Water flow detection but not for leaks

[picta]

What?  
 
What do you think is coming into the tank when hot water gets used from a tap?  COLD water.  How would that be any different than water being recirculated?  If anything the in-wall water would likely be a bit warmer than what's coming in from underground.  So, no, there's no more risk of thermal shock than present during normal operation.  
 
That and when circulating there's no changes in pressure involved.  The water is already in the system and it's all operating at the same pressure.  As it pumps water out, it's pulling water in.  The same pressure out, the same pressure in.   Water recirculating systems like this have been around for decades.

 
Not quite. There is a pressure differential in the pipe proportional to the pipe's length and the flow velocity, and it increases with the number of fittings and bends. The pump's performance will depend on the plumbing configuration used. I know we have a few elbows on the recirc line, and it also runs up in the attic, while the pump is in the house. I could probably upgrade the pump, but the next model up is too pricey to justify the change. I use automation rules instead and the water is ready most of the time.

 

[KeepItSimpleStupid]

I too am planiing a Metlund Demand system.  Been planning for years and have most of the parts.  That system places a pump at the furthest fixture that bridges the hot and cold lines.  It runs on a push button event.  However, the fixture method wonn't be used.  It will be below the fixture in the ceiling below.
 
The plan at this point is a motion detector in the bathroom.  Nearly every time you enter, you wash your hands or take a shower, so that will run the pump prior to your need to use it.
 
The kitchen will use an RF remote that has some other lighting functions.  This is X-10 based.  I should be able to intercept X-10 signals and bridge them to UPB if needed.  If I wanted to, the automation controller, Webmountain, could do it my timer.
 
It will be internet enabled, so smart phone activation too.
 
The wireless will be a system from Linear which uses their DXS alarm modules.  The actual mechanism for smart phone activation isn't settled yet.
 
In any event, the "system" is starting out to be activation of the circulation pump,  driveway lights  and the creation of a wireless doorbell. Common features are an 8 channel DXS rreceiver module (doorbell, pump) and activation of the driveway light from an RF remote in car and on kitchen wall and from cell phone.
 
An opportunity will present itself to email me when the PERS (Medical Alert system goes off) and it might be possible to issue a ALL light on on that event and cause the outside lights to blink which could be turned off from the cell.
 
How things integrate, I don;t really care,  The X-10 driveway light system broke,  Both cars don;t support Homelink and I don;t have the range with another,
 
Other automation issues include adding a porch and post light.  Astronomic control of the lighting would be possible too.
 
On the subject of showers, I have to fix the vent and add a check valve to keep the birds out.  The fan is too noisy - probably duct related,  Later in the plan is to detect flow and use a frequency switch from Jaycar (.au) to detect if hot water is being used in the shower.  Currently, the exhaust fan runs for 15 minutes after it's turned off.
 
I have lots of issues to solve first.  Pump fittings have been installled.
 
So, that's my plan.