Solar Drainback and variable speed pumps

rayplumber

Finishing up a small drainback system and considering using the Taco VT solar series pumps. I have a 20 ft head, and that includes a 20% allowance for extra friction, and have used 1" supply and return. I am concerned that using the 009 at 35 ft head would "cool" my single panel during the initial 3 minute start up. I've also thought of using a smaller VT, like the 008 in series with a 007 booster in order to have less velocity at start up. The 007 dropping out after 3 minutes. Or to confuse matters, should I use a Grundfos Alpha pump in series with a 15-42 as booster and rely on pressure rather than temperature delta T? I have on hand a Goldline controller and realize I won't use it if I go w/the Taco VT series. But couldn't I use the Goldline with a Taco VDT? Any suggestions appreciated!

hot_rod

single collector?

what size and type? Typically 1- 2 gpm per collector is about right. 1" tube is a bit large for that flow rate, you need to be able to maintain a siphon, oversized tube on the return may need to be balanced down. 1/2" may have been large enough?



What is the actual lift from the DB static level to the top of the collector? If it is under 20 feet you might be able to run a single circ like a 15-58.



Consider a controller with drainback function. That gives you a fill at 100% speed with adjustable time. Then a stabilization period, also adjustable. This helps prevent short cycling on start up. Once it is up and running the VS function will adjust the pump speed based on ∆T.



Time how long it takes to fill and establish the siphon, then adjust the filling time so the pump ramps to the required speed as soon as possible, once the siphon is estabilished.



The stabilization function ignores a shut off condition for the time period you select. This prevents the pump from frequent on/ off cycles on start up. Also handles that cooling concern.



You could use dual pumps, but you would not be able to VS an Alpha, so two 15-58 or whatever brand you prefer.



Install a schrader valve somewhere to pressurize the system, after you fill.



If the Goldline cannot vary the speed, don't bother. The new digital solar controls offer so much more for adjustability, basic data logging, nice displays, etc.



Include a lightning arrestor for the collector sensor, cheap insurance to prevent controller damage. i learned the hard way on two systems this summer.

rayplumber

After Xmas

Thanks for the reply. I'll give some thought to your pointers. But the heating demands have put this project off until the new year. People are soo irrational about heat!