Sue,
If you don't have the humidity set high enough it can irritate the mucus membranes and cause congested nose. My machine has to have the humidifier part turned on separately from the other part of it, and if I ever forget to push that button, or let it run out of water, then I can have a runny nose for days and days (and sneezing) as a result. Also, if you have the humidity set high enough, but your lips come open and you're breathing through your mouth instead of nose, not only will you not benefit from the bipap, but you'll get dried out and have problems with runny nose. Last month I noticed (but then promptly forgot) that my humidifier was not turned on, and about a half hour after falling asleep, I awoke with a cold nose, even though I turned the heat on after discovering it, I still had a super runny nose and sneezed what seemed like a thousand times a day for a couple of days afterward.
So, crank up the heat, but then, if you have (because of this) "rain out" where water condenses and builds up in hose/mask, you can prevent the rain out (or at least lessen) by putting a hose snuggly (insulator) on it. You can purchase online, or I've made my own too. It also helps (if you don't move around much) to tuck the hose under the covers with you, to help keep it warm. The condensation forms from contrast between room air and air inside hose. Cooler room with warmer air causes it to happen quicker than warm room with warm air.
Hope this helps.