Of all the components you will come across in your trade, valves are perhaps the most common component. They are everywhere! Almost every HVAC application and installation uses valves. Plumbing, heating, refrigeration, solar, drains, fire protection, lawn sprinklers installations – all use valves.

In some cases, the valve will be a code requirement or part of a listed manufactured assembly. It may serve as a safety, isolation, balance, mixing or any combination, valve. It may require listings and certifications required by the application. For example, valves used for fire protection might need FM listings. 

WOG (water, oil and gas) was a common denotation on valves. It was intended to imply any nonflammable, compressible fluid in a gaseous state. Perhaps to eliminate some confusion, some brands are now using the term cold-water pressure – the maximum pressure, as a nonshock pressure, at which the valve can operate. To be safe, check with the manufacturer when using fluids other than water.

Selecting valves requires some important data-gathering. The type of fluid, temperature, pressure and flow rate are commonly needed to select even a basic shut-off or isolation valve. For example, the valve may have an electrical component, as a zone or control valve, where the operating voltage needs to be known. 

If the valve has a control or end switch, you need to ensure that the rating of the component meets the requirement. Balance valves may have a range of working flow rates as they are designed to work within the system. If the valve is applied in a potable water application, low-lead requirements must be met and the valve listed to that standard.

Of course, like any product, there is often a wide range of pricing. Those among us who touch and work with valves on a regular basis can usually tell a quality valve from an inexpensive knock-off. High-volume contractors may refer to their suppliers for brand selection. Others contractors use a brand their dad or granddad used, just because. 

Flow coefficient value

So here are some important criteria to consider. The flow coefficient value (Cv) is a term you need to be familiar with. Simply defined, it is the flow rate of plain water at 60 degrees, measured in gallons per minute, which creates a pressure drop of 1 pounds per square inch. This information is sometimes printed or stamped on the device. It should be in the technical and submittal sheets or be searchable at the manufacturer’s website. Tables and spreadsheets are available online that show your selected valves and their relationship to flow and pressure drop. 

Suppose you are sizing a thermostatic mixing valve to install on a water heater. This valve will blend the hot water from the tank or heater with some cold water to achieve the desired supply and outlet temperatures. Three-way valves are becoming common on water heaters. These valves allow a high tank temperature to discourage bacteria growth while providing a lower, safe “mixed” supply temperature, sometimes code-dictated, to the building. 

A common thermostatic mixing valve may have a Cv of 3. So, 3 gpm flowing through the valve would present a drop of 1 psi. No problem. Suppose at a design load condition, the home or building required 8 gpm. Would a 3 Cv valve be appropriate for that application? I use a spreadsheet to predict what the pressure drop will be (see Figures 1 and 2). It will work for any valve. With a spreadsheet, if you know any two values, you can solve for the third.

Ideally, you would find a valve with a Cv at or close to what the flow rate will be. Manufacturers generally do not make valves for every Cv. A chart or spreadsheet will help you determine how well a valve will perform and if the pressure drop is acceptable. 

For a home plumbing system, 75 psi is a commonly agreed upon maximum pressure. It keeps the faucet and toilet fill valve folks happy. If I install a system with a flow of 8 gpm through a 3 Cv valve, is it an acceptable application? See Figure 1.

Suppose you are on a private well and your system pressure varies from 30 to 50 psi, is a 7 psi pressure drop still acceptable? I have seen specs on 3 Cv mixing valves indicate a max of 20 gpm! Figure 2 shows what that would look like. 

An engineer friend referred to 20 gpm flow through a 3 Cv mixing valve as technical insanity. In addition to excessive pressure drop, the velocity passing through may cause the valve to “sing” to you. Velocity noise is, well, maybe not a tune you expect or enjoy. Excessive wear may be experienced, especially with hot, hard water flowing. 

If you use a three-way thermostatic mixing valve for a hydronic application, the Cv plays into the circulator sizing. The pressure drop through the valve needs to be in the pump selection criteria, usually expressed as feet of head. The spreadsheet also shows the psi pressure drop, more commonly used in potable water sizing terminology.

Another valve to pay attention to is a check valve. The Cv rating on valves is generally in the full-open position. Consider a swing check if the valve has an 8 Cv and you are flowing at 3 gpm. The flapper may be floating somewhere between the full-open and the closed positions. You may experience chatter or flow noise under that condition. 

While it may look unusual, be sure to size the check valve to the flow rate, not the pipe size. I prefer a hydronic-specific check valve for hydronics. While commonly used, a typical off-the-shelf, brass-to-brass seal swing check will not provide a bubble-tight seal. A swing check needs a pressure differential to be able to shut and seal. In hydronics, when the circulator is off the pressure is the same on both sides. The seal would be questionable. Save the swing checks for your sump pumps. 

Some piping distance between a valve and a pump intake/discharge is suggested. There are specialty check valves built with unique engineered features and functions. For industrial applications, you will find application-specific valves. Look for all the needed data online, or consult with your manufacturers’ rep or the factory directly.

Valves are your friends! Search for the valve that best meets the application; many choices are available. Size, apply and install them properly so you can maintain a happy and healthy relationship