3x Oversized ModCon with Concrete Slab, New Home

Hello, this is my 1st post here. Great Site BTW.

I'm designing my new ICF home. 2900 ft2 slab heat 3-zones with master/slave zone synchronization.

I did a ResCheck DOE heat loss estimate with my proposed plans & got UA = 400

Location is Cripple Creek Colorado so my design Temp is about -30*F, thus at 70* indoor my Delta T is about 100* 400 x 100 = 40,000 BTU/hr.

I have a new in box Buderus GB142-45.

Looks like this boiler is about 3x oversized after being derated for 10,000' altitude.

I did the math on my slab, 40yds concrete x 3,800#/yd (with rebar) = 152,000#

With a Specific heat of .2 it will take 30,400 btus to raise the slab 1*f

So at -30* my boiler duty cycle will be about 33%, at freezing (this is a guess) possibly a 10-15% duty cycle?

So my question is: Should I sell the Buderus & buy smaller ModCon OR will slab act like a large buffer tank & Buderus will perform fine with low duty cycles?

Thanks BBP