it will encounter pressure loss due to friction in the piping. The circulator needs to produce the required amount of flow while overcoming that pressure loss. Pressure is generally measured in pounds per square inch, or psi. In hydronics, head loss is the common term. Converting psi to head loss is very simple. A column of water 2.31 feet, or 28 inches high, will have a gauge pressure at the bottom of 1 psi. Therefore, 1 psi of pressure drop in a system equals 2.31 feet of head. It’s important to remember when sizing a circulator there is no need to take into account the height of the building. In in a closed loop system, the circulator does not need to lift the water to the top of the building, it only has to overcome the friction loss of the piping and components or the head loss of the system. To size a circulator properly, start with a heat loss analysis of the structure. The flow rate needed for a particular job or a particular zone is based solely on the heat loss. The heat loss analysis should be conducted on a room-byroom basis, since each room will have its own unique heating requirements and will require terminal units (typically baseboard) sized for its unique needs. After each room is calculated, group the rooms together into zones to determine what the zone's total heat loss is. This is important when zoning by circulator, since each circulator will be based on the zone flow requirement. When zoning with zone valves, it will be important to know the flow rate for the entire job so the circulator will be sized correctly. Once the Btuh load of the zone or of the entire job is known, depending on whether zoning will be accomplished with zone valves or circulators, calculate the actual flow rate using the Universal Hydronics formula: gpm = Btuh/(ΔT x 500). 26 National Oilheat Research Alliance For example, when zoning with circulators, and there is a zone that has 27,000 Btus’ worth of baseboard, or roughly 45 feet. Since it is baseboard, use a 20° ΔT, (100% water—no glycol). To determine the flow rate, divide the load, 27,000, by the ΔT of 20 times 500. GPM = Btuh/ΔT x 500 GPM = 27,000/20 x 500 = 2.7 The flow rate for the zone is 2.7 gallons per minute. What size pipe should be used for this zone? The rule of thumb for pipe sizing is two to four gallons per minute of flow, so use ¾" M copper pipes. These will keep flow velocities at no less than two feet per second and no more than four feet per second. At velocities greater than four feet per second, the system will produce unacceptable noise. At a velocity less than two feet per second, dissolved oxygen may be released and cause air problems within the system. Head Loss of a zone To determine the head loss of a zone, measure the total length of the zone, including both piping and heat emitters. Fittings and valves produce pressure drop in a system equivalent to a few feet of pipe. To accommodate basic fittings and valves, multiply the total length of pipe by 1.5. This is the total developed length of the circuit. Other head loss items, such as flo-valves (aka flowchecks) three-way valves and other high head loss items may be evaluated later. Next, take that number and multiply by .04. This number represents four feet of head per 100 feet of copper pipe. That head number applies as long as the pipe has been sized according to the velocity guidelines discussed above. The end product is the head loss for the zone.
NORA Advanced Hydronics
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