Too big of a Buderus?

Jarno7

I am planning to replace the 65 year old Columbia boiler, with tankless coil, in my 1200 square foot house. I have had two contractors look at the layout of my system,  and one recommended a Biasi B-10/4 , and one recommended a B-10/5. To add to the confusion, I have been offered a slightly used Buderus G215/3 in exchange for removing it from it's present location, due to the owner switching to gas.



The house is a single zone, with about 13 feet of baseboard in one room, and 19 feet of Beacon Morris convectors throughout the rest of the rooms. I would like to incorporate a Triangle Tube Smart 40 indirect water heater in the new installation.



The contractors I spoke to recommended the Biasi primarily due to it performing well with a poor draft situation- the existing boiler exhausts via a 12 foot long horizontal run to a 8" sq tile lined chimney. I have since decided to install a new triple wall chimney adjacent to the new boiler, whatever it may be, to assure the best possible draft.



They both recommended a boiler with low mass since there are no big radiatiors- just the baseboard and convectors. The water capacity of the Buderus seems quite large at 12.9 gallons and I am concerned that that might be a major source of inefficiency in my case. I'm sure it was an asset in the original location in an older home with big radiators.



Is the Buderus an acceptable boiler for my new application, since it is available for the removal, or is it too oversized for my application to result in any substantial economy over my old boiler.?The claim made for the Biasi is the boiler can "go cold" except when heat is required due to it's low water volume. I am making the assumption that this would not be the case with the high water volume in the Buderus. I value any comments and advice you wish to offer!

Eastman

What does the house need?

Maybe all the options are too big.  Has someone done a heat loss calculation?

Jarno7

Heat loss

I did a heat loss calculation using the Smart/Fin application and came up with 31914 Btu. Neither of the contractors recommending the Biasi did any measurements- they just had the total square footage, and I did not want to contaminate their thinking with my results since I lack confidence in my ability in that area. I simply measured the rooms accuratly and entered the info- without any practical experience I would not be able to know how realistic the number I got was, and why I am hoping someone here can offer some insight. This is the number the application returned, no "safety factor" applied.



My concern is what size furnace would be ideal to handle both the house heating and the Smart 40 indirect water heater, with normal "factors of safety" applied, and whether the Buderus G215/3, with it's large water volume, is an acceptable choice given my relatively low volume radiation. I realize it appears to be too large for my small house, but how much is "too big"?

SWEI

Sounds about right

for an old house of that size with no envelope upgrades.



Higher water content in the boiler is a good thing, especially if the radiation is low mass.



One of our oil pros should be able to tell us how far can you safely downfire a 215-3.  Even at 30k, you'd still have the same recovery capacity as a standard gas-fired tank water heater.  Unless you have a huge soaker tub, I would not worry about DHW capacity.

Robert O'Brien

Heat loss

No need for a heat loss on an oil heated 1200 sq/ft home. The smallest oil boilers are at least double what you need. What type of chimney? A 215 can't be downfired by any appreciable degree,the stack temp will be too low. i'd go with a B10-3 or 115/3

Aaron_in_Maine

215/3 way too big

The heat loss you have pretty good. I have a 1300sqft home in Maine where it gets real cold. My heat loss is 42,500btu. I am putting in a baisi b10-3 this fall. Mr. O'Brien is right the b10-3 or 115/3 is the right boiler for you.