I've called Honeywell's customer service in the past regarding hooking up new replacement 'stats and received immediate help. YMMV

Turn the temperature adjustment all the way to its lowest setting, 42 degrees, and give the system 10 minutes or so to stabilize. Does the heat still stay on?

Did you follow the instructions that were packed with the thermostat? They seem pretty clear to me here.

I haven’t tried calling customer service but can. I did follow the instructions which basically tell you to connect the old lettered wires to the corresponding letters in the new unit. I just don’t know what the old B wire would be for and thinking that is my problem. Looking online older heat only systems only had 2 wires, R and W. I also don’t have a heat pump. The only thing I have seen is the B could be a C but I don’t want to try that and potentially break something, I also didn’t see a C wire when I replaced a unit in another zone a few years back. It also wouldn’t make sense why the old mercury unit would even have a terminal for a C.

Unfortunately, there wasn't much standardization in wire colors (especially early on). It was always a good idea to look at how the PREVIOUS thermostat was connected before removing it completely.

At this point, you're going to have to go find the other end o the wires at the furnace and figure out what they are connected to,.

I don't think the CT87N thermostat is going to work with your oil fired boiler. Honeywell makes several variations of the old style round thermostat. The one made for heat only hot water systems is the CT87K thermostat. Page 3 shows a 3-wire installation. I believe your current B wire should be connected to the Y terminal. Both Y and W should be energized when the thermostat calls for heat. If that doesn't work then you can swap Y and W connections. The R terminal is always the 24 V power from the transformer.

If you can't return the CT87N then call Honeywell and see if could be used. If you are going to buy another thermostat then consider buying a programmable type so you can program set back settings and save some fuel costs.

Is there any harm in trying to swap the wires or attach the B to Y, I am not sure if you can see the picture I attached in my original post but the wire was connected to a B terminal. I did see the heat only version had only three terminals R, W, and Y, so I contemplated trying that but was being cautious.

Do you have any idea what the third wire would be controlling?

Thank you all who commented. Mike I think this is exactly what I was looking for, i did some searching and I can clearly see how to wire this on an old CT87A thermostat it has the letters of my old wires and how they get wired in that model (W -> Y, B -> W). It seems like the heat only models are compatible with 3 wire zones. I just bought an Emerson model that states they support 3 wire zone to be safe, plus that one is programmable.

The problem is the "B" designation has two different meaning between old and new style thermostats. You would think with 26 letters in the alphabet thermostat manufacturers could have come up with better nomenclature.

Which Emerson model did you buy? I am skeptical an Emerson programmable thermostat will work with your system, but it is possible.

I bought this one https://climate.emerson.com/en-us/shop/1/white-rodgers-sku-1f95ez-0671

Model 1F95EZ-0671, said it was compatible with 3 wire zone value connections.

Seems like White Rodgers manufactures 3 wire zone values and Emerson now owns them. Here was the picture from the manual:

Now the only question is whether I connect the W-->W, or B-->W and the other wire to "6", guess there is only one way to find out.

This Emerson model looks like it will work. Most of the default program settings have to be changed for your needs.

You don't think I will just need to connect the wires to the appropriate locations, you think there are more Menu settings that will need to be adjusted? Can you point me to the sections in the manual for this or if you know off hand. I just need to know what I should be looking for to change. I do appreciate all of your help on this.

Google on Model and get manual.

Manual

PS I didn't read it to see if there is a separate Installer's Manual.