Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
The discontinued SSB255 and SSB399 are the same 609 x 1564 x 550 mm floor-standing stainless condensing cabinet with 5:1 turndown, 80 psi MAWP and 194 °F maximum operating temperature. The SSB255 is the 96% AFUE model with 255,900 BTU/hr input and 206 MBH net; the SSB399 moves to 399,000 BTU/hr input, 386 MBH gross output and 336 MBH net with 97.6% combustion / 96.9% thermal efficiency instead of an AFUE row.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Buderus SSB255 | Buderus SSB399 |
|---|---|---|
| Input, maximum | 255.9 MBH | 399 MBH |
| Input, minimum | 51.18 MBH | 79.8 MBH |
| Gross output (AHRI) | — | 386 MBH |
| Combustion efficiency (AHRI) | — | 97.6 % |
| Thermal efficiency (AHRI) | — | 96.9 % |
| Net rating (AHRI) | 206 MBH | 336 MBH |
| Weight (dry) | 198 lb | 220 lb |
| water-volume | — | — |
| Gas connection | 1" | 1" |
| Water connections | 1 1/2" | 1 1/2" |
Both sizes are documented in the SSB255/399/512 installation manual with the same dimensions (23 31/32 x 61 9/16 x 21 21/32 in; 609 x 1564 x 550 mm), 5:1 turndown, 80 psi MAWP and 1" gas connection. The SSB255 uses 1 1/2" water connections; the SSB399 also uses 1 1/2".
The rating rows differ in format: the SSB255 prints 255,900 BTU/hr input, 237.0 MBH heating capacity, 96% AFUE and 206.0 MBH net rating; the SSB399 prints 399,000 BTU/hr input, 386.0 MBH gross output, 97.6% combustion efficiency, 96.9% thermal efficiency and 336.0 MBH net rating. Both have a 3" or 4" vent connection respectively (SSB255 3", SSB399 4").
Dry weights are 198 lb and 220 lb, and both are discontinued with manuals in Bosch's US commercial-discontinued archive. The larger SSB512 continues the same cabinet with 512,000 BTU/hr input and 4" venting.
Bosch Thermotechnology
The Buderus SSB255 is a discontinued floor-standing commercial/residential stainless steel condensing boiler rated 255,900 BTU/hr maximum input with 5:1 turndown, 96% AFUE and 206 MBH net rating.
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.
The AHRI certified ratings table prints 96% AFUE for the SSB255 but prints combustion (97.6%) and thermal (96.9%) efficiency rows for the SSB399 instead. Boilers.ca reproduces exactly what each column prints.
Gross output rises from the SSB255's 237.0 MBH heating capacity to 386.0 MBH, with net rating rising from 206.0 to 336.0 MBH.
No - the SSB255 uses a 3" (80 mm) vent/air connection and the SSB399 a 4" (100 mm) connection, both compatible with CPVC, PVC, PP, PP Flex and AL29-4C.
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.