It depends on your sun. New England winter sun is unlikely to burn phals, but southern California winter sun is quite bright.

I agree with the previous posts about the plants needing to feel a dip in temperature for a few weeks - but honestly, if your light exposure isn't sufficient, this won't matter much. Make sure the plant is healthy, potted nicely in media that isn't broken down, and give that plant good light. I have all my phals in south windows with fans on them to keep the leaves from burning. That is the advice I was given when I started: most orchids flower best when given as much light as they can handle without burning the leaves. I have had phals for 10 years that flower reliably every spring - you can do it too!!

Well, I am in New England. Brrr. Anyway, I am providing the temp differential each day and I now have 2 phals with spikes, which is so exciting, since I have never been able to spike this type - I have done cattleyas, and dendrobiums but not phals. As to light, I have a very bright and well lit home with lots of windows so they get plenty. Thanks for all input.

All my phals are flowering on a North window with a shear. At night the temp drops about 10 degrees, I think that is the key.
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Beauty of a plant. Like it that you have a northern window exposure and a 10 degree only drop. And blooms!! Seems there are many scenarios in which to live and thrive for these plants. Yours certainly is thriving. Thanks for help.

A study at Texas A&M showed that it's not the day/night difference that causes spike initiation in phals, it's an 10°-15°F overall reduction in average growing temperature for about ten days to two weeks that does it.

A few years back, I tracked the temp daily for a whole year. I had that degree of day/night difference occur in every single month, but spikes were not initiated until the plants saw two weeks of that reduced average. Read more here.

All I know is during the day my temp in my grow area is 70 but at night when we shut the woodburner down it drops to 60. All my phals are blooming. The temp during the day has not changed much except when it is extremely hot in summer.

I agree that it is the overall temp drop that would occur in fall. Is your grow area somewhat warmer in summer, both during day and night?

It only changes when it is extremely hot or cold outside. Most of the time it stays 70.