Compatibility with older Honeywell

[strider841]

Trying to replace older Honeywell thermostat. Heading system only. Gas burning to heat water for base board radiators. Apparently the gas burner replaced an earlier oil burner.

I only see very large wires, black and white. No clear labels on them either.

Picture included shows face and wires beneath the face.

Any suggestions?

[HVAC]

You need 24v. To operate most thermostats. 
From the pic I see yellow and black wire that’s possibly a millivolt system. Unfortunately won’t work. 

[strider841]

Thanks. After your reply, I investigated a little further on the boiler side of the system and found a label (albeit facing the floor!) that clearly says for 750 mV only. So that solves that, at least. I would post the picture, but somehow google's forum looks like it only allows the photo posting on the first post. Ah well.

[zoeuvre]

Hi folks,

Thanks for posting in the Google Nest Community forum. I'm glad to hear that you found an answer to your main concern. If you still wish to post another photo, please click this link, scroll down to your response, and hit reply. Check for the camera icon or click the three horizontal dots to expand the toolbar. From there, you'll be able to reply and share a photo.

I appreciate your input, @HVAC.

Best,

Zoe