I currently have the Watts Recirculating system, pump Model 500803, on my water heater with the bypass sensors located under two areas in the house. We have enjoyed it. The con on this is that the cold water now takes too much time to make it to the spigot due to the lag time for the sensor to stop the reverse flow. We are doing a remodel and I am now running a dedicated line back to the water heater from the the farthest spigot and will remove the under sink bypass sensors making it a closed system now. I was told to purchase a new pump, Grundfos Type UP15-10B5, for the return line to be mounted on the water heater cleanout spout as per instructions. Rather than spend the money on a new Groudouf pump, could I just leave the Watts pump on top of the water heater? Or move the Watts down to the spigot area. Both pumps look familiar and have same flow direction.
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Water – Recirculator pump hot water
water-heater
Related Solutions
I suspect you have a set-up like this?
You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?
The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)
In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.
I've been looking into a heat pump replacement.
You said "However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter."
That will depend on the location of the internal connections. The HP models I have seen with a desuperheater ( hot water generator ) place the HWG after the compressor before the reversing valve. That means the pump always runs forward, the HWG always has hot refrigerant to use and it works in summer and in winter. The reversing valve does just that - in summer it sends the leftover heat to the outside and extracts heat (cools) the inside. In winter it sends the heat to the inside and extracts heat from the outside. 
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Best Answer
I believe that you could use that pump as you describe pumping through a dedicated return line. For reference, this is what I did for my house. I have 3 bathrooms, so I ran a 1/2" dedicated return line from each bathroom to a point near the hot water tank. At that point, I tied all the lines together and ran that line thru the smallest bronze pump I could buy that was for domestic water. On the discharge of the pump I installed a 1/4" orfice to limit the pumps flow and from there to the bottom of the tank. I use a timer to only run the pump at specific times. This set up has worked for 20+ years. One last thing, When running the dedicated line, make sure that you install an isolation valve and a check valve to stop any reverse flow.