drain back pump sizing

radmix

I know how to size a residential pump but how does one size a pump for a drainback system. I have 8- 4x8 panels, 1" lines going to panels, 25' of rise from tank to panel and approximately 120' of 1" pipe. I would like to use one pump with a variable speed control.

Thanks

Rich

hot_rod

the lift

from the water level in the drainback tank to the top of the collector, and the flowrate required.



So 6 collectors at .75 gpm = 6 gpm. If the lift is 25' then that is the pump spec. With a variable speed solar controller the pump will run at 100% for the time you program. then the pump drops down to say 30- 40%, whatever you determine and program.



Then the pump speed modulates based on the temperature rise in the collector. That % is also programable. So the control maximizes the solar harvest and fill the system.



On this pump curve the 15-100 is a nice drainback pump. Most solar controllers will switch that size pump from the on-board relay.



Remember a pressurized drainback has some unique advantages. it reduces the risk of flashing when the fluid hits a hot collector, and helps overcomes the lift head.



the second attachment shows the min pressure at various fluid temperatures for the UP series Grundfos pumps.



hr

matt_sunwaysolar

size your pump

based on the recommended flow rate provided by the collector manufacturer. Typically you are around .75 - 1.5 gpm per panel. You'll probably be around 8 gpm recommended flow rate (8 4x8 Collectors).

What Hot Rod says is true. Add the lift, 25' in your case. Make sure that the pump can handle a little more than 25' of head at 8 gpm on a pump curve. I typically like run the sizing calcs for a pressurized, closed loop as well to make sure that the pump can handle the head and flow of a standard circuit once a siphon has been achieved. It almost always does, but I like to make sure. Likely, once the pump forces the air from the collectors into the drainback tank, the gpm will increase and the variable speed control will ramp it down to hit your delta T.



The Grundfos 15-100 pump is a nice pump for up to about 5 collectors. As you can see on the pump curve for it, at 8 gpm, it can push ZERO feet of head. Above about 5 gpm on a DB system, it can't typically provide the necessary push.