Picked up my hot water recirculating pump last night to install today after lunch/dinner (or linner). I turned off the supply valve to the water heater & ran the sinks in the house, after 5 minutes they had slowed slightly but still had good pressure. I checked the supply valve by hand, it was as tight as I could get it by hand. I got my pipe wrench and gave it another quarter-turn & checked the sinks, the flow had slowed slightly. I gave the valve another half turn with the wrench, it was tight but moved, sinks slowed further. I did this a quarter-turn at a time after that and had the closest sink down to just more flow than a trickle. When I tried to give the valve another quarter-turn, it snapped. The handle and shaft spin, but I guess it's not attached to the internals of the valve anymore.
So now I'm calling a plumber out, hopefully tomorrow, to replace that supply valve and see if they'll mount the pump for me while they're at it. I also couldn't get the nut on the output line to budge.
I have a few choice words for how upset I am right now.