my mistake on the number of loops jackall but the total still somes out to whati have posted
All circulator pumps are typically sized based on the heat load and head loss (pressure drop) for a given zone.
Knowing the heat load (in BTU's) for a given zone, allows to calculate the required circulator pump's flow rate in gallons per minute (GPM).
For hot water hydronic or radiant heating applications, the following equation can be used:
GPM = 0.002*BTU/(Temperature Drop),
where Temperature Drop is the difference between supply and return temperatures in the system and GPM is the amount of flow the circulator must produce.
Since most of the radiant heating systems utilize a 20F temperature drop, the formula can be changed to:
1 GPM = 10,000 BTU/hr,
meaning that for every 10,000 BTU's of heat load the circulator must output a 1 gallon per minute flow.
Assuming that system calls for 100,000 BTU/hr, a circulator pump should have a minimum 10 Gallons Per Minute flow rate at a given pressure drop.
For snow melt systems with a 50/50 mix of glycol and water, the above equasion is slightly different:
1 GPM = 11,000 BTU/hr
The next step is to calculate the head loss, or pressure drop in the system.
Head loss is associated with friction of the water against the internal surface of the pipes/tubing in the hydronic or radiant heating system and restricts flow rate a circulator can produce.
Although radiant heat manifold and PEX tubing sizing are a different topic, let's assume, for example, that a manifold has 8 outlets with 1/2" PEX tubing installed at 300ft length per loop and the system calls for 72,000 BTU's.
Using the formula above, we can determine the flow rate required for our given zone: 72,000 / 10,000 = 7.2 GPM.
Flow rate through every selected circuit of the manifold equals Flow Rate divided by number of Circuits:
7.2 GPM / 8 circuits = 0.9 GPM per circuit (assuming that the circuits are equally balanced).
Using a Pressre Drop Table or Pressure Drop Chart, supplied by the PEX tubing manufacturer, a pressure drop per ft of tubing can be calculated at a given GPM flow rate.
NOTE: Pressure drop data supplied by manufacturers may be available both in PSI (lbs per square inch) and in foot (ft) of head.
For conversion, use the following equation: 1 psi = 0.434 ft of head (for fresh water).
In this example, pressure drop per 1 ft of 1/2" PEX tubing at 0.75 GPM flow rate would be approximately 0.03 ft of head).
Considering that each individual PEX tubing circuit is 300 ft long, pressure drop per circuit would be 0.03 x 300 = 9.0 ft of head.
Since PEX tubing circuits are in parallel to each other, pressure drop per circuit is always the same as the total zone pressure drop. So, the total pressure drop is: 9.0 ft of head.
We now have the complete specification for the circulator pump available: 7.2 GPM flow at 9.0 ft of head pressure drop.
It is important to understand that other components installed within a given zone (such as the radiant heat manifold itself, fittings, check valves, mixing valves, balancing valves, heat exchangers, etc.) also have to be considered when sizing a circulator pump. Pressure drop information is usually available in a form of technical specifications or submittal sheet supplied by the manufacturer.
Given real conditions, we may add extra 2 ft of head just in case, making pressure drop 11ft of head.
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Topic: need help sizing pump (Read 6279 times)