Am I probably looking at a new boiler?

Hello! I just found out that my natural gas bill for the month of March was absolutely atrocious. December, January and February weren't far behind. The gas services a larger hot water heater as well, and a range. We cook a fair bit between my wife and I.

The boiler system that came with the house when we bought it 3 years ago is a Burnham Holiday; from what I've read it's a reliable and durable boiler. The panel has a Water BTU/Hr rating of 120,000 and Input and Output BTU/Hr of 200,000 and 160,000 respectively. There's two circulator pumps, one for each zone. An expansion tank that's nearly rotted out (going to replace very soon), a much older gas valve that seems ok. I recently installed a new thermocouple myself because the pilot wouldn't stay lit- and that has been fine since. I also removed the lower tray and cleaned out the cast iron gas jets last summer- a lot of dirt/rust/debris was in there! In addition, as I can see the entire collection of gas jets, I adjust the main line every so often to get a soft, blue flame. But to be honest, I have no idea what I’m supposed to do with the air vents for each burner. I have them open a little less than mid-way.

I don't think the system has been professionally inspected/serviced in a very long time. That’s where I’m headed at this point with it- but I’d like to know what to expect going into it as well.

Functionally, the loop for our first floor doesn’t seem all that good. I have the “aquastat” set for nominal 180 degrees and the overtemp at 200 degrees, but in the several times this winter I peeked at the temp gage, it registered around 145-160, never above that. The boiler must run constantly to maintain our drafty old house at a balmy 67- I don’t think the system is currently able to heat it to 68 unless outside temperature is 40 degrees or greater. The upstairs loop seems fine; but we hardly ever run it. It seems the downstairs more or less heats the whole house. However- I think I should mention that our upstairs is carpeted, and also has siding installed on the exterior. The first floor is brick exterior with horsehair plaster interior walls- and nothing in between- and hardwood flooring. Newly installed windows (white plastic-type frame) throughout the house, but I think a lot of the rest is working against me.

I’m trying to get myself more comfortable with the idea of having to buy a new boiler/furnace. In the meantime, I’ll have a professional inspect the entire system.

Could my boiler be scaled up inside, and this is preventing it from working efficiently? Could my rotted expansion tank be contributing to my problem? (That’s probably a stupid question). The house was originally built with a fireplace and chimney which may have been functional when the boiler and baseboard radiators were installed (but aren’t now)…would that factor into the size of the boiler? Or should it have been sized as the only source of heat for the whole house regardless of any other heating sources?

Your advice, tips, help or general anecdotes are greatly appreciated.