Viessmann · Vitogas 100 (GS1 / GS10)
Viessmann Vitogas 100 GS1-35
The Vitogas 100 GS1-35 is the middle GS1 size: 145 MBH input (42 kW), 123 MBH output and 85.5% AFUE, stepping up to a wider four-section chassis.
Key figures
- Max input
- —
- Max CSA output
- —
- AFUE
- 85.5 %
- Min input
- —
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Viessmann Vitogas 100 GS1-35 in practice
The GS1-35 fires at 145 MBH (42 kW) input with 123 MBH (36 kW) output and a 107 MBH (31 kW) Net I=B=R rating at 85.5% AFUE. It is the first GS1 size on the wider chassis: 864 x 840 mm (34 x 33 in) overall with four cast-iron sections and five burners.
Weight is 375 lb (170 kg) and boiler water content 3.6 USG at the series' 60 psig maximum operating pressure. The GS1 remains a fully assembled atmospheric boiler for chimney venting with a draft hood and vent damper.
Best suited to
- Existing GS1-35 installations needing printed ratings
- Mid-size residential loads between 115 and 165 MBH
- Documentation for the first wide-chassis GS1 size
Applicability and support
- Model number
- GS1-35
- Canadian availability
- historical
- Production status
- discontinued
- ROMA supplies
- Confirmed at consultation
- ROMA services
- Confirmed at consultation
- Warranty
- Not verified
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Specifications
Published specifications
Values as published by Viessmann. Units are shown with every value; notes flag conflicting sources or missing figures.
| Specification | Value | Note |
|---|---|---|
| Input | ||
| CSA input (natural gas) | 145 MBH | 42 kW, per the technical data manual |
| Output | ||
| CSA output / DOE heating capacity | 123 MBH | 36 kW |
| Net I=B=R rating | 107 MBH | 31 kW; based on piping and pick-up allowance of 1.15. |
| Efficiency | ||
| Efficiency (AFUE) | 85.5 % | With intermittent pilot electronic ignition and vent damper. |
| Venting and air | ||
| Vent pipe collar | Per model | 5 to 7 in. outside diameter per the technical data manual. |
| Physical and connections | ||
| Weight (boiler with insulation, burner and control) | 375 lb | 170 kg |
| Overall length | 864 mm | Printed as 34 in; heights 33¼ in (845 mm) with control in operating position. |
| Overall width | 840 mm | Printed as 33 in |
| Overall height (operation) | 845 mm | 33¼ in; 955 mm with control in operation position, 1180 mm for servicing. |
| Operating limits and other | ||
| Fuel | Natural gas or propane | 5 in. w.c. minimum NG supply pressure; 11 in. w.c. for propane. |
| Cast iron sections | 4 | 5 burners. |
| Boiler water content | 3.6 USG | |
| Maximum operating pressure | 60 psig | |
| Boiler supply / return | 1½ in. male thread | Gas supply ½ in. NPT. |
Manuals
- Technical Data Manual — Vitogas 100 GS1 Series, doc 5167 462 v1.55167 462 v1.5 · 2011-02
- Installation Instructions — Vitogas 100 GS1, rev 1.31.3 · 2011-02
- Technical Data Manual — Vitogas 100 GS10 Series, doc 5167 466 v1.35167 466 v1.3 · 2011-02
- Service Instructions — Vitogas 100 GS10, rev 1.11.1 · 2007-11
Alerts documented for this model
None documented yet.
Frequently asked questions
What is the GS1-35 rated at?
145 MBH (42 kW) input, 123 MBH (36 kW) output, a 107 MBH (31 kW) Net I=B=R rating and 85.5% AFUE.
How does the GS1-35 differ from the GS1-29?
It adds a section and a burner (4 / 5 vs 3 / 4), raises input from 115 to 145 MBH, and moves to the wider 864 x 840 mm chassis at 375 lb.
Does the GS1-35 have a vent damper?
Yes. The GS1 technical data manual notes AFUE figures are with intermittent pilot electronic ignition and a vent damper; each boiler is ordered with one to comply with minimum AFUE requirements.
Questions about the Viessmann Vitogas 100 GS1-35?
Describe what the boiler is doing and get a straight answer about the next step.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Compare and explore
- Viessmann Vitogas 100 GS1-22Same series
- Viessmann Vitogas 100 GS1-29Same series
- Viessmann Vitogas 100 GS1-42Same series
- Viessmann Vitogas 100 GS1-48Same series
- Viessmann Vitogas 100 GS1-60Same series
- Viessmann Vitogas 100 GS10-72Same series
- Viessmann Vitogas 100 GS10-84Same series
- Viessmann Vitogas 100 GS10-96Same series
- Vitogas 100 (GS1 / GS10) series hub
- Buying guide
Sources and verification
- Technical Data Manual — Vitogas 100 GS1 Series, doc 5167 462 v1.5 · 5167 462 v1.5 · accessed September 9, 2026
- Viessmann Canada — Historic manuals · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.