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Snail · 06-07-12, 05:49 PM

I should Google first. see here:

Pump and Pipe Sizing for a Solar Water or Space Heating System

Quoting from that page:

Step 2: Measure the Pump Vertical Head Requirement

For a drainback system, when the sun comes on the collectors, the controller will turn the pump on, and the pump must be able to pump water all the way up to the top of the collector to start the flow. In order to make sure the pump can do this, you must carefully measure the VERTICAL distance between the water level in the tank, and the top of the collector. If the top of the collector is 30 ft over and 11 ft up from the tank water level, the number you want is the 11 ft -- this is the vertical distance from water level to top of collector. Measure this carefully -- don't eyeball it. Note that you measure from the water surface level in the tank, not from the pump level.

This requirement at startup to pump water from the tank all the way to the top of the collector is a tough requirement for pumps, and it will likely be the main consideration in picking the pump to use.

For the my Solar Shed system, the vertical distance from the water level in the tank to the top of the collector is 11 feet 3 inches.

So, the pump must be capable of pushing water up at least this vertical distance. Some margin should be allowed about the vertical distance. In other words, don't choose a pump that just barely has enough static head to make the top of the collectors. People vary on how much margin should be added, but Alan R.ushforth, who has done a number of drainback systems and has had experience with pumps that don't quite get the flow started recommends that the pump have a static head capability that is 20 to 30% greater than the vertical distance from tank water level to the top of collector. I would use that recommendation.

So, for the Solar Shed, the minimum pump static head would (11.25ft)(1.25) = 14 ft of static (startup) head.

Once flow is well established, and the return line is running full, the only pressure that the pump has to overcome is the friction losses in the pipe -- that's what the next step is about.

06-07-12, 05:49 PM	#36
Snail Lurking Renovator Join Date: Jul 2011 Location: Palmerston North Posts: 20 Thanks: 3 Thanked 9 Times in 7 Posts	I should Google first. see here: Pump and Pipe Sizing for a Solar Water or Space Heating System Quoting from that page: Step 2: Measure the Pump Vertical Head Requirement For a drainback system, when the sun comes on the collectors, the controller will turn the pump on, and the pump must be able to pump water all the way up to the top of the collector to start the flow. In order to make sure the pump can do this, you must carefully measure the VERTICAL distance between the water level in the tank, and the top of the collector. If the top of the collector is 30 ft over and 11 ft up from the tank water level, the number you want is the 11 ft -- this is the vertical distance from water level to top of collector. Measure this carefully -- don't eyeball it. Note that you measure from the water surface level in the tank, not from the pump level. This requirement at startup to pump water from the tank all the way to the top of the collector is a tough requirement for pumps, and it will likely be the main consideration in picking the pump to use. For the my Solar Shed system, the vertical distance from the water level in the tank to the top of the collector is 11 feet 3 inches. So, the pump must be capable of pushing water up at least this vertical distance. Some margin should be allowed about the vertical distance. In other words, don't choose a pump that just barely has enough static head to make the top of the collectors. People vary on how much margin should be added, but Alan R.ushforth, who has done a number of drainback systems and has had experience with pumps that don't quite get the flow started recommends that the pump have a static head capability that is 20 to 30% greater than the vertical distance from tank water level to the top of collector. I would use that recommendation. So, for the Solar Shed, the minimum pump static head would (11.25ft)(1.25) = 14 ft of static (startup) head. Once flow is well established, and the return line is running full, the only pressure that the pump has to overcome is the friction losses in the pipe -- that's what the next step is about.