inch concrete on 12 inch centers with 1/2 inch pex 1200 sq ft.  another question I have is how important is it that my zone circulator pump be after the expansion/air scrubber since I have alot of room to mount the pump before the inlet of the hx which also makes it easy to point the pump up.  The expansion etc would be right after the hx outlet and I have already mounted the actuators on the return manifold. Does this setup raise any red flags I could mount the pump after but would be a lot more difficult due to lack of space. Thanks for advice.  I am planning on running the alpha as a ciculator pump with no check valve

The expansion tank & air-purging work best if you're pumping away from them as opposed to toward them. If the pump is pulling directly from the HX + radiation the head of the loop creates an impedance resulting in higher-frequency pressure fluctuation & cavitation at the bump impellers. If you place the expansion tank & air scrubber near the pump input , the tank behaves as a shock-absorber for that high-frequency impeller "noise", and the air scoop can do it's job, and the pump will run more efficiently. It doesn't matter where in the loop the pump & expansion tank live- it can pump toward the radiation & away from the HX, or toward the HX and away from the radiation, as long as it pumps way from the expansion tank & scoop.

On the Navien side loop it might matter- it depends on your water pressure. If your water pressure is under 20psi you may need to pump toward the Navien, since it may lower the pressure on the Navien's HX to below it's operating minimum (~15psi) with the pump running. I don't recall if that's pre-plumbed feature of the 210A with it's own internal pump, but if it is, just assume that they got it right.

If there's a power outage, none of it works anyway unless you're running on an emergency backup generator. In that case, simply putting a switch in series with the pump power that you'd flip if your shower ran cold would do it. There are various flow detectors etc. out there that you could use, but if the concern is only for during emergency operation it's hard to rationalize the expense.

If the concrete isn't yet poured, a 100' loop of PEX in the slab to pre-heat the cold feed to the Navien makes the system "share" better, without needing extra controls to give priority to the DHW flow.

Dana a loop through the slab to the cold side, that a great idea I never thought of It the slab was poured a long time ago though,  I hope to try and re route some stuff to get pump in the right spot. Are Y strainers a must on the closed side since it will mostly be the same water?  Another thing is that one of the 1/2 inch loops is 345 feet for one zone in the sunroom I have calculated 12.42 feet of head for the loop plus another 2 feet for hx and manifold does this seem right for 1 gallon per min at 100 degrees if so do you think the alpha in auto adapt will do the job the other zones are 1 loop or 150 a 2 loops of 265 is 1 gpm enough flow?  Thanks for all of you help it makes it alot easier to feel confident in my install.       
                           Heath      p.s The 1 gallon is just through the long loop