I'm replacing a old thermostat with a new digital Honeywell one. On the old one both the Yellow and Blue wires are connected to the Y screw. I also have a green, red, and white. The new ones instructions say do not use wires marked B. This one is not marked B but I figure it refers to the blue wire. Should I just not hook up or put the two together like on the old one? I would rather not just hook up and see what way works because I rent and Landlord would be very unhappy with me if I broke anything.
Thanks for any help

There is no way to tell from here since the way the old thermostat was wired was not to the standard color code. You will have to trace all wires to where they go then note which colors do what at the furnace. When you have all that info we can try to help. BTW The yellow wire to the Y screw is usually used for the contactor in the outside A/C unit. I have a bad feeling that someone put the blue on with the yellow to activate a add on relay for the blower. But this is just a guess.

I pulled the plate off the wall and there are just 4 wires(bundled) coming out of the wall. The blue comes out of the wall and not the yellow. The yellow wire that is connected to the yellow screw in front is soldered(?) to a spot on the back of the plate. Looks like the plate was made this way. Think I should connect the blue wire that is connected to Y on old to Y on new? I have not looked at the board at furnace. It was not visible and I did not want to open the furnace up if I did not have to.
Thanks again.

It was not visible and I did not want to open the furnace up if I did not have to

That is your decission. More than likely they used the blue but the only way to tell for sure is to look.

I hooked the blue to the Y. New thermostat works well. Thanks for your help. I was reluctant to open the furnace because Landlords live next door, I have to go outside and through a cellar door to get to furnace. They would not like me messing with it even though just looking would not hurt anything.

Glad you got it working