Sharon,
The amount of chlorine you need without the SWG running is the same as you would need if you were just using bleach and didn't have the SWG and these amounts may be found in Ben's Best Guess CYA chart. If you are losing chlorine quickly, then this may be due to your low CYA if your pool is exposed to sunlight. Just because it's winter doesn't mean the sun doesn't ever shine (especially if you're lucky). Chlorine will still get used up in the winter, but it lasts longer due to the less strong (lower in the sky) and less frequent sunlight.
I suggest getting your CYA level up to at least 30 ppm and then you could probably get away with the 3 ppm "Min" column number for Free Chlorine (FC) level since your water temperatures are cold. Just use ordinary unscented bleach to raise the FC level. If you do need to increase your CYA, using the Trichlor pucks is probably the easiest way to do that as this will provide both CYA and FC for you, but with the cold temps the tabs will be slow to dissolve. You'll also have to add base (such as Borax) to compensate for the drop in pH that will come from using the Trichlor tabs. You could instead use Washing Soda (Sodium Carbonate -- same stuff in pH Up) to raise the pH since you'll probably want a higher Total Alkalinity (TA) level when you are using the pucks (say, closer to 120 ppm instead of the 80 ppm you may be at now).
I am a bit surprised that your chlorine level went away so quickly with the water being so cold and the sun in a low winter position (and fewer hours of daylight, etc.). Perhaps you actually have high chlorine levels and don't know it. What kind of chlorine test kit are you using? Is it the OTO test that has a yellow color? Or is it the DPD test that has a pink/red color? Or is it the FAS-DPD test that counts drops until the pink/red disappears? You can dilute your pool water with distilled (or bottled, but not tap) water and retest to see if it shows a chlorine level.
If you give us a complete set of water chemistry parameters, we can tell you what you need to do. That would include pH, Total Alkalinity (TA), Calcium Hardness (CH), Cyanuric Acid (CYA). It sounds like you may not have a good test kit and should consider a Taylor K-2006 or similar kit as recommended on this forum.
By the way, if you had a pool cover, then that would cut down the loss of chlorine to almost nothing.
Richard
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