Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
The GS1-22 and GS1-29 are the two smallest Vitogas 100 GS1 boilers and share the same 844 x 650 mm footprint. The GS1-29 adds a cast-iron section and a burner — three and four against two and three — raising input from 90 to 115 MBH and weight from 273 to 326 lb, while AFUE eases from 85.6% to 85.5%.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Viessmann Vitogas 100 GS1-22 | Viessmann Vitogas 100 GS1-29 |
|---|---|---|
| CSA input (natural gas) | 90 MBH | 115 MBH |
| CSA output / DOE heating capacity | 76 MBH | 98 MBH |
| Net I=B=R rating | 66 MBH | 85 MBH |
| Efficiency (AFUE) | 85.6 % | 85.5 % |
| Cast iron sections | 2 | 3 |
| Weight (boiler with insulation, burner and control) | 273 lb | 326 lb |
| Boiler water content | 2.6 USG | 3.1 USG |
Both boilers are atmospheric cast-iron sectional units at 60 psig maximum operating pressure in the same 844 x 650 mm (33¼ x 25½ in) cabinet with a 845 mm height. Inputs print at 90 MBH (26 kW) for the GS1-22 and 115 MBH (34 kW) for the GS1-29, with outputs of 76 and 98 MBH and Net I=B=R ratings of 66 and 85 MBH.
The build difference is one cast-iron section and one burner: 2 sections / 3 burners on the GS1-22 against 3 / 4 on the GS1-29. That takes weight from 273 to 326 lb (124 to 148 kg) and boiler water content from 2.6 to 3.1 USG.
Printed AFUE is 85.6% on the GS1-22 and 85.5% on the GS1-29, both with intermittent pilot electronic ignition and a vent damper. The choice is simply the calculated load: below about 90 MBH the GS1-22 is sufficient, and its lower weight suits tighter mechanical spaces.
Viessmann
The Vitogas 100 GS1-22 is the smallest of the discontinued GS1 residential cast-iron boilers: 90 MBH input (26 kW), 76 MBH output and 85.6% AFUE in a two-section atmospheric boiler.
Viessmann
The Vitogas 100 GS1-29 is the second GS1 size: 115 MBH input (34 kW), 98 MBH output and 85.5% AFUE with four burners and a 3.1 USG water content.
They share the 844 x 650 mm footprint and 845 mm height; weight differs at 273 lb (124 kg) for the GS1-22 and 326 lb (148 kg) for the GS1-29.
90 MBH (26 kW) input / 76 MBH output on the GS1-22, and 115 MBH (34 kW) input / 98 MBH output on the GS1-29, both natural gas with propane ratings printed in the manual.
The GS1-22 prints 85.6% AFUE against the GS1-29's 85.5% — a tenth apart, both with intermittent pilot ignition and a vent damper.
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.