With a recirculation return line, several things must be considered:
- you don't want the flow to be too fast...IOW, you don't want or need a huge pump as it takes more power, can tend to become noisy, and can literally wear out the pipes and shorten the life of the WH
- you do need a check valve to prevent sucking water from both the hot and return line at the same time when drawing water. AFter you've used some hot water, the bottom of that tank where the return usually goes will be cold, or at least tepid. Without a check valve, you'll be sucking it from both pipes, diluting your hot water.
- you want the lines insulated
- depending on the layout, you may need more than one return line so water will flow to each fixture, or at least close enough to minimize the waste and time to get it there
- for economy, it can help if you have a thermostatic control to stop the flow
- water will take the path of least resistance, so if you do end up with multiple loops, you may need balancing valves so all get hot water.
- sensing the water temperature at the fixture can be less expensive than back at the WH, since you don't normally need the return line to be fully hot..just hot at or close to the point of use.
- if you're using a tempering valve, unless you're running hot water, the tempering valve may allow full hot out into the hot line...IOW, it may not be able to actually temper that water coming out of the tank. some tempering valves have a port on them for use with a recirculation system to overcome that problem. When drawing hot water, some cold goes into the tank, and some will mix with the hot to the house, limiting the maximum outlet temperature. WHen recirculating...there's NO flow in from the cold line to the WH, so there's no tempering action. It will either block flow, or let all hot from the tank out. If it has a recirculation port, the cooler, recirculation water will temper the outlet. Running the pump constantly without a sensor in the system would eventually get that hot line the same temperature as the WH, and that may be too hot per code. Returning the water to the bottom of the tank may not work properly without some futzing around if you have a tempering valve. You could simply put a T in before the tempering valve for it verses going into the bottom of the tank.
The system I have, once it gets things primed after the first timer activation of the day, only needs to run maybe 1-minute 4-5 x per hour to maintain it since I was able to insulate most of the pipes. Some systems run the pump constantly and use sensors at the point of use (POU) to block it once hot there, some just have a sensor that remotely turns the pump off (this can get more complicated if you need or want to sense at multiple locations), and, there are options about where to actually install the pump. The most common place is back at the WH since you often have power readily available, but the simpler way, is to put the pump and control at the POU. In my case, everything was a short run off of a main line, with the upstairs vanity the last POU. It was easy to drop a line down, install a receptacle from the one above the vanity into the cabinet, and once there, it took all of about 10-minutes to install it. Mine's been running fine for about 13-years now with no problems or maintenance. It has an integrated check valve, pump, and sensor in a small box. You could use either the cold line as the return, or a dedicated one. I can only hear it if I really am listening for it, but some will put it at the WH to avoid any possibility of that. It does take up a little space underneath the vanity, but that hasn't been an issue for me. It would have been a major pain to use a dedicated return line. I find that flushing the toilet essentially flushes all of the hot/warm water out of the cold line. Part of that is the way I have mine adjusted...it has an adjustable aquastat (most don't) that I set to get that vanity warm. Since everything else in the house is before it, those are hot, and hot's not far away, but I usually don't need immediate full hot at that vanity. Warm works just fine to wash my hands.