three way control valve balancing
three way control valve balancing
(OP)
Hi guys,
According to my CIBSE reference, a three way valve on a coil needs a balancing valve on the bypass line along with one in the return line back to the chiller/boiler. That bypass valve gets set to provide the same dP as the coil when in full bypass mode.
Whats the point? at part load say half the water going through the bypass the hydraulic resistance would be half (two resistances in parallel) so you will still be in overflow. On the other hand if you have a high dP coil on the index run (small dP on the balancing valve) and no bypass bal valve then you will be in even more serious overflow.
Does anyopne have any thoughts on this? Has anyone seen an arrangement without a balancing valve in the bypass??
According to my CIBSE reference, a three way valve on a coil needs a balancing valve on the bypass line along with one in the return line back to the chiller/boiler. That bypass valve gets set to provide the same dP as the coil when in full bypass mode.
Whats the point? at part load say half the water going through the bypass the hydraulic resistance would be half (two resistances in parallel) so you will still be in overflow. On the other hand if you have a high dP coil on the index run (small dP on the balancing valve) and no bypass bal valve then you will be in even more serious overflow.
Does anyopne have any thoughts on this? Has anyone seen an arrangement without a balancing valve in the bypass??





RE: three way control valve balancing
There is no ideal proportionality but balancing valves would help having it within reasonable limits.
The main reason for having balancing valve on main line is that too often dp on coil is quite small and you cannot find three-way valve with kv small enough to reach adequate hydraulic authority.
Yes, I saw many installation without balancing valve in bypass, in past when that was not mandated, and big trouble in commissioning as well, which often included later addition of balancing valve.
RE: three way control valve balancing
RE: three way control valve balancing
With no balancing valve in the bypass, the branch in question could divert even more water from reaching other coils on the main when the 3-way valve is in full bypass.
Question: Which has the greater annual energy cost?
RE: three way control valve balancing
Waramanga, older installation never had bypass valve, and even now, some "old school" engineering manuals still have their example setups without it. The reasoning is that you need only one branch to balance, to adapt it to other, but, as mentioned, long-term practice showed lot of troubles with it. There is already problem how to measure line without valve to compare it to another.
That would apply to real three-way mixing, modulating. If you use three-way valve only for on-off work with terminal units, than you would not mind having so precise balance, on branch regulating valve would be sufficient, for two reasons:
- you do not care for modulating control preciseness
- effect on eventual change would likely be negligible on hydronic loop level.
RE: three way control valve balancing
RE: three way control valve balancing
Too low flow in chiller = freeze.
Too low flow in pump = overheating.
Good on ya,
Goober Dave
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RE: three way control valve balancing
One can also use a single automatic flow balancing valve (Nexus, Flow Design Inc, Griswold) installed in the return (down stream of the bypass connection) in lieu of two manual valves. The automatic flow device will 'restrain' the water when the valve goes into bypass mode.
RE: three way control valve balancing