Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
Boilers.ca is a brand of ROMA Heating & Cooling · Licensed gas contractor LGA0202903
Comparison
Verdict in brief
The GBX-105 and GBX-140 are adjacent sizes of Smith's discontinued residential cast-iron water boiler line: 105 versus 140 MBH AGA input, 86 versus 115 MBH DOE heating capacity and 75 versus 100 MBH net I=B=R. AFUE is essentially identical (82.1% versus 82.0%) and both use 1-1/2" supply and 1-1/4" return tappings, so the difference is capacity for the home's heat loss.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Smith GBX-105 | Smith GBX-140 |
|---|---|---|
| AGA input (as printed) | 105 MBH | 140 MBH |
| DOE heating capacity | 86 MBH | 115 MBH |
| Net I=B=R water rating | 75 MBH | 100 MBH |
| AFUE | 82.1 % | 82 % |
| Supply / return tappings | 1-1/2" NPT supply / 1-1/4" NPT return | 1-1/2" NPT supply / 1-1/4" NPT return |
Both models come from the GBX line that Smith's official navigation marks '(Discontinued)'. The BSG-10 selection guide prints 105 MBH AGA input for the GBX-105 against 140 MBH for the GBX-140; DOE heating capacity is 86 versus 115 MBH and net I=B=R water is 75 versus 100 MBH. Both run on natural or propane gas with intermittent ignition.
Efficiency does not separate them: 82.1% AFUE for the -105 and 82.0% for the -140. The GBXIOM-4 manual documents the shared installation arrangement — supply to the 1-1/2" NPT tapping at the top and rear, return to the 1-1/4" NPT tapping near the bottom of the left section — so the larger size does not require different connection planning.
Water content rises from 6.00 to 7.28 gallons and the jacket is larger, but the meaningful difference is capacity. For legacy replacement planning, match the existing boiler's net rating to the home's current heat loss; a newer condensing boiler at a lower input can often serve the same load, which is a conversation for a heating professional.
Smith Cast Iron Boilers
The Smith GBX-105 is a discontinued residential cast-iron water boiler rated 105 MBH AGA input with 86 MBH DOE heating capacity, 75 MBH net I=B=R and 82.1% AFUE, on 1-1/2" supply and 1-1/4" return tappings.
Smith Cast Iron Boilers
The Smith GBX-140 is a discontinued residential cast-iron water boiler rated 140 MBH AGA input with 115 MBH DOE heating capacity, 100 MBH net I=B=R and 82% AFUE, on 1-1/2" supply and 1-1/4" return tappings.
Only a fraction of a point: BSG-10 prints 82.1% AFUE for the GBX-105 and 82.0% for the GBX-140.
Yes. GBXIOM-4 documents 1-1/2" NPT supply and 1-1/4" NPT return tappings for the GBX line.
No. Smith's official site navigation marks the GBX line '(Discontinued)'. The published ratings are useful for matching a replacement, which is a job for a licensed heating contractor.
A consultation includes a heat-loss review so the size is right the first time.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.