Zone Valves When one circulator is serving multiple zones, the gpm requirement and head loss for each zone must be determined. To find the total gpm, add all of the zones together. However, to determine head loss, size the largest head loss. Example: Zone 1 requires 2.5 gpm with 6 foot of head loss Zone 2 requires 3.5 gpm with 4 foot of heat loss Circulator demands will be 6.0 gpm with 6 foot of head loss. GPM is cumulative; head loss is not cumulative—if it can overcome a high head loss zone, then the same circulator can overcome a lower head loss zone. Cv—coefficient of flow An additional factor in determining total head that a circulator may need to overcome is consideration of the different components in the system. In assessing those components, coefficient of flow (Cv) through those components must be determined. 28 National Oilheat Research Alliance Cv is a function of the pressure drop in devices installed in hydronic systems. Every component has an associated Cv value. This is important to understand because it represents the pressure drop in that device for a certain flow rate. For example, if a device has a Cv value of 3.5, the device has 3.5 gpm are flowing through it, will cause a 1 psi pressure drop. To equate that to foot of head loss, multiply the psi pressure drop by 2.31. A device with a Cv of 3.5 and with a design flow rate of 3.5 will have a pressure drop of 2.31 foot of head: (Flow/Cv)2 x 2.31 If the Flow rate for the system is 5 gpm, then there will be (5/3.5)2 x 2.31 = 4.7 foot of head for that device. Add that additional head loss to the head loss of the associated piping and other devices for proper circulator sizing. The higher the Cv rating for a device, the lower the pressure drop. Does the Zone Valve affect the Circulator Choice? In earlier calculations, it was determined that the hard piping head loss is 3.6 feet. If you are using zone valves, the head loss produced by the zone valve must also be included. Assuming a 1” valve has a Cv rating of 3.8, that means it takes 3.8 gallons per minute to create 1 psi of pressure drop through that particular valve. Different valves have different Cv ratings, so a valve that had a Cv rating of 12 would produce 1 psi of pressure drop at a flow rate of 12 gallons per minute. A valve with a Cv rating of 47 would create 1 psi of pressure drop at a flow rate of 47 gallons per minute.
NORA Advanced Hydronics
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