Putting three way valves in a variable flow system. 2

[hollandhvac]

Gents,
I have a design where three way vales have been added in a variable flow system, inorder to maintain a minimum flow.
This is againt what I was told; constant flow is three way valves and variable flow is two way valves for capacity control.
Who can tell me his experience with this. Will this work and what are the possible problems?

 

[MisterDonut]

In a variable flow system, sparing use of three-way valves is a technique used to maintain a minimum of flow throug the pumps at all times. This is not unusual and should work fine if applied correctly. The flow throught the three-way valves in the bypass position should only be enough to maintain the minimum flow through the pump. Any more than that will waste energy and defeat the benefits of the variable flow system.

 

[Drazen]

for such kind of reason you actually need way of bypassing by either three-way valve of bypass valve. in longer lines variable flow + intelligent pump can easily lead to insufficient differential pressure - or overflow - on more distant terminals at part loads.

this can be far from simple problem, while picv valve marketeers tend to put it under carpet as it does not fit well with their "silver bullet" stories.

 

[CarolinaPE]

You could shut the balancing valve in the bypass leg. What you have been told is generally correct (2-ways on a variable flow system, 3-ways on constant flow). Some engineers put a 3-way valve at the end of runs to "keep the pipe cold" in a 2-way valve system but generally it is only a matter of a few minutes to get the cold water out to the furthest coil on start-up. If too many 3-ways get put on a system you could wind up with "low delta T" and reduce efficiency.

 

[Wilbur55]

I think it depends on the size of the system. On a small system, the extra cost of the controls for a modulating bypass valve probably can't be justified. On a large system, the continual waste through 3-way valves may be evidence of a cookie-cutter design philosophy that worked on smaller projects. The designer has not provided proper service to their customer, who now has to live with (and pay for the operation of) a system that has not been given an amount of thought commensurate with the higher fee for a larger system.

 

[Drazen]

in earlier phases of picv valves introduction we have to modify variable flow system by adding either three-way valves or bypasses to longer branches to allow system to properly work.

variable-flow pump cannot operate properly at very low flows, and it either does not provide proper flow when too few fan coils are on or bypass if undesirable forced through some open fan-coils where there is no other bypass path.

moreover, large dead part of circuits needs time to cool down again which causes erratic temperatures in many rooms, not only those who are switched on, but also those that are on already as they suddenly receive water with higher temperature.

variable systems will work perfectly when switching scheme is not too demanding, but in many real situations designer has to make additional provision. this is far from simple recommendations which "solve all problems" according to some manufacturers.

 

[urgross]

You will need a minimum pressure for the 2-way PICV's to operate. Bypass or 3-way is needed at end to maintain the minimum pump and PICV operating pressure. On a large system, it may be desirable to include an in-line booster pump to maintain minimal operating PICV pressure, such as with a variable primary with distributed booster central chilled water system.