Like many other things concerning fluid dynamics, pumps (and fans) do not just pump they suck!
When nothing is turned on the static pressure throughout the system is equal.
Turn on the pump motor and a negative pressure is created in the hot water line.
Hot water flows from the heater down the hot water supply line because it now has a lower pressure than the cold water line.
If you turn on a hot water valve, a lower pressure is created in the hot water supply pipe and hot water from the tank flows towards the valve and cold water enters the hot water tank because it has a higher pressure.
Because the pump is on, that does not mean the pressure on the hot water side is higher than the pressure on the cold water supply. The sucking negative pressure is proportionate to the pumping positive pressure and circulation only occurs in the hot water loop.
Ok
Now I am beginning to grasp the concept.
I assumed that the 40 lb (for visual purpose) pressure is equal at both outlet and inlet but since the pump is not creating say 50 lb pressure it will not fight the incoming water.
In other words only the cold supply is giving resistance. WHACK! to the side of my head.
That is how the after market (construction) design gets back.
I misread this also (at first). It takes a kit at the furthest fixture and uses the cold water line as a return.
Not true. It either enters the cold inlet at the top which has a dip tube that leads to near the bottom of the tank or the modified drain valve assembly at the bottom.
Perhaps we are talking about different systems. I thought that you were responding to R Elliot’s post #11 regarding circulating pump systems rather than the passive type. The pump driven recirc systems I am familiar with are hot supply to drain side loops.
Chuck…did you see post #30? There are a couple of different designs for recirc systems. I think I tend to see more hot supply to drain out also but I have run across the style I showed in post #30 a few times.
OK, I guess I don’t encounter these under-sink add-on systems. So instead of waiting to flush the cold water from the hot water lines, when opening a hot faucet, they get to wait to flush the hot water from the cold lines when using the cold faucet.
What a deal!
I guess in TX it wouldn’t really matter too much, because the water in the cold lines sitting in the hot attic is going to be about 130 F degrees anyway:).
