Hydronic system: 3-way valve piping

[government kid]

Hi.
I was wondering if anyone knew why I have a cross connection from supply to return at every 3 way valve in my HW & CW systems.
They are constant volume systems with 3-way valves at each branch.
Thanks for any help in advance.

[Dallas Duster]

Is it setup for diverting or mixing? It's most likely the bypass of the coil I assume.

[Cagey57]

Since it is a "Constant Volume" pumping system(s) the bypass created by the 3-way valve lets that "Constant Volume" delivered by the pump(s) to continue to flow through the bypass port of the 3-way valve. Normally that is done so that the Pump(s) don't "Dead Head" if more than a certain percentage of the valves are closed, there by keeping the "Volume" (flow) constant through the piping system.

On a "constant Volume" system when 2-way valves are used if more than about 35% of the valves close the pump discharge pressures increase and cause unwanted issues on the other parts of the system.

[government kid]

Used for mixing.

[hcong]

I second Cagey57, your constant speed pump is going to pump the same amount of water no matter if you are cooling or heating a single fan coil or all of them... so your water can bypass all the fan coils that are shut off and run back to the return. Variable speed pumps won't need the 3 way valves... 2 way will do.

[19characterusername]

If there are balance valves on the bypass legs (there should be to mimic the pressure drop of the coil) then you can add VFDs to your pumps and go to variable flow pumping and save a boatload of money on pump energy. Typical paybacks 1.5 to 4 years. A little longer if you have to isolate the piping and insert a shut off valve in or cut and cap the bypass leg. You can replace the three way valves with two way as they fail at end of life or with equipment replacement.

[government kid]

Variable speed would be nice. A few years back a consultant was hired to give us system options, I'm sure the price came in at close to 500k.

Constant flow seems to work fine with 1 chiller running (which is about 85% of the year). Chilled water temperatures great in the distribution system until system variables come into play. Like when OAT shuts off pumps for fresh air units, or summer loads bring on multiple chillers.

[maxster]

chillers and boilers need to see constant flows no matter what position the 3-way is moving to as the AHU reaches setpoint the valves moves to shut coil entering and by pass that water into the returns thru the by pass.seeing 2-wy valves on chilled water sytems is a flag where engineers go with the idea water will pass thru the coils to make the loop into that return..the chiller picks up low flow returns and reacts unloading or tripping flow switch or going off on freeze protection