Another heat source (electric heaters) or insulation being added (even putting a blanket over a large window or glass door may help some) can help, but the reality is that properly sized furnaces in our area are not designed to keep your home 70 degrees in -20F weather. Design conditions for our area are Zero degrees which means that below that there is a chance that your furnace may not keep your home at 70 degrees or higher. Thankfully you're able to maintain 67 and the end of this weather is in sight. Make sure your flue stays clear and your filter is clean/new-ish and your furnace should keep doing what it can to keep you as comfortable as possible until the weather breaks.
67-68 degrees is good for today's temps. Wait it out and see that would be my advice.
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