Mod Con Sizing Dilemma

StewBrewer

Circa 1990 2 story single family house w/ 5300 sf of heated space (4100 sf above grade + 1200 basement) in Chicago suburbs
Manual J says design day heat loss is 120K (2 degrees outside, 70 inside)
Smaller boiler AHRI Heating Capacity = 129K w/ low fire of 13k
Larger Boiler AHRI Heating Capacity = 181K w/ low fire of 18k
30 gallon buffer tank will be connected to either boiler to address small zone loads
Q: Is there a drawback to selecting the larger boiler considering the 30 gal buffer is sized to allow at least a 10 minute run time at low fire with the larger boiler?

hot_rod

Why go bigger, if the small one covers design load? You get a lower turndown, possibly a less expensive boiler, with the smaller one?

If you are using it with an indirect the larger may be useful if your DHW loads are high, or in quick succession.

What type and how do the heat emitters match the load?

Do they have good options on the control?
Rate limiting, step fire, boost are all handy to fine tune a system. Remote reading and adjustability?

StewBrewer

The only reasons for going with the bigger boiler would be lack of experience with the results of the manual j vs reality(the worst monthly gas furnace consumption in the last eight years adjusted for that month’s degree days shows 65k btuh output), and the possibility that the boiler's listed AHRI heating capacity might not be achievable due to lack of condensing at max conditions or decline in performance with age.
No indirect planned.
Emitters are 3100 sf of retrofit pex-al-pex in plates screwed underneath subfloor with ¾ hardwood on top, 1000 sf radiant slabs, 1200 sf basement (emitters tbd – 100 ft baseboard?) I am converting the house from 2 furnaces forced hot air.
Boiler info is for Laars mascot firetube (heat only). Manual notes boost capability, unfamiliar with other features.

hot_rod

I think the smaller one, 140,000 input would be adequate. Or shop for a 150 or 160,0000 input, something between the 140 and 180.

lkstdl

Normally I'd agree with Hot Rod here, but for the Laars FT, I believe the MFTHW199 actually has a lower minimum fire (19,900 Btu/h) than the MFTHW140 (28,000 Btu/h).

Manual: https://www.laars.com/images/uploads/products/1342E-NH.pdf

Where are you getting the low fire of 13K?

StewBrewer

I may have mistakenly referred to the boiler as a "Mascot". The newest versions are just called "FT Series". All but the smallest version now have 10:1 turn down. The new version of MFTHW140 is now 13,000 Btu/h min fire.

https://www.laars.com/images/uploads/products/1486B.pdf

https://www.laars.com/images/uploads/products/1483A.pdf