Sizing a Buderus GB142

[chillwater]

This new home calls for a heating load of 65,000 btuh @ 0 degrees F outside ambient. Load calcs done using wrightsoft software. There has been 18000 btuh of radiant staple up installed. The entire house has been ducted and conected to a first co a/h with a hydro coil supplying 65000 btuh @ 140 F water temp or 103000 btuh @ 180 F water temp. There will also be a 50 gal. indirect water heater connected to this boiler. The boiler does have priority zoning. Which GB would you recomend? The GB142/30 has a rated output of 28,100-91,500 btuh @ 176/140 F water temp or 30,700-102,400 btuh @ 122/86 F water temp.
The GB142/45 has a rated output of 42,500-142,000 btuh @ 176/140 F or 47,200-158,000 btuh @ 122/86 F .

[skippedover]

One of the beautiful things of the mod/con boiler is that it can modulate down to match the connected load for any given temperature. I'm not sure why the hydro-air when there's so much radiant load connected but with staple-up, you generally have to run a higher temperature than you would for masonry or top of floor. Having said that, you're still way ahead with the raidant, IMO. With the connected load you've got, I'd go with the large of the two boilers. That'll cover you for cold temperatures. I'm not clear on what weather zone the installation is but looking at it from a New England perspective, that's what I'd do. :D

[BadgerBoiler MN]

oversizing any gas-fried appliance; it wastes fuel.

If your heat load is 65m the GB142-24 is the one. If you oversize the boiler it will only run to the load and will not condense if fired at its lowest output.

I run my fan coils off the indirect water heater. It has to be hot say 140F which is perfect for fancoils and presents a constant load to the boiler and steady source of comfortable water for the fan coil.

Reset will determine more moderate and efficient water temperatures for your radiant floors.

Sizing the boiler accurately will allow it to modulate and condense in a wider range.

MA