First floor window went down to 54° last night. (Paphs)
Second floor down to 52°. (Phrags) House thermostat is set to have the heat come on at night at 58°.

Bordering on too low?

I could open some vents partway, or not. Fans still running on the first floor, just two little ones. Averaging 70-75° in both windows during the days.

Very cold and windy here tonight! Should I go up to 60° at night???

It's dependent on the species that you're growing. I suspect that intermediate and cool growing paph species and most hybrids will do just fine. In fact, the cooler night temps are ideal for species such as armeniacum, micranthum, malipoense, tigrinum, etc. that need a cool winter rest. Some warmer growing varieties might not like it so cool.

For multiflorals, statistics show average night temps for most to be in the low 70's to mid 60's, with the exception of rothschildianum, which has night temps in the lower 60's, and parishii and dianthum, which have winter lows in the 50's and 40's. That said, who knows how adaptable the plants can be?

As an aside, when my armeniacum shows a sheath in the winter, I move it every night into an unheated room and close the door. In the morning I bring it back and put it under the lights. The average winter temp in that room is 45F with the door closed.

Heather

All of my plants survived Winter this year. Usually I lose the one/two phallies I buy the season before because it's too cold. I had no major problems with my Paphs with night temperatures averaging in the mid 40's.

In one way I envy you your cold weather as it's when I personally thrive. Day time temps here in Winter go to the mid 80's though :) That'll probably be way different to you.

My observations were that light was the governing factor on plant health. I have a callos-rothschildianum which I repotted in April as it was showing signs of distress (roots in poor shape) - when I repotted it again in Spring there was a mass of new root growth on the plant. Tried S/H on the strength of this and have recently reverted to pot culture after noting the same distress.

My point? - This plant was in what was basically full sun all Winter. Even with the cold nights it powered.

Cheers
Stephan