I have 2 options for heating my in-law apartment, forced air gas furnace in a 4' craw space with concrete floor and insulated, or a h/w boiler inside a utility closet inside the home. I can't use forced air inside the home because the utility closet is sitting right over an I-beam, can't run down flow ducting. Looking to see what would be the least cost, furnace with all the duct work for the craw space or a boiler running pex piping in the craw space. I would need a 40,000 btu output furnace, can anyone tell me what size boiler I would need in place of this furnace? I believe the hot water output would have to be greater, am I correct?

 

If you need 40,000 Btu/hr, either a hot-water boiler or forced air furnace with that output rating would be satisfactory. Why do you think a hot-water boiler would need to have a larger rating?

 

Correction, I needed a 45,000 btu furnace,
I worked for a plumbing/heating company years ago, my boss did all the sizing and I helped with the installs, never knew to much on the sizing part of it. A company from Home Depot wanted to come out and size up my addition for free so I let them, figured I'd let them tell me what I needed and I'd buy a furnace or boiler and install it myself. I was given a quote for a 45,000 btu furnace and when I asked what size boiler I would need if I decided to go that way they told me a 50,000 btu. I was wondering why I would need a larger btu boiler than the furnace, that's why I was asking, figured maybe it had something to do with forced air registers and hot water fin tubing, thought I'd ask to get a second opinion.
I found a boiler with a rating of:
Max input capacity 52, 000
DOE heating capacity 44,000
Net I=B=R capacity 38,000
The furnace quote I got had a rating of:
Max input btu 44,000
Max heating output btu 42,000
For the little extra money I'd rather have h/w heat than forced air. Would the boiler I found be the correct size I would need? I appreciate all the help...

 

I must say you are a gentleman offering an inlaw apartment of about 1600 sq. ft. as that is about what size the 40K boiler would heat. I believe the boiler stated is way oversized.