That looks like a really nice unit. Possibly a bit overkill, but I see the TDR (Turn Down Ratio) is 11:1 on the thing so it's capable of running at ~9% of total capacity when demand is lower. I'd still prefer it was smaller, but a boiler like this isn't as dependent of sizing correctness as a furnace would be. For a furnace it's simply the end of the world. They don't have the ability to turn down (modulate) the way a boiler does. If they make a smaller one, I'd be getting that.
Without looking closer, I'm guessing that's a 250KBTU water heater and a 150KBTU Boiler, simultaneously. Again, I do not own a combi, so they may have some nuance of sizing that means they need to be bigger than I expect given what I know about the demand. But let's get to the demand.
I looked at the files you sent in the PM. Your company does things a little different than mine, but it's basically the same and I'll explain. They also give more info on the bill, with each one showing the usage history. That's cool. I'll look up the site for the weather history when time allows. But, we can make a pretty good guess from this.
So, your usage: They use "Therms", where I (and my gas company) use CCF. A CF is a cubic foot of gas. That has 1030 BTUs of energy. A CCF is 100 Cubic Feet of gas, which has 103000 BTUs of energy. The first C = Roman numeral C = 100 (ie: a C note = 100 bucks). Now, a "Therm" is a relatively new unit they started using to make comparisons easier. It represents 100000 BTUs of input energy. So, more or less a CCF. In this way, it's easier to compare to say Propane which has 2544 BTU's per cubic foot, but where it's sold by the gallon and a gallon represents 35.97 cubic feet of propane "Gas", and makes it require a BS in mathematics to really determine which is cheaper, gas or propane. Therm's makes it consistent across fuel types (oil @ 138500/Gallon, wood, coal, whatever.) I pay by the CCF, so to compare to propane requires some math. Gas is way cheaper, but propane companies like to play games with the words to make it seem that's not so.
Ok, so your peak "Month" was February (as was everyone else's) and the billing cycle showed 29 days of usage between meter reads ending on the 26th. CCFs were 223 on the meter (reads in volume), Therms was 232. So you used 232 x 100,000 = 23,200,000 BTU's of input energy during that 29 day period. Take 23,200,000 and divide by 29 days, then divide by 24 hours and we get 33,333 BTUH's (BTU Hours) of total gas input for the month (average).
So there is the 'Load Calc' for the house. Done. Irrefutably accurate, and for the coldest winter we've had possibly in our lifetimes. Don't care about the windows, don't care about the walls, what direction it faces, none of that. I turned the heat on and this is what it the gas company says I used, done. Only questions that remain are, how much does the demand increase when the temps are at their lowest, is that a linear progression, and do I have enough baseboards to get that heat out of the heating appliance? The last one being the big one.
In theory, and on very nice new construction, yes the graph is a straight line. But if the house is not tight and there is a measure of draft (stack effect), where hot air rises through the structure and out, then it becomes non linear. My house for instance, the "new" section was built in 1915, so they were not concerned with stack effect so mine is quite non-linear. But I've seen other folks online who have real-time usage data show that some homes literally use gas in such a way that a graph of the temp and a graph of the gas consumption, the lines cover each other all but when it's windy.
For my house where I have data of usage by day and temps by day, it shows that I used 58,796 BTU's on my Feb bill, ended around the 8th I think. However, I had 2 days where demand was 80,240 avg. So blindly looking at my 58KBTU average won't work any better than your 33K. Gotta add at least another 1/3, and online I've seen recommendations of 40%. Whatever, still puts you in the 50KBTU range for heating load, and don't forget those useage numbers include your kitchen and hot water. For me, kitchen is electric (induction/resistive ovens) but water is also gas (tankless).
The third of those questions was about baseboard. If you have the typical stuff, copper pipe and fins, that's about 620 BTUs per lineal foot at 180F. Measure how many feet in total in the house, and you have the max energy you can remove from that boiler. If your capacity of the baseboard is low, and I'm betting it is, like way low, then the boiler short cycles something horrible and that's why you've had issues with the previous boiler. They are designed to turn on and stay on. Not turn on and off. This situation is far worse with a furnace and improper ductwork, but that's another story. I could type here for hours about all the issues these two problems cause.
The biggest issue most times is people think replacing the boiler / furnace is the answer, when often the issue is to fix the house or change the ductwork / radiators. If those won't allow the boiler / furnace to put the entire heating capacity into the living space, no amount of sizing and installing is going to fix it. In CA, they have made it illegal to install a duct "Bypass" on a heating system. It's against code now, and damn well about time. Seems everyone has some mickey-mouse rigg'n on a heating system that's industry std. Drives me crazy.
Long story short, I'm not thrilled about the 250K / 150K numbers on that model. Not going to say outright that it's wrong, but I have a bad feeling about it. I would like to learn more about it. If there is a smaller model, get that. My water heater is a 200Kbtu tankless, way overkill for my needs. However, my house is a 7 BR, 7 full bath with 2 kitchens and it services all of it. Capacity 13gpm @ 130F. Usual summer gas use is 2-3 CCF, so about $3-4 / month for hot water. So I don't care. I also had no choice in availability when I bought it, so its fine. But with time, choices and more info, I might have chosen differently.
When you were typing your reply, a friend stopped in my office to ask about a similar problem he's going through, sizing a gas boiler and water heater, and he's the second one to stop here in the last 7 days, lol. I'll cut this off here and follow up later.
