Stainless pump for DHW

Yes, you do unless you want a corroded pump leaking iron into your DHW!!

A composite circ, mostly used for DHW recirculation systems is an option over stainless or bronze casings. different strokes for different folks on how they connect to the system. Composite bodies will be a union connection, stainless can be threaded, flanged, or union. Bronze bodies typically are sweat versions.

So yes, on the heating side of the heat exchanger, you can use a cast iron flanged circ. Just remember to add air elimination, expansion tank and all the accruements on a heating system.

High flow rate wi transfer more heat energy , more quickly. If the pump can then be turned off the tank will stratify for the next heat call

If the water side pump also serves as the dhw recirc pump, that complicates it

The recirculating flow is based on the temperature drop of the dhw recirc loop, usually 7-10 degrees. That may take a very low flow rate. So the two tasks may be at odds with one another

Im curious what the heat load is and the water heater btu. Those hydronic combicor tanks were 65,000 or larger

You may need to turn off the heat loop to get a hot shower on cold days?

yes the HX work best with counter flow piping . The heating circ wants to be downstream of the expansion tank

Pump away not at the expansion tank

On your drawing, the stainless pump should have an IFC or a check valve in series with it. The bit of cracking pressure a check valve stops convective flow when the pump is not running.

Add a drain connection to the domestic hot water in of your HX so you can easily descale it if needed.

You need some fill setup (back flow preventer and regulator) and pressure relief valve on the heat side.

Not diagram specific, but pumps should be somewhere physically lower down, pumping sideways or up, so gravity will help in priming it.

it depends on how accurately you want the radiant temperature controlled. A typical water heater could have a 20 degree differential. For comfortable radiant a 10- 15 degree temperature drop is desireable. A mixing valve would regulate the SWT to the radiant within 2-4 degrees.Regardless of the WH temperature

In. Perfect world, all the info, gpm. Flow rate and temperature drops. Both on the A and B side of the HX would be entered into the sizing software, then the exact HX spec would be available.

Without this step you could measure temperature drops and add a balance valve to trial and error your way to the results you want.

KISS as Alan mentioned would be to run it without the mix valve, see what you get.

I would either add two valves or a Webstone purge ball valve on the potable side to acid flush it, or make the HX removable to descale it.

Webstone valves on the radiantside to fill into and purge out of.

they are less $$ because they are not paying for the testing and the right to use the UPC and NSF shields.

Without those certifications I suppose they can say whatever they want as far as “similar to”

Installing a non hello hlisted device is against code and opens you up to a liability issue for the life of that product to future owners. Is it worth it?