Sorry about the disaster, that's gotta really stink.
Many, many thoughts... Might not be enough room on the internet for them all. May also cost you some money, as I'll probably point out some stuff your contractor either doesn't know, or won't tell you, and it'll mean different equipment.
Let me list a few things as a checklist. No particular order after the first few.
1: Do not allow the contractor to size the system without you verifying that it's correct. Do not pay some joker for a manual J calc, and if anyone says they want to do one, run. They know 0 about heating systems. You can do your own load calc in about 30 seconds, and if the contractor doesn't know that or doesn't tell you that, they are either too stupid to install a system, or they're a crook who's trying to sell you a bill of goods. No third option.
2: Daughter's 45 minute showers would cease in my house after the first one reached about 10 minutes. That's what they invented the hot tub for. Water bills must be horrible, and if you have a well, that's a lot of demand on it imho. Either way, I'm not paying to dump hot water down the drain 45 minutes at a time. Ymmv.
3: If your current system isn't a direct vent system with both vents installed, do not even consider using whoever installed that. Meaning, if it goes out a chimney, that contractor would be buried somewhere out in my backyard.
4: I'll assume you have a boiler from the terms you said. You didn't specify if it's a furnace or a boiler (unless I'm supposed to look up that model). I ripped out a Burnham boiler when I moved into the house I'm in now. Horrifyingly oversized, and inefficient on a scale that's hard to describe. Literally, I own the house because the utility costs bankrupted the previous owners (I bought it repo'ed for cash) and since 2018, I've cut the gas consumption by >85%. It's a BIG house, and there's 3 heating systems in it. Boiler for one side, forced air for the other, and gas radiant in the garage.
5: LI NY, you might get a little coastal influence I don't, but largely we've got the same weather. If you can PM me some PDFs of your gas bills last winter, I'll tell you what they say about the house. Or just give me all the pertinent data. # days billed, CCF, Avg temp, Degree Days if shown, what gas company (they will most likely have the weather history on their site). Take note, I don't care what the Sq Ft'age is and I don't care what sizes your windows are. That's all a scam where they bill you hourly to do a s*** load of math that not only means nothing, it's dead wrong in every case and intentionally so. That calculation is horse___ and everyone knows it. I've seen stats that 98% of all heating systems in the US are oversized. Not a question in my mind the troubles you've had with that old one were from exactly that. And don't let them tell you that modulation is going to fix that oversize issue.. It doesn't. And I don't care what the turndown ratio is.
I have a Bosch boiler, and if you know about Bosch, you know it wasn't the cheapest one I could get. Difference in price was unknown, and I don't care. It is SILENT. Literally, put your head against it and you can't tell if it's running or not. It is efficient, it has been bulletproof, worth every penny. I did install EVERYthing new though. Not the baseboard and lines themselves, but every valve, trinket, zone pump, controller, plumbing, everything.
Mine is not a combi, I had already installed a tankless water heater. They come with some considerations... Some may not have a good time with new faucets. While they're super fast, and super efficient, you must use commercial faucets because any new faucet in the past decade or so won't flow enough water for them to go into high fire. I installed a Moen kitchen faucet, and the thing would not flow enough, so the water heater took 30 seconds to heat up. Other faucets would raise to full temp (130F) in 3 seconds. Be aware of this in advance. In the kitchen, you might even want a tiny 1Gal tank under the sink if you cook a lot.
Not sure who your gas company is, but check into a rebate. UGI here has a $1200 rebate on a new combi boiler. That's a govt program so I assume all gas companies have similar programs, but sometimes they're geographically limited, so look into that.
I have a spreadsheet for tracking my usage from the last couple years. I installed a new furnace in March of 25 and wanted to quantify what effect that had. I'd be happy to send a copy if you were interested. Might be a bit daunting, (it's big) but, shows usage by day, factored for heating degree days (temp) and all manner of comparisons from previous years. Also has notes for when changes were made and what change, which allows one to see what a particular change can do.
I also have an electric sheet which shows that removing a York 93.3% condensing HE furnace with a single stage blower, and replacing with a Goodman 96.5% with a variable speed blower, reduced the electric cost in the house by >$700 in one year. And that was NOT changing out the central air unit with is still the old one from 2007. Furnace paid for itself in 1 year in gas and electric. Never mind the comfort difference.
Anyhoo, if interested, send me those gas bills if you want. I have to stop typing. I'm at work, and if I worked for me, I'd be fired...
