pexwater
I am interested in getting a hot water recirculation pump installed so that I can get hot water faster to each of my faucets.
From looking at the picture and researching I believe I have PEX in a "home run" configuration:
When doing my walk through with my warranty guy I mentioned I was interested in getting a recirculation pump, but he told me he wasn't sure how that works with a manifold and that I should investigate first.
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 have heard of issues like this with Moen "pressure balance" valves, in circumstances where there is a pressure difference between the hot and cold supply (call Moen, they will tell you about it too). You said the valve was fairly new so it is likely a pressure-balancing unit, and this sound like a pressure-balance feature malfunction. Turn the pump OFF and test the system to see if the problem goes away.
If the test proves the pump is the cause then it may be oversized for the small single circuit on which it has been installed, maintaining an increased pressure on the hot-water side even with the water running, whereas optimal would be for the pressure to drop to normal system pressure when the valve is opened. As a matter of fact, removing any restriction to flow, as suggested already by others, might help (though, counterintuitively, for a totally different reason).
Grundfos re-circulators are powerful pumps, putting one on a small loop is not commonly done. More often a thermosiphon circulating loop is established, without a pump, and they work quite well. You could convert yours just by yanking out the pump and connecting the pipes up (make sure there is a check valve on the return line near the boiler, as mentioned by @d.george.
Diagram shamelessly stolen from the internet
Best Answer
I'm aware of two types of recirculation pumps.
The traditional system runs a connection from the most distant hot water tap (often the kitchen or master bath) back to the hot water heater. From there, you would have a check valve and a pump that moves the hot water from the tank, to that distant fixture, back through the return line that was added, and into the hot water tank via the cold inlet. With a home run PEX system, you don't have a most distant fixture.
A second type avoids the need to run the return line to the hot water tank by connecting the distant fixture by using the cold water line for the return. This means your cold line would end up being filled with water that had gone through the hot water tank, which is a significant drawback to this system (you really don't want to drink the water that's been through a hot water tank with all the sediment).
Since PEX is typically a home run install, this means you'll need to install a device on each fixture you want to have recirculated. If, for example, you only hook up the bath tub, you won't see any benefit at the sink if they are each a separate home run. Each of these connections would be independently controlled, either with a thermostat, timer, or some other on demand switch, so that your cold water lines don't fill with hot water. You will also need to have a power source at each fixture to run the pump.