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I need to know the total glass area of all the windows. I'm assuming 2 exterior insulated doors?
Ok, I'll get back to you in a bit once I do the calcs
I get a heat loss based on these specs of 54,500 BTUh. Since you're using hot water radiators I would upsize the boiler output a minimum of 30% to 50% to accommodate the large water content of the system. If you don't it could take forever for the radiators to warm up from a cold start. Since most condensing boilers modulate burner output according to system demand upsizing the boiler has no downside. In fact an even higher output capacity has no downside if the boiler modulates burner output.
No way the heat loss is anything near 180. That said you will be dealing with this guy (I assume) after the install and I can virtually guarantee that he will point to the smaller boiler (than he recommended) as the cause. The Prestige series will modulate down to as low as 25% of the max output so 25% of the 175k model is ~44k. Probably more than what you want for optimal efficiency but not terrible either. The other consideration is DHW. If you are using an indirect fired water heater (it is the best option) and you have a lot of hot water demand then the 180k boiler output would be beneficial and is even required to achieve rated DHW tank output in some cases. If you don't have any other options for a second opinion from another plumber and you have a decent relationship with your plumber (he isn't likely to be offended by your shopping for my opinion) then mention my calcs to him. The thing is that low water volume condensing boilers were not designed for high water volume systems so bigger isn't necessarily a bad thing.
Also Triangle Tube is a good product.
Not a huge fan of Navien. We actually prefer Buderus but Triangle is also a good option. I'd get a second opinion if you can
Slant fin makes great baseboard their boilers are ok but I have no experience with Slant Fin condensing boilers.I'd appreciate a favorable rating