How's your gas meter?
In my neck of the woods they usually top out at 250 btus if I remember correctly. The tankless would put us over and that would require a bigger meter at my cost as well as a new line to the house and re-piping the whole house! Again it was a while ago.
Water heater 200
Furnace 80
stove/dryer/fireplace/grill/etc
You can see where this goes. Its one of the reason those things are expensive as hell to install. I had an electric one. It was pretty great but an energy hog and it just encourages you to take longer hotter showers IMO.
Have you looked into a steam shower? Honestly might be cheaper then re-piping your whole house.
Is there an easy way to check this?
Given that it's a northern 'large' home, my guess is that we can supply 200k BTU, but unsure if furnace and heater are on at the same time?
Your meter should have a tag with the information on it, or a model number you could Google for information. Your gas company owns the meter, so you could call and ask them what the standard meter can supply.
For pipe inside you house, Page 40 & 41 of the installation instructions have tables to estimate the capacity different pipe sizes can provide for various lengths.
Correct. In general you can simply google the model number and it will tell you.
The way they do it is they assume every gas appliance you have is running full out. So if that puts you >250 you need to upsize. Do you have any other gas appliances in your house? Stove, dryer, fireplace, grill etc.
I put in a new direct vent 75 gallon hot water tanks. For me it was a good compromise. Its not quite tankless but I have no run out of hot water yet.
IMO it is one of the reasons we don't see more of these installed. In the south you can get away with an electric one because it does not need nearly as much temperature gain. But in zones 4-6 the water is pretty cold and it takes a ton of energy to get it warmed up.