Delta T Winter settings Questions?

[solarhotairpanels]

I installed a drain back solar hot water system consisting of:

2 - 20 tube evac units. Total 40 tubes
6 gallon drain back tank
40 gallon storage tank
Grundfos variable speed pump

My question is has anyone here had better luck creating hot water in the storage tank by changing the Delta T settings to what I am calling a winter setting?

I'm talking during 20 degree weather and colder.

During the warmer months I leave mine set at: 12/8
When the collectors are 12 degrees warmer then the storage tank my pump kicks in and pumps until the collector is within 8 degrees of storage tank temp then pump shuts down and waits for heat to build up again.

Has anyone experienced better results heating their storage tanks by using different setting during winter and if so what settings are you utilizing to accomplish your goals?

What's happening right now with my system is if the sun is beaming my pump fires up on cue and hardly ever shuts down. Water just keeps circulating from my storage tank up thru the collectors and back but the collector temp never gets cooled down enough to shut the pump down.

Initially, when the controller turns on the pump I am pumping 45 degree water from my storage tank up thru the collectors which are staying right around 74 degrees.

My storage tank temp increases to 65 degrees and sometimes a little warmer but pump just keeps on cranking..

Your thoughts, experimentations, test results during winter months would be appreciated.

Thanks!