A recirculating pump is a small pump that goes on the furthest hot/cold junction in the house from the water heater - typically a suite bathroom. It pulls hot water from the hot line and pushes it back to the heater through the cold line - and yes, this will work even though the pipes are pressurized to 40-75 psi.
When the pump runs, you fill the line with hot water so there’s no wait for warm water when you turn on the tap. It will work for all fixtures along that line, so if you choose the right point, all sinks and showers will have boosted hot water.
There are two (and a half) ways to use such a pump: more or less continually running, continually running with a thermostatic shutoff at the far point (so it runs every half hour or so), or only “on demand” - when you push a start button at the far point. Many hotels and retirement homes use something like continuous operation for the convenience and safety of their guests. But that wastes at least some of the heat as it continually radiates out of the pipes.
So the on-demand mode is the same as flipping on the hot tap for a minute or two to run hot water to your sink, but without wasting that 1-5 gallons of water. It’s also convenient in that you don’t have to pay any attention; you can push the button and continue getting undressed, taking a whiz, whatever and the pre-hotted water will be waiting for you.
Like that. What the hard part for most people (I know it was for me) is wrapping your head around how it can pump water from the hot side into the cold side. As long as you can understand that, the rest should be fine. Just think that the entire system is under pressure and all it’s doing is pushing water around in a circle, that’s it. From the water heater through a loop (that the pump created) and back to the heater.
I’ve had my system for 12 years with only one repair. In my system there’s a check valve to prevent hot water from thermosyphining into the cold water side when the pumps not running. I had to disassemble the pump and replace that part once. Other than that it’s been great.
Just keep your actual goals in mind… if you just want to save the water from going down the drain then yes it’s all feasible.
But remember you’re spending/using extra energy to run the pump and losing a bit of heat through the pipes by “charging” them with hot water periodically; even if you only do that for a few minutes a day (not to mention the cost of the unit and installation it’s self), so overall cost wise I don’t know if you’re saving anything… quite possible you’d be losing a bit. Again though, if it’s a combination of convenience and water savings you’re after then yes these ideas would work.
