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Learn More About HVAC Three-Way Valves

There are two types of three-way valves used in the HVAC industry: Mixing Valves and Diverting Valves.  In order to prevent any misunderstanding due to terminology, we will consider mixing valves to have two inlets and one outlet, and diverting valves to have one inlet and two outlets.

Figure 1.

Many people will call all three-way valves mixing valves.  Three-way valves can also be referred to as bypass valves, constant flow valves, and many other terms. 

Note:  Improper use of one for the other will cause chattering, water hammer, vibration, and damage to the system.

Mixing valves are more commonly used in the HVAC field.  Mixing valves make good modulating valves although they can be used like two-position valves, taking the full flow from one or the other inlet to the common outlet.

Diverting valves are usually used as two-position.  The flow is totally diverted either one way or the other.  Generally speaking, diverting valves do not make good modulating valves, although some valve manufacturers are putting characterized plugs into three-way diverting valves so they can be used to modulate.  Valve manufacturers normally specify in their catalogs if a valve is for mixing or diverting service.

Once the determination has been made as to what three-way valve you’re dealing with, mixing or diverting, modulating or two-position, the selection should proceed much like two-way valves.  Find the CV factor.  As before, you need to know full flow rate and DP. 

Three-way valves are used in many closed-system applications. Examples include:

1.      Flow temperature variation

2.      Flow volume variation

3.      Primary / secondary pumping systems

4.      Two / four pipe distribution systems

There are no “rule of thumb” ways to determine flow rate or available pressure for a three-way valve.  All the specifications must be known to determine the flow rate for a three-way valve.

Figure 2.

Figure 2 shows a three-way valve varying the temperature of the flow.  Notice the amount of water to the system (shown here as a coil) doesn’t change.  In this case, a low DP is desired.  Use 20% of the available pressure.  In this example, 20 psi is available.  4 psi would be the DP to use to find a CV.

Figure 3.

In Figure 3, we are varying the amount of flow through the coil.  In this application, a high DP across the valve is desirable.  Use 50% of available pressure, 5 psi minimum if possible.  In the example, 18 psi is available, so 9 psi is the DP to use to find the CV.  If the available pressure fell below 10 psi, say 8 psi, use 5 psi as DP.

As in two-way valves, if the three-way valve selected is less than line size, don’t forget the FP factor.  Resize the valve applying the FP factor to find the new CV.

For three-way valves used in chilled water-hot water, summer-winter changeover, mixing two-position or diverting, use a line size valve.  This is a low DP application.  Full flow is desired.

To find the static pressure a valve must be rated for, the following formula is used:


 Static Pressure Rating (in psig)      =     [(HFP + HT) + (HP - HF)]  /   2.31

Where     HFP = Fill pressure at low point of system in feet of water.

HT = Distance of valve above low point of system.

HP = Total pump head in feet of water.

And          HF = Friction loss in piping between valve and pump in feet of water.

Unfortunately, not all of the information may be known to solve the Static Head Pressure Rating (SHPR).  A method can be used to determine an approximation of the SHPR. Take the fill pressure and add the pump head pressure of the largest pump in the system.  Make sure the valve body static pressure rating equals or exceeds that sum. You do need those two pieces of information.

Close-off pressure ratings for three-way valves in a closed loop must equal or exceed the total pressure difference that can occur across either port when that port is closed.

Figure 4.

In Figure 4, the maximum pressure the valve would have to close-off against would be equal to the sum of the pressure drops in the coil, coil pumping legs, and the valve with full flow from B to AB.  This is because when there is no flow through the bypass, X to A, the pressures at X and A are the same.  The maximum DP the valve must close off against is equal only to the DP from X to whichever circuit (A or B) has the highest resistance to maximum flow plus the pressure drop through the valve.

Figure 5.

In Figure 5, the situation is the same.  The valve must close off against the highest pressure drop from X to AB.  Unfortunately in the real world of valve sizing, the DP’s necessary to check a three-way valve’s close off pressure is almost never known.  Usually, one might even say luckily, the valve selected by flow rate and DP will have close off ratings high enough to work.

Three-way valves used on cooling towers present special problems.  We are no longer dealing with closed loops, but open loops.  Open loop systems are systems open to the atmosphere in some portion of the system.

When the condenser is at the same level or above the cooling tower, a three-way diverting valve is recommended in the bypass section.  A three-way mixing valve is not recommended at point A, since it would be on the pump suction side and create vacuum conditions rather than maintain atmospheric pressures.  See Figure 6.

Figure 6.

When the condenser is below the level of the cooling tower a bypass using a two-way valve is recommended.

The DP from A to B at full flow should equal the head C-D.  See Figure 7.

Figure 7.

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