Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
The discontinued SSB399 and SSB512 share the 609 x 1564 x 550 mm cabinet, 5:1 turndown, 80 psi MAWP, 4" vent and 1" gas connection. The SSB399 is rated 399,000 BTU/hr input, 386 MBH gross output and 336 MBH net at 97.6% combustion efficiency; the SSB512 raises input to 512,000 BTU/hr with 495 MBH gross output, 430 MBH net and 95.4% combustion / 96.8% thermal efficiency, and moves to a 2" water connection.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Buderus SSB399 | Buderus SSB512 |
|---|---|---|
| Input, maximum | 399 MBH | 512 MBH |
| Input, minimum | 79.8 MBH | 102.4 MBH |
| Gross output (AHRI) | 386 MBH | 495 MBH |
| Combustion efficiency (AHRI) | 97.6 % | 95.4 % |
| Thermal efficiency (AHRI) | 96.9 % | 96.8 % |
| Net rating (AHRI) | 336 MBH | 430 MBH |
| Weight (dry) | 220 lb | 242 lb |
| water-volume | — | — |
| Gas connection | 1" | 1" |
| Water connections | 1 1/2" | 2" |
Both models occupy the same floor-standing cabinet (23 31/32 x 61 9/16 x 21 21/32 in; 609 x 1564 x 550 mm) and share the 4" (100 mm) vent/air connection, 1" gas connection, 5:1 turndown, 80 psi MAWP, 194 °F maximum operating temperature and 210 °F maximum heat-exchanger temperature.
Ratings from the technical data table: SSB399 at 399,000 BTU/hr input, 79,800 minimum, 386.0 MBH gross output, 336.0 MBH net, 97.6% combustion and 96.9% thermal efficiency; SSB512 at 512,000 BTU/hr input, 102,400 minimum, 495.0 MBH gross output, 430.0 MBH net, 95.4% combustion and 96.8% thermal efficiency.
The SSB512's higher flow needs the larger water connection (2" against 1 1/2") and holds 6.0 US gallons against 4.5; dry weights are 242 lb and 220 lb. Both are discontinued and documented in the SSB255/399/512 installation manual, still served from Bosch's US commercial-discontinued archive.
Bosch Thermotechnology
The Buderus SSB399 is a discontinued floor-standing commercial/residential stainless steel condensing boiler rated 399,000 BTU/hr maximum input with 5:1 turndown, 96.9% AHRI thermal efficiency and 336 MBH net rating.
Bosch Thermotechnology
The Buderus SSB512 is a discontinued floor-standing commercial/residential stainless steel condensing boiler rated 512,000 BTU/hr maximum input with 5:1 turndown, 96.8% AHRI thermal efficiency and 430 MBH net rating.
The SSB512 at 512,000 BTU/hr maximum input, with 495.0 MBH gross output and 430.0 MBH net rating.
Its higher firing rate needs more flow: 2" against the SSB399's 1 1/2", per the technical data table.
Yes - both are stainless steel condensing boilers with CPVC/PVC/PP/PP Flex/AL29-4C venting and condensate management described in the installation manual.
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.