Posted By sailawayrb on 09 Jan 2024 01:45 AM
Well, you first need to accurately determine the required heat load before selecting the heat source.

How should I go about determining that given that my current boiler is of unknown efficiency and function?

Your boiler efficiency is most likely between 55 and 75%...maybe just use 65%. Since this is an existing building, you have two options...a heat loss analysis (AKA ACCA Manual J) or an energy usage analysis. We have free DIY calculators on our website that could accomplish both:

https://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

https://www.borstengineeringconstruction.com/Existing_Building_Energy_Usage_Analysis_Calculator.html

Alternatively, you could hire someone to figure this out for you. Again, you might want to post on Green Building Advisor to get help with this.

Just came across your thread, and glad to see you've provided an update. I'm also in PA with an old boiler (A.O.Smith) for the radiant, and a 19.9k BTU condensing tankless water heater (Takagi TK-540P-NIH). Considering our options for if/when the boiler fails. Did you get any info from the city as to whether an isolated loop using the TACO X Block is permissible? What is the reason you've focused on combi heaters instead of the loop now? Other than it does seem the models you've mentioned are priced less than just the TACO X Block system.