Why don't we pump away from expansion tank on dhw recirc systems?

I'm sure many better educated than me can go into better detail. With a recirc on an open DHW system, we don't have a fill valve located at the expansion tank, and since the system is open we aren't going to be eliminating entrained air from the system. For my knowledge these are two primary advantages to establishing a proper PONPC in a closed loop hydronic system, purging air and maintaining proper fill pressure when a fill valve is present. neither of these are relevant really in an open DHW piping system

On your tank mixer documents you would see your choices for tank location are either on the outlet hot port, or the inlet cold port, both on the tank side of the piping, you can't locate it past the mix valve because the valve can become isolated from the burner, and you have to locate the recirc in the location shown. Of the two available locations to pipe the expansion tank, the cold side is preferred because the tank should have a longer life being subjected to lower water temperatures, and prevents the tank lining from being damaged in the event the water heater overtemps on the hot side.

I'd love for someone maybe a bit better educated on the subject than me to chime in, this is all from my understanding of the PONPC I typically don't dabble in the DHW side of things and usually stick to closed loop so there may be things I don't fully understand at work here also.

Probably the simplest reason is that as many have implied there is very little if any chance of the pump cavitating, never mind pulling air. It would be a very rare domestic water setup which had a static pressure less than around 20 psig; 30 psig would be more normal. With that much static pressure you would need 60 feet of head loss or more between the expansion tank and the pump inlet to get in trouble. Not going to happen…

It's an open system. The PONPC concept does not apply. the pressure is always changing as you open and close faucets. The reason for the ThermXTrol is to allow for expansion of the cold water in the water heater to have a place to go when it heats up after all the faucets are closed. in the past that expanded water would just back up into the cold side and force the pressure back into the city water main. Today, however, many systems have a check valve or backflow preventer at the water meter location so that expanded water can't get back into the city water main. With no place to go, it expands and builds up pressure until the relief valve releases the pressure and who wants all that hot water spilling all over the laundry room floor?